The The ss-Block Elements-Block Elements
40.140.1 Characteristic Properties of the Characteristic Properties of the ss-Block -Block
ElementsElements
40.240.2 Variation in Properties of the Variation in Properties of the ss-Block -Block ElementsElements
40.340.3 Variation in Properties of the Variation in Properties of the CompCompounds of the ounds of the ss-Block Elements-Block Elements
4040
The The ss-Block Elements-Block Elements
• Elements of Groups IA* (the alkali metals) and IIA* (the alkaline earth metals)
constitute the s-block elements
their outermost shell electrons are in the s orbital
*Note: In the following, Groups IA and IIA are abbreviated as Groups I and II respectiv
ely.
The The ss-Block Elements-Block Elements
• The two groups of elements have many similarities
highly reactive metals
strong reducing agents
form ionic compounds with fixed oxidation states of +1 for Group I
elements and +2 for Group II elements
The s-block elements
Group I elementsGroup I elements
• Lithium
Group I elementsGroup I elements
• Sodium
Group I elementsGroup I elements
• Potassium
Group I elementsGroup I elements
• Rubidium
Group I elementsGroup I elements
• Francium
Group I elementsGroup I elements
• Beryllium
Group I elementsGroup I elements
• Magnesium
Group I elementsGroup I elements
• Calcium
Group I elementsGroup I elements
• Strontium
Group I elementsGroup I elements
• Barium
Group I elementsGroup I elements
• Radium
40.140.1Characteristic PrCharacteristic Pr
operties of theoperties of thes-Block Elementss-Block Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.38)
Some characteristic properties of Group I elements
Group I element
Atomic number
Electronic configuration
Electronegativity value
Oxidation state in compounds
Li
Na
K
Rb
Cs
Fr
3
11
19
37
55
87
[He] 2s1
[Ne] 3s1
[Ar] 4s1
[Kr] 5s1
[Xe] 6s1
[Rn] 7s1
1.0
0.9
0.8
0.8
0.7
–
+1
+1
+1
+1
+1
–
40.1 Characteristic Properties of the s-Block Elements (SB p.38)
Some characteristic properties of Group I elements
Group I element
Oxide formedHydroxide
formedFlame colour
Li
Na
K
Rb
Cs
Fr
Li2O
Na2O, Na2O2
K2O, K2O2, KO2
Rb2O, Rb2O2, RbO2
Cs2O, Cs2O2, CsO2
–
LiOH
NaOH
KOH
RbOH
CsOH
–
deep red
yellow
lilac
bluish red
blue
–
40.1 Characteristic Properties of the s-Block Elements (SB p.38)
Some characteristic properties of Group II elements
Group II element
Atomic number
Electronic configuration
Electronegativity value
Oxidation state in compounds
Be
Mg
Ca
Sr
Ba
Ra
4
12
20
38
56
88
[He] 2s2
[Ne] 3s2
[Ar] 4s2
[Kr] 5s2
[Xe] 6s2
[Rn] 7s2
1.5
1.2
1.0
1.0
0.9
–
+2
+2
+2
+2
+2
–
40.1 Characteristic Properties of the s-Block Elements (SB p.38)
Some characteristic properties of Group II elements
Group II element
Oxide formedHydroxide
formedFlame colour
Be
Mg
Ca
Sr
Ba
Ra
BeO
MgO
CaO
SrO, SrO2
BaO, BaO2
–
Be(OH)2
Mg(OH)2
Ca(OH)2
Sr(OH)2
Ba(OH)2
–
–
–
bluish red
blood-red or crimson
blue
–
Metallic CharacterMetallic Character
• All Group I elements
silvery solids and tarnish rapidly in air at room temperature and pressure
stored under paraffin oil or in vacuum-sealed ampoules
(to prevent contact with oxygen and water vapour in air)
40.1 Characteristic Properties of the s-Block Elements (SB p.38)
Metallic CharacterMetallic Character
• All Group I elements
weak metallic bonds
only one valence electron per atom delocalized into the electron s
ea for the formation of metallic bonds
soft and can be cut with a knife easily
40.1 Characteristic Properties of the s-Block Elements (SB p.38)
Metallic CharacterMetallic Character
• All Group I elements
low melting points and boiling points
40.1 Characteristic Properties of the s-Block Elements (SB p.38)
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
sodium
Sodium is stored under paraffin oil
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Caesium and rubidium are stored in vacuum-sealed ampoules
caesium rubidium
Metallic CharacterMetallic Character
• All Group I elements
body-centred cubic structures
much empty space
comparatively low densities
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Some information about Group I elements
Group I element
Atomic radius (nm)
Ionic radius (nm)
Metallic structure
Melting point(C)
Lithium
Sodium
Potassium
Rubidium
Caesium
Francium
0.152
0.186
0.231
0.244
0.262
0.270
0.060
0.095
0.133
0.148
0.169
0.176
b
b
b
b
b
–
180.5
97.8
63.7
39.1
28.4
27
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Some information about Group I elements
Group I element
Boiling point(C)
Density at 20 C (g cm–3)
Abundance on Earth (%)
Lithium
Sodium
Potassium
Rubidium
Caesium
Francium
1 330
890
774
688
690
680
0.53
0.97
0.86
1.53
1.87
–
0.002 0
2.36
2.09
0.009 0
0.000 10
Trace
Metallic CharacterMetallic Character
• All Group II elements
greyish solids at room temperature and pressure
also be cut with a knife, but harder than the alkali metals
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Metallic CharacterMetallic Character
• All Group II elements
two valence electrons per atom
smaller atomic sizes
metallic bonds are stronger
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Metallic CharacterMetallic Character
• All Group II elements
the melting points and boiling points are higher than those of Group I
elements
show different metallic structures
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Metallic CharacterMetallic Character
• Beryllium and magnesium
hexagonal close-packed structures
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Metallic CharacterMetallic Character
• Calcium and strontium
face-centred cubic structures
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
Metallic CharacterMetallic Character
• Barium
body-centred cubic structure
40.1 Characteristic Properties of the s-Block Elements (SB p.39)
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
Some information about Group II elements
Group II element
Atomic radius (nm)
Ionic radius (nm)
Metallic structure
Melting point(C)
Beryllium
Magnesium
Calcium
Strontium
Barium
Radium
0.112
0.160
0.197
0.215
0.217
0.220
0.031
0.065
0.099
0.113
0.135
0.140
h
h
f
f
b
–
1 278
648.8
839
769
729
697
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
Some information about Group II elements
Group II element
Boiling point(C)
Density at 20 C (g cm–3)
Abundance on Earth (%)
Beryllium
Magnesium
Calcium
Strontium
Barium
Radium
2 477
1 100
1 480
1 380
1 640
1 140
1.85
1.75
1.55
2.54
3.60
5.0
0.000 28
2.33
4.15
0.038
0.042
Trace
Low ElectronegativityLow Electronegativity
• All s-block elements
low electronegativity values
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
Low ElectronegativityLow Electronegativity
• All s-block elements
electropositive elements
their atoms have a relatively high tendency to lose their outermost s
hell electrons
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
Low ElectronegativityLow Electronegativity
• The outermost shell electrons
effectively shielded from the nucleus by the fully-filled inner electron sh
ells
the outermost shell electrons are only loosely held by the nucleus
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
Low ElectronegativityLow Electronegativity
• Going down both Groups I and II
the elements become more electropositive
the atoms tend to lose electrons more readily
the outermost shell electrons are much further away from the
nucleus
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
Low ElectronegativityLow Electronegativity
• Group II elements
more electronegative than the Group I elements
the increase in effective nuclear charge
the attractive force between the nucleus and the outermost shell electrons becomes stronger
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
Electronegativity values of Groups I and II elementsGroup I element
Electronegativity value
Group II element
Electronegativity value
Li
Na
K
Rb
Cs
Fr
1.0
0.9
0.8
0.8
0.7
–
Be
Mg
Ca
Sr
Ba
Ra
1.5
1.2
1.0
1.0
0.9
–
Formation of Basic OxidesFormation of Basic Oxides
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
• All alkali metals form more than one type of oxide on burning in air (except lithium)
1. Group I Elements1. Group I Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
• Three types of oxides:
normal oxides
peroxides
superoxides
1. Group I Elements1. Group I Elements
• They are all ionic
• They can be related as follows:
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
1. Group I Elements1. Group I Elements
2O21
2OO2–
oxide ion
O22–
peroxide ion
2O2–super
oxide ion
• Lithium
when it is burnt in air, it forms normal oxide only
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
1. Group I Elements1. Group I Elements
C180
4Li(s) + O2(g) 2Li2O(s) lithium oxide
• Sodium
when it is burnt in an abundant supply of oxygen
forms both the normal oxide and the peroxide
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
1. Group I Elements1. Group I Elements
C180
4Na(s) + O2(g) 2Na2O(s) sodium oxid
e C300
4Na2O(s) + O2(g) 2Na2O2
(s) sodium peroxi
de
• Potassium, rubidium and caesium
form the normal oxide, the peroxide and superoxides when they are b
urnt in air
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
1. Group I Elements1. Group I Elements
• Potassium:
4K(s) + O2(g) 2K2O(s)potassium oxide
2K2O(s) + O2(g) 2K2O2(s)potassium peroxide
K2O2(s) + O2(g) 2KO2(s)potassium superoxide
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
1. Group I Elements1. Group I Elements
• Rubidium:
4Rb(s) + O2(g) 2Rb2O(s)
2Rb2O(s) + O2(g) 2Rb2O2(s)
Rb2O2(s) + O2(g) 2RbO2(s)
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
1. Group I Elements1. Group I Elements
• Caesium:
4Cs(s) + O2(g) 2Cs2O(s)
2Cs2O(s) + O2(g) 2Cs2O2(s)
Cs2O2(s) + O2(g) 2CsO2(s)
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
1. Group I Elements1. Group I Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.41)
Oxides formed by Group I elementsGroup I element
Normal oxide Peroxide Superoxide
Li
Na
K
Rb
Cs
Li2O
Na2O
K2O
Rb2O
Cs2O
–
Na2O2
K2O2
Rb2O2
Cs2O2
–
–
KO2
RbO2
CsO2
• Lithium
does not form the peroxide or superoxide
the size of lithium ion is very small
leading to its high polarizing power
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
1. Group I Elements1. Group I Elements
• When a peroxide ion or superoxide ion approaches a lithium ion
the electron cloud of the peroxide ion or superoxide ion would be greatl
y distorted by the lithium ion
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
1. Group I Elements1. Group I Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
1. Group I Elements1. Group I Elements
The electron cloud of the superoxide ion is greatly distorted by the small lithium ion
• The greater the distortion of the electron cloud
the lower the stability of the compound
lithium peroxide and lithium superoxide do not exist
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
1. Group I Elements1. Group I Elements
• Potassium ion, rubidium ion and caesium ion
larger sizes
relatively low polarizing power
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
1. Group I Elements1. Group I Elements
• The electron cloud of the peroxide ion or superoxide ion
not be seriously distorted by the metallic cations
pack around them with a higher stability
able to form stable peroxides or superoxides
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
1. Group I Elements1. Group I Elements
• Beryllium, magnesium and calcium
form normal oxides only on burning in air
2Be(s) + O2(g) 2BeO(s)
2Mg(s) + O2(g) 2MgO(s)
2Ca(s) + O2(g) 2CaO(s)
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
• Strontium and barium
able to form normal oxides and peroxides of the formula MO2 whe
n the metals are burnt in air
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
• Strontium
2Sr(s) + O2(g) 2SrO(s)strontium oxide
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
2SrO(s) + O2(g) 2SrO2(s) strontium peroxide
• Barium
2Ba(s) + O2(g) 2BaO(s) barium oxide
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
2BaO(s) + O2(g) 2BaO2(s)
barium peroxide
500C
700C
• All these oxides are basic in nature (except beryllium oxide which is amphoteric)
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
Oxides formed by Group II elements
Group II element
Normal oxide Peroxide Superoxide
Be
Mg
Ca
Sr
Ba
BeO
MgO
CaO
SrO
BaO
–
–
–
SrO2
BaO2
–
–
–
–
–
• Beryllium peroxide does not exist
the small size of beryllium ion
high polarizing power of beryllium ion
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
• The small beryllium ion
greatly distorts the electron cloud of the peroxide ion
results in instability of the compound
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
• Beryllium ion
smaller in size
higher charge than lithium ion
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
• Beryllium ion
distorts the electron cloud of the peroxide ion to a greater extent th
an lithium ion
beryllium peroxide is very unstable
40.1 Characteristic Properties of the s-Block Elements (SB p.42)
2. Group II Elements2. Group II Elements
• The ionic radii of potassium ion and barium ion
very similar
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
2. Group II Elements2. Group II Elements
• Potassium
forms stable superoxide on burning in air
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
2. Group II Elements2. Group II Elements
• Barium
does not forms stable superoxide
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
2. Group II Elements2. Group II Elements
• Barium ion
higher charge than potassium ion
higher polarizing power than potassium ion
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
2. Group II Elements2. Group II Elements
• The electron cloud of the superoxide ion
greatly distorted by barium ion
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
2. Group II Elements2. Group II Elements
• Barium ion
difficult to pack with the large superoxide ions in a stable ion
ic lattice
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
2. Group II Elements2. Group II Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
Formation of HydroxidesFormation of Hydroxides
• All Group I elements (except lithium)
react with water to form metal hydroxides and hydrogen gas
1. Group I Elements1. Group I Elements
2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
2K(s) + 2H2O(l) 2KOH(aq) + H2(g)
2Rb(s) + 2H2O(l) 2RbOH(aq) + H2(g)
2Cs(s) + 2H2O(l) 2CsOH(aq) + H2(g)
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
1. Group I Elements1. Group I Elements
• They are basic oxides
react exothermically with water to form the corresponding hydrox
ides
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
1. Group I Elements1. Group I Elements
• For normal oxides
M2O(s) + H2O(l) 2MOH(aq)
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
1. Group I Elements1. Group I Elements
• For peroxides
M2O2(s) + 2H2O(l) 2MOH(aq) + H2O2(a
q)
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
1. Group I Elements1. Group I Elements
• For superoxides
2MO2(s) + 2H2O(l) 2MOH(aq) + H2O2(aq) + O2(g)
where M2O, M2O2 and MO2 represent the normal oxides, peroxides and superoxides of Group I elements respectively
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
1. Group I Elements1. Group I Elements
• Hydroxides of the Group I elements
the strongest bases known (except lithium hydroxide)
40.1 Characteristic Properties of the s-Block Elements (SB p.43)
1. Group I Elements1. Group I Elements
• All Group II elements (except beryllium and magnesium)
react with water to form metal hydroxides and hydrogen gas
less vigorous than the Group I elements in the same period
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
2. Group II Elements2. Group II Elements
Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g)
Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g)
Ba(s) + 2H2O(l) Ba(OH)2(aq) + H2(g)
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
2. Group II Elements2. Group II Elements
• Beryllium
does not react with water or steam
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
2. Group II Elements2. Group II Elements
• Magnesium
reacts very slowly with water
but reacts more quickly with steam to form magnesium oxide and hydrogen gas
Mg(s) + H2O(g) MgO(s) + H2
(g)
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
2. Group II Elements2. Group II Elements
• Calcium and strontium
react readily with water at room temperature and pressure
the reactivity of Group II elements with water increases down the gro
up
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
2. Group II Elements2. Group II Elements
• The hydroxides of calcium, strontium and barium
prepared by reacting the normal oxides with water
CaO(s) + H2O(l) Ca(OH)2(aq)
SrO(s) + H2O(l) Sr(OH)2(aq)
BaO(s) + H2O(l) Ba(OH)2(aq)
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
2. Group II Elements2. Group II Elements
• Magnesium oxide
only slightly soluble in water
but it dissolves in acids to form salts
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
2. Group II Elements2. Group II Elements
• Beryllium oxide
almost insoluble in water or in acids
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
2. Group II Elements2. Group II Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
Calcium reacts readily with water at room temperature and pressure
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
Ionic Bonding with FixedIonic Bonding with FixedOxidation State in their CompoundsOxidation State in their Compounds
• s-Block elements form compounds
predominantly ionic in nature
show constant oxidation states of +1 for Group I elements and +2 for
Group II elements
• For Group I elements
form ions with an oxidation state of +1 only
their atoms have only one outermost shell electron
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
1. Group I Elements1. Group I Elements
• Once this outermost shell electron is removed
a stable fully-filled electronic configuration is obtained
the first ionization enthalpies of Group I elements are low
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
1. Group I Elements1. Group I Elements
• The second ionization
involves the removal of an electron from an inner electron shell
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
1. Group I Elements1. Group I Elements
• Once this electron is removed
the stable electronic configuration will be disrupted
their second ionization enthalpies are very high
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
1. Group I Elements1. Group I Elements
• Group I elements
form predominantly ionic compounds with non-metals
by losing their single outermost shell electrons
form ions having a fixed oxidation state of +1
40.1 Characteristic Properties of the s-Block Elements (SB p.44)
1. Group I Elements1. Group I Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
Chemical formulae of some Group I compounds and the oxidation states of Group I elements in the
compoundsGroup I element
Oxide Hydride ChlorideOxidation state of Group I element in the compound
Li
Na
K
Rb
Cs
Li2O
Na2O2
KO2
RbO2
CsO2
LiH
NaH
KH
RbH
CsH
LiCl
NaCl
KCl
RbCl
CsCl
+1
+1
+1
+1
+1
• For Group II elements
they form ions with an oxidation state of +2 only
their atoms have two outermost shell electrons in the s orbital
2. Group II Elements2. Group II Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
• Once these two outermost shell electrons are removed
a stable fully-filled electronic configuration is obtained
the sum of the first and second ionization enthalpies of Group II
elements is relatively low
2. Group II Elements2. Group II Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
• The third ionization
corresponds to the removal of an electron from an inner fully-filled
electron shell
2. Group II Elements2. Group II Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
• The third ionization enthalpy
an extremely large positive value for these elements
these elements do not form ions with an oxidation state of +3
2. Group II Elements2. Group II Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
• Group II elements
form predominantly ionic compounds with non-metals
by losing their two outermost shell electrons
form ions having a fixed oxidation state of +2
2. Group II Elements2. Group II Elements
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
Chemical formulae of some Group II compounds and the oxidation states of Group II elements in the
compoundsGroup II element
Oxide Hydride ChlorideOxidation state of Group II element in the compound
Be
Mg
Ca
Sr
Ba
BeO
MgO
CaO
SrO
BaO
BeH2
MgH2
CaH2
SrH2
BaH2
BeCl2
MgCl2
CaCl2
SrCl2
BaCl2
+2
+2
+2
+2
+2
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• Most s-block elements
give a characteristic flame colour in the flame test
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• Method:
putting a sample of the elements or their compounds into a non-lu
minous Bunsen flame
40.1 Characteristic Properties of the s-Block Elements (SB p.45)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• The outermost shell electrons of atoms of both Groups I and II elements
weakly held by the nucleus
the electrons are easily excited to higher energy levels upon heating
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• When these electrons return to their ground states
radiation is emitted
falls into the visible light region of the electromagnetic spectrum
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• The amount of energy of the emitted radiation is quantized
the flame colour is a characteristic property of the element
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• Example:
Sodium chloride
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• When sodium chloride is heated in a Bunsen flame
the ions are converted to gaseous atoms
Na+ Cl–(g) Na(g) + Cl(g)
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• The electrons in the gaseous sodium atoms
excited to higher energy levels
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
Characteristic Flame Colours of SaltsCharacteristic Flame Colours of Salts
• When the excited electrons return to their ground state
light of golden yellow colour is given out
gives a golden yellow flame on burning
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
The characteristic flame colours of some Groups I and II elements in the flame test
Group I element Flame colour Group II element Flame colour
Li
Na
K
Rb
Cs
Deep reed
Golden yellow
Lilac
Bluish red
Blue
Ca
Sr
Ba
Brick-red
Blood-red or
crimson
Green
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
Characteristic flame colours of some Groups I and II elements in the flame test: (a) lithium-containing compounds give a deep red flame; (b) sodium-containing compounds give a golden yellow flame; (c) potassium-containing compounds give a lilac flame; (d) calcium containing compounds give a brick-red flame
(a)
(b)
(c)
(d)
Weak Tendency to Form ComplexesWeak Tendency to Form Complexes
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
A complex is formed when a central metal atom or ion is surrounded by other molecules or ions (called ligands) which form dative covalent bonds with the central metal atom or ion.
Weak Tendency to Form ComplexesWeak Tendency to Form Complexes
• Complex formation
a common characteristic of d-block metal ions
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
Weak Tendency to Form ComplexesWeak Tendency to Form Complexes
• When a d-block metal ion is surrounded by ligands (such as NH3 and Cl–)
the lone pair electrons of the ligands can be donated to the central d-bl
ock metal ion
form dative covalent bonds
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
Weak Tendency to Form ComplexesWeak Tendency to Form Complexes
• Example:
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
Weak Tendency to Form ComplexesWeak Tendency to Form Complexes
• The presence of low-lying vacant d-orbitals in the d-block metal ions
accept the lone pair electrons from the surrounding ligands
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
Weak Tendency to Form ComplexesWeak Tendency to Form Complexes
• s-Block metal ions
also be surrounded by polar molecules
but there is only electrostatic attraction between the metal io
n and the negative ends of the dipoles
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
Weak Tendency to Form ComplexesWeak Tendency to Form Complexes
• s-Block metal ions
do not have low-lying vacant orbitals available for forming dative covale
nt bonds
rarely form complexes
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
Check Point 40-1Check Point 40-1
Variation in Physical PropertiesVariation in Physical Properties
40.2 Variation in Properties of the s-Block Elements (SB p.49)
1. Atomic Radius and Ionic Radius1. Atomic Radius and Ionic RadiusThe atomic radii and ionic radii of most Groups I
elementsGroup I element Atomic radius (nm) Ionic radius (nm)
Li
Na
K
Rb
Cs
0.152
0.186
0.231
0.244
0.262
0.060
0.095
0.133
0.148
0.169
40.2 Variation in Properties of the s-Block Elements (SB p.49)
The atomic radii and ionic radii of most Groups II elements
Group II element Atomic radius (nm) Ionic radius (nm)
Be
Mg
Ca
Sr
Ba
0.112
0.160
0.197
0.215
0.217
0.031
0.065
0.099
0.113
0.135
40.2 Variation in Properties of the s-Block Elements (SB p.49)
Variations in atomic radius and ionic radius of Groups I and II elements
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• Atoms of all Groups I and II elements
form their respective M+ and M2+ ions by losing their outermost s electro
ns
1.1. The ionic radius of any Groups I or II The ionic radius of any Groups I or II element is smaller than its atomic raelement is smaller than its atomic radiusdius
• There is one electron shell less in the cation than the atom
the nucleus pulls the electron cloud more closely towards it
the ionic radius is smaller than the atomic radius
1.1. The ionic radius of any Groups I or II The ionic radius of any Groups I or II element is smaller than its atomic raelement is smaller than its atomic radiusdius
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• The atoms or the ions
more electron shells occupied
the outermost electron shells become further away from the nucleus
2.2. Going down both Groups I and II, boGoing down both Groups I and II, both the atomic radii and ionic radii inth the atomic radii and ionic radii increasecrease
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• The outermost shell electrons
more effectively shielded by the inner shell electrons from the nuclear
charge
2.2. Going down both Groups I and II, boGoing down both Groups I and II, both the atomic radii and ionic radii inth the atomic radii and ionic radii increasecrease
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• A decrease in the attractive force between the nucleus and the outermost shell electrons
both the atomic radii and ionic radii increase down a group
2.2. Going down both Groups I and II, boGoing down both Groups I and II, both the atomic radii and ionic radii inth the atomic radii and ionic radii increasecrease
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• Each atom of the Group II element
one more electron in the outermost shell
one more proton in the nucleus than each atom of the Group I element
in the same period
3.3. Going from Group I to Group II in eaGoing from Group I to Group II in each period, the atomic radii and ionic ch period, the atomic radii and ionic radii decreaseradii decrease
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• The additional electron
enters the same shell
at approximately the same distance from the nucleus
3.3. Going from Group I to Group II in eaGoing from Group I to Group II in each period, the atomic radii and ionic ch period, the atomic radii and ionic radii decreaseradii decrease
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• The repulsion between electrons
relatively ineffective to cause an increase in both the atomic radius
and ionic radius
3.3. Going from Group I to Group II in eaGoing from Group I to Group II in each period, the atomic radii and ionic ch period, the atomic radii and ionic radii decreaseradii decrease
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• There is only very little or even no increase in the shielding (or screening) effect of the inner shell electrons on the outermost shell electrons
3.3. Going from Group I to Group II in eaGoing from Group I to Group II in each period, the atomic radii and ionic ch period, the atomic radii and ionic radii decreaseradii decrease
40.2 Variation in Properties of the s-Block Elements (SB p.50)
• The additional proton in the nucleus of the atoms of Group II elements
stronger attractive force to the electrons
the atomic radii and ionic radii of the Group II elements are smaller
3.3. Going from Group I to Group II in eaGoing from Group I to Group II in each period, the atomic radii and ionic ch period, the atomic radii and ionic radii decreaseradii decrease
40.2 Variation in Properties of the s-Block Elements (SB p.50)
2. Ionization Enthalpy2. Ionization Enthalpy
40.2 Variation in Properties of the s-Block Elements (SB p.50)
Ionization enthalpies of Groups I elements
Group I elementFirst ionization
enthalpy (kJ mol–1)Second ionization enthalpy (kJ mol–1)
Li
Na
K
Rb
Cs
Fr
519
494
418
402
376
381
7 300
4 560
3 070
2 370
2 420
–
40.2 Variation in Properties of the s-Block Elements (SB p.50)
Ionization enthalpies of Groups II elements
Group II element
First ionization enthalpy (kJ mol–
1)
Second ionization enthalpy (kJ mol–
1)
Third ionization enthalpy (kJ mol–1)
Be
Mg
Ca
Sr
Ba
Ra
900
736
590
548
502
510
1 760
1 450
1 150
1 060
966
979
14 800
7 740
4 940
4 120
3 390
–
40.2 Variation in Properties of the s-Block Elements (SB p.51)
Variations in the first and second ionization enthalpies of Group I elements
40.2 Variation in Properties of the s-Block Elements (SB p.51)
Variations in the first, second and third ionization enthalpies of Group II elements
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The outermost s electron
located in a new electron shell
1.1. The first ionization enthalpies of GroThe first ionization enthalpies of Group I elements are relatively lowup I elements are relatively low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The attractive force between thiss electron and the nucleus
relatively weak
1.1. The first ionization enthalpies of GroThe first ionization enthalpies of Group I elements are relatively lowup I elements are relatively low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The outermost s electron
effectively shielded from the attraction of the nucleus by the full
y- filled inner electron shells
1.1. The first ionization enthalpies of GroThe first ionization enthalpies of Group I elements are relatively lowup I elements are relatively low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• Once this electron is removed
a stable octet or duplet electronic configuration is attained
1.1. The first ionization enthalpies of GroThe first ionization enthalpies of Group I elements are relatively lowup I elements are relatively low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The outermost s electron
relatively easy to be removed
the first ionization enthalpies for Group I elements are relatively
low
1.1. The first ionization enthalpies of GroThe first ionization enthalpies of Group I elements are relatively lowup I elements are relatively low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The second ionization of Group I elements
involves the loss of an inner shell electron
closer to the nucleus
1.1. The first ionization enthalpies of GroThe first ionization enthalpies of Group I elements are relatively lowup I elements are relatively low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The removal of this electron
disrupts the stable electronic configuration
the second ionization enthalpies of Group I elements are extremely high
1.1. The first ionization enthalpies of GroThe first ionization enthalpies of Group I elements are relatively lowup I elements are relatively low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The two outermost s electrons of atoms of Group II elements
relatively easy to be removed
these two electrons are effectively shielded from the nucleus by i
nner electron shells
2.2. The first and second ionization enthThe first and second ionization enthalpies of Group II elements are relatialpies of Group II elements are relatively lowvely low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• Once these two electrons are removed
a stable octet or duplet electronic configuration is attained
2.2. The first and second ionization enthThe first and second ionization enthalpies of Group II elements are relatialpies of Group II elements are relatively lowvely low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The third ionization of Group II elements
involves the loss of an inner shell electron
closer to the nucleus
2.2. The first and second ionization enthThe first and second ionization enthalpies of Group II elements are relatialpies of Group II elements are relatively lowvely low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The stable electronic configuration
disrupte after the removal of this electron
the third ionization enthalpies of Group II elements are very hig
h
2.2. The first and second ionization enthThe first and second ionization enthalpies of Group II elements are relatialpies of Group II elements are relatively lowvely low
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• There is an increase in atomic radius down the groups
3.3. Going down both Groups I and II, thGoing down both Groups I and II, the ionization enthalpy generally decre ionization enthalpy generally decreaseseases
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The outermost shell electrons of the atoms
far away from the nucleus
experience less attraction from the positively charged nucleus
less energy is required to remove the outermost shell electrons of the atoms
3.3. Going down both Groups I and II, thGoing down both Groups I and II, the ionization enthalpy generally decre ionization enthalpy generally decreaseseases
40.2 Variation in Properties of the s-Block Elements (SB p.52)
3. Hydration enthalpy 3. Hydration enthalpy
Hydration enthalpy (Hhyd) is the amount of energy released when one mole of aqueous ions is formed from its gaseous ions.
40.2 Variation in Properties of the s-Block Elements (SB p.52)
M+(g) + aq M+(aq) H = H
hyd
always has a negative value
3. Hydration enthalpy 3. Hydration enthalpy
40.2 Variation in Properties of the s-Block Elements (SB p.52)
M+(g) + aq M+(aq) H = H
hyd
the amount of energy released resulting from the attraction betwe
en ions and water molecules
depends on the charge to radius ratio of the ion
3. Hydration enthalpy 3. Hydration enthalpy
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The greater the charge on the ion
the stronger the attraction between the ion and the water molecule
s
the larger the amount of energy given out
3. Hydration enthalpy 3. Hydration enthalpy
40.2 Variation in Properties of the s-Block Elements (SB p.52)
• The smaller the size of the ion
the higher the effective nuclear charge
the stronger the attraction between the ion and the water molecule
s
a large amount of energy is released
3. Hydration enthalpy 3. Hydration enthalpy
40.2 Variation in Properties of the s-Block Elements (SB p.53)
Hydration enthalpies of the ions of Groups I and II elements
Group I ionHydration enthalpy
(kJ mol–1)Group II ion
Hydration enthalpy (kJ mol–1)
Li+
Na+
K+
Rb+
Cs+
Fr+
–519
–406
–322
–301
–276
–
Be 2+
Mg2+
Ca2+
Sr2+
Ba2+
Ra2+
–2 450
–1 920
–1 650
–1 480
–1 360
–
40.2 Variation in Properties of the s-Block Elements (SB p.53)
Variations in hydration enthalpy of the ions ofGroups I and II elements
40.2 Variation in Properties of the s-Block Elements (SB p.54)
1.1. Going down both Groups I and II, thGoing down both Groups I and II, the hydration enthalpy of the ions decre hydration enthalpy of the ions decreases (becomes less negative)eases (becomes less negative)
• The ions
larger in size down both groups
the charge density of the ions falls
40.2 Variation in Properties of the s-Block Elements (SB p.54)
1.1. Going down both Groups I and II, thGoing down both Groups I and II, the hydration enthalpy of the ions decre hydration enthalpy of the ions decreases (becomes less negative)eases (becomes less negative)
• The electrostatic attraction between the ions and water molecules
weaker
40.2 Variation in Properties of the s-Block Elements (SB p.54)
1.1. Going down both Groups I and II, thGoing down both Groups I and II, the hydration enthalpy of the ions decre hydration enthalpy of the ions decreases (becomes less negative)eases (becomes less negative)
• The hydration enthalpy
less negative on going down the groups
40.2 Variation in Properties of the s-Block Elements (SB p.54)
2.2. The ions of Group II elements have The ions of Group II elements have more negative hydration enthalpies tmore negative hydration enthalpies than those of Group I elements in the han those of Group I elements in the same periodsame period
• Group II ions
a charge of +2
a smaller ionic radius
higher charge density than the Group I ions in the same period
40.2 Variation in Properties of the s-Block Elements (SB p.54)
2.2. The ions of Group II elements have The ions of Group II elements have more negative hydration enthalpies tmore negative hydration enthalpies than those of Group I elements in the han those of Group I elements in the same periodsame period
• The electrostatic attraction between the Group II ions and water molecules
stronger than that between the Group I ions and water molecules
40.2 Variation in Properties of the s-Block Elements (SB p.54)
• The melting points of Groups I and II elements depend on
the strength of the metallic bonding
how the atoms are arranged in the metallic crystal lattice
4. Melting Point4. Melting Point
40.2 Variation in Properties of the s-Block Elements (SB p.54)
• The stronger the metallic bond
the higher the melting point of the element
4. Melting Point4. Melting Point
40.2 Variation in Properties of the s-Block Elements (SB p.54)
• The strength of metallic bond depends on:
1. the ionic radius of the metal ion;
2. the number of valence electrons per atom of the element
4. Melting Point4. Melting Point
40.2 Variation in Properties of the s-Block Elements (SB p.54)
The melting points of Groups I and II elements
Group Ielement
Melting Point (C)
Group IIelement
Melting Point (C)
Li
Na
K
Rb
Cs
Fr
180
97.8
63.7
38.9
28.7
24
Be
Mg
Ca
Sr
Ba
Ra
1280
650
850
768
714
697
40.2 Variation in Properties of the s-Block Elements (SB p.55)
Variations in melting point of Groups I and II elements
40.2 Variation in Properties of the s-Block Elements (SB p.55)
• In each group
the number of electrons participating in metallic bonding remains the same
the ionic radii of the metal ions increase
1.1. Going down both Groups I and II, thGoing down both Groups I and II, the melting points generally decreasee melting points generally decrease
40.2 Variation in Properties of the s-Block Elements (SB p.55)
• The attractive forces between the electron sea and the metal ions
weaker
the metallic bond becomes weaker going down the groups
1.1. Going down both Groups I and II, thGoing down both Groups I and II, the melting points generally decreasee melting points generally decrease
40.2 Variation in Properties of the s-Block Elements (SB p.55)
2.2. The melting points of Group II elemeThe melting points of Group II elements are much higher than those of Grnts are much higher than those of Group I elementsoup I elements
• The ions of Group I elements
carry one positive charge only
• The ions of Group II elements
carry two positive charges
40.2 Variation in Properties of the s-Block Elements (SB p.55)
2.2. The melting points of Group II elemeThe melting points of Group II elements are much higher than those of Grnts are much higher than those of Group I elementsoup I elements
• The Group II elements
the greater number of valence electrons
the smaller ionic radii
40.2 Variation in Properties of the s-Block Elements (SB p.55)
2.2. The melting points of Group II elemeThe melting points of Group II elements are much higher than those of Grnts are much higher than those of Group I elementsoup I elements
• The attractive forces between the metal ions and the electron sea
stronger in Group II elements
40.2 Variation in Properties of the s-Block Elements (SB p.55)
2.2. The melting points of Group II elemeThe melting points of Group II elements are much higher than those of Grnts are much higher than those of Group I elementsoup I elements
• The metallic bond
stronger in Group II elements
40.2 Variation in Properties of the s-Block Elements (SB p.55)
3.3. The general decrease in melting poiThe general decrease in melting point down Group II elements is broken nt down Group II elements is broken with an irregularity with an irregularity ―― the melting p the melting point of magnesium is lower than that oint of magnesium is lower than that of calciumof calcium
• Melting point of a metal depends on
how the individual atoms are packed
in the crystal lattice
40.2 Variation in Properties of the s-Block Elements (SB p.55)
3.3. The general decrease in melting poiThe general decrease in melting point down Group II elements is broken nt down Group II elements is broken with an irregularity with an irregularity ―― the melting p the melting point of magnesium is lower than that oint of magnesium is lower than that of calciumof calcium
• Magnesium atoms
not particularly well packed in the
crystal lattice as compared to calci
um atomsCheck Point 40-2ACheck Point 40-2A
40.2 Variation in Properties of the s-Block Elements (SB p.56)
Variation in Chemical PropertiesVariation in Chemical Properties
• s-Block elements have strong reducing power
reflected by their low ionization enthalpies
40.2 Variation in Properties of the s-Block Elements (SB p.56)
Variation in Chemical PropertiesVariation in Chemical Properties
• The lower the ionization enthalpy
the stronger the reducing power of the metal
40.2 Variation in Properties of the s-Block Elements (SB p.56)
Variation in Chemical PropertiesVariation in Chemical Properties
• Going down both Groups I and II
the atomic size increases
easier for the atoms to lose the outermost shell electrons
the ionization enthalpy decreases accordingly
the reducing power of the metals increases down the groups
40.2 Variation in Properties of the s-Block Elements (SB p.56)
Variation in Chemical PropertiesVariation in Chemical Properties
• Group I metals
react readily with oxygen and water by losing their single outermos
t shell electron
40.2 Variation in Properties of the s-Block Elements (SB p.56)
Variation in Chemical PropertiesVariation in Chemical Properties
• Group II metals
generally less reactive than Group I metals
their higher ionization enthalpies
40.2 Variation in Properties of the s-Block Elements (SB p.56)
• Most s-block elements
show a silvery white lustre when they are freshly cut
they tarnish rapidly upon exposure to the atmosphere
they react with oxygen in the air to form an oxide layer
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.56)
Sodium shows a silvery white lustre when freshly cut
40.2 Variation in Properties of the s-Block Elements (SB p.56)
• s-block elements (except beryllium and magnesium)
very reactive
stored under paraffin oil or in vacuum-sealed ampoules
prevent contact with oxygen and water vapour in the air
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.56)
• Beryllium and magnesium
form an oxide layer with oxygen
but they tarnish comparatively slowly
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.56)
• All s-block elements
burn in air to form one or more of the three types of oxides
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.56)
• Three types of oxides
normal oxides
peroxides
superoxides
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.56)
“Dot-and-cross” diagrams of the three types of oxide ions
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• On burning in air, lithium
forms only lithium oxide (Li2O)
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• On burning in air, sodium
forms a mixture of sodium monoxide (Na2O) and sodium peroxide (Na2
O2)
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• On burning in air, potassium, rubidium and caesium
form the normal oxides, peroxides and superoxides
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.57)
1. Reaction with Oxyg1. Reaction with Oxygenen
4K(s) + O2(g) 2K2O(s) potassium oxide
C180
• Example:
C300
2K2O(s) + O2(g) 2K2O2(s)potassium peroxi
deK2O2(s) + O2(g) 2KO2(s)
potassium superoxide
C3000
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• Group II elements
form the normal oxides when they are burnt in air
2Be(s) + O2(g) 2BeO(s)
2Mg(s) + O2(g) 2MgO(s)
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• Beryllium and magnesium
relatively unreactive towards oxygen at room temperature and pressure
burn with a brilliant white flame when ignited
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.57)
Magnesium burns vigorously in air
Magnesium
40.2 Variation in Properties of the s-Block Elements (SB p.57)
Magnesium oxide is formed after burning
Magnesium oxide
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• Strontium and barium
form the normal oxides and peroxides on burning in air
2Ba(s) + O2(g) 2BaO(s) barium oxide
1. Reaction with 1. Reaction with
OxygenOxygen
2BaO(s) + O2(g) 2BaO2(s)
barium peroxide
500C
700C
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• Strontium and barium
very reactive at room temperature and pressure
stored under paraffin oil to prevent contact with air
1. Reaction with Oxyg1. Reaction with Oxygenen
40.2 Variation in Properties of the s-Block Elements (SB p.57)
Oxides formed by the s-block elements
Type of oxide FormulaMetals that form oxides when exposed to air or burnt in air
Normal oxide O2– All Groups I and II elements
Peroxide O22– Na, K, Rb, Cs, Sr, Ba
Superoxide O2– K, Rb, Cs
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• All Group I metals
react with cold water to form metal hydroxides and hydrogen
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.57)
• Example:
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g) (vigorous)
2Na(s) + 2H2O(l) 2NaOH(aq) + H2
(g) (violent)
2K(s) + 2H2O(l) 2KOH(aq) + H2(g) (explosi
ve)
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• All Group I metals
reduce cold water to form hydroxide ions and hydrogen
2. Reaction with Water2. Reaction with Water
H2O(l) + e– OH–(aq) + H2(g)21
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• The reactivity of Group I metals with water
related to the relative ease of the metal atoms to lose the outermost
shell electron
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Going down the group
the atomic size increases
the outermost shell electron becomes easier to be removed
the reactivity of Group I metals towards water increases down the
group
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Lithium
reacts with water vigorously
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Sodium
reacts with water violently
moves on the water surface with a hissing sound
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Potassium
reacts with water explosively
producing a lilac flame
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Rubidium and caesium
react with water explosively
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
Lithium reacts with water vigorously
40.2 Variation in Properties of the s-Block Elements (SB p.58)
Sodium reacts with water violently
40.2 Variation in Properties of the s-Block Elements (SB p.58)
Potassium reacts with water explosively
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• For Group II metals
less reactive than the Group I metals in the same period
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Beryllium
does not react with water
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Magnesium
reacts with steam rapidly to form magnesium oxide and hydrogen g
as
Mg(s) + H2O(g) MgO(s) + H2
(g) (vigorous)
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Magnesium
reacts with cold water very slowly to form magnesium hydroxide and
hydrogen gas
Mg(s) + 2H2O(l) Mg(OH)2(aq) + H2(g)
(very slow)
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Calcium
reacts with water steadily
Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g)
(moderate)
2. Reaction with Water2. Reaction with Water
40.2 Variation in Properties of the s-Block Elements (SB p.58)
• Strontium and barium
react with water vigorously
Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g)
(vigorous)
2. Reaction with Water2. Reaction with Water
Check Point 40-2BCheck Point 40-2B
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
Reactions of Oxides of Reactions of Oxides of ss-Block El-Block Elementsements
• The oxides of all Group I elements
react with water to form the corresponding hydroxides
it is exothermic
1. Reaction with Water1. Reaction with Water
• For the normal oxides of Group I elements
react with water to form metal hydroxides
as the only products
Li2O(s) + H2O(l) 2LiOH(aq)
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• For peroxides and superoxides of Group I elements
other products are formed
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• For peroxides of Group I elements
react with water to form metal hydroxides and hydrogen pero
xide
Na2O2(s) + 2H2O(l) 2NaOH(aq) + H2O2(a
q)
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
Dissolution of sodium peroxide in water containing phenolphthalein (The red colour is due to the formation of hydroxide ions which turn phenolph
thalein from colourless to red)
• For the superoxides of Group I elements
react with water to form metal hydroxides, hydrogen peroxid
e and oxygen
2KO2(s) + 2H2O(l) 2KOH(aq) + H2O2(aq) + O
2(g)
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• The basicity of the oxides of Group I elements increases down the group
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• The oxides of Group II elements
generally less basic than those of Group I elements
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• The oxides of all Group II elements (except beryllium oxide and magnesium oxide)
react with water to form weakly alkaline solutions
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• Example:
CaO(s) + H2O(l) Ca(OH)2(aq) (weakly alkaline)
SrO(s) + H2O(l) Sr(OH)2(aq) (weakly alkaline)
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• The basicity of the oxides of Group II elements also increases down the group
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• Beryllium oxide
amphoteric
almost insoluble in water or in acids
its amphoteric nature is shown only in its reactions with hot acids or alkal
is
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
BeO(s) + 2H+(aq) hot
Be2+(aq) + H2O(l)
BeO(s) + 2OH–(aq) + H2O(l) hot
[Be(OH)4]2–(a
q)beryllate io
n
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• Magnesium oxide
only slightly soluble in water
dissolves in acids to form salts
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• The oxides of other Group II elements
soluble in water
solubility increases down the group
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.59)
• Barium peroxide
reacts readily with cold water to form barium hydroxide and hydrogen
peroxide
BaO2(s) + 2H2O(l) Ba(OH)2(aq) + H2O2
(aq)
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• Metal peroxides and metal superoxides
strong oxidizing agents
indicated by their reactions with water to give hydrogen peroxide
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• Sodium peroxide
oxidizes green chromium(III) hydroxide to yellow sodium
chromate(VI)
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• Metal peroxides and metal superoxides
useful in both qualitative and quantitative analysis
2Cr(OH)3(s) + 3Na2O2(s) 2Na2CrO4(aq)
+ 2NaOH(aq) + 2H2
O(l)
1. Reaction with Water1. Reaction with Water
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• All oxides of Groups I and II elements (Except beryllium oxide is amphoteric)
basic
react readily with dilute acids
2. Reaction with Dilute Acids2. Reaction with Dilute Acids
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• Their normal oxides
neutralize the acids to form salts and water
CaO(s) + 2HCl(aq) CaCl2(aq) + H2
O(l)
2. Reaction with Dilute Acids2. Reaction with Dilute Acids
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• Their peroxides
react with dilute acids to form salts and hydrogen peroxide
Na2O2(s) + 2HCl(aq) 2NaCl(aq) + H2O2(a
q)
2. Reaction with Dilute Acids2. Reaction with Dilute Acids
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• Their superoxides
react with dilute acids to form salts, hydrogen peroxide and oxyge
n
2KO2(s) + 2HCl(aq) 2KCl(aq) + H2O2(aq) + O2
(g)
2. Reaction with Dilute Acids2. Reaction with Dilute Acids
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• In general, the oxides of s-block elements do not react with dilute alkalis
3. Reaction with Dilute Alkalis3. Reaction with Dilute Alkalis
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
• Except beryllium oxide
amphoteric
reacts with sodium hydroxide toform sodium beryllate
BeO(s) + 2NaOH(aq) + H2O(l) Na2Be(O
H)4(aq)
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
3. Reaction with Dilute Alkalis3. Reaction with Dilute Alkalis
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements
Thermal stability refers to the resistance of a compound to undergo decompositionon heating.
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• The higher the thermal stability of a comp
ound
the higher the temperatureneeded for the compound to
decompose thermally
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.60)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• A compound is often said to be
thermally stable
not decomposed at the temperature of the normal Bunsen flame
(approximately 1300 K)
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• The thermal stability of an ionic compoun
d depends on
the charges of its constituent ions
the sizes of its constituent ions
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• For compounds with relatively large polar
izable anions
the thermal stability is affected by the polarizing power of the cations
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• If the cation has strong polarizing power
distort the electron clouds of the neighbouring anions to a greater
extent
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• The compound
less stable
more likely to decompose on heating
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
The large anion is polarized by the small cation
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• Most carbonates and hydroxides of Grou
ps I and II metals
undergo decomposition on heating to give oxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• Example:
MgCO3(s) MgO(s) + CO2(g)
Ca(OH)2(s) CaO(s) + H2O(g)
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• Oxide ions
smaller in size than carbonate ions and hydroxide ions
less polarizable
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• The electrostatic attraction between the c
ations and oxide ions
stronger
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• The oxides of Groups I and II metals
more stable than the corresponding carbonates and hydroxides
decompose to give oxides upon heating
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
When a compound with large anions undergoes thermal decomposition, a compound with small anions will be
formed since small anions are less easily polarized
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• Group II metal ions
smaller in size
higher charge
stronger polarizing power
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.61)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• The large carbonate ions and hydroxide i
ons
distort to a greater extent
more readily to undergo thermal decomposition
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
Effect of sizes of the cations on thermal stability of the carbonates and hydroxides of both
Groups I and II metals
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
Relative Thermal Stability of the CarRelative Thermal Stability of the Carbonates and Hydroxides ofbonates and Hydroxides ofss-Block Elements-Block Elements• Going down each group
the size of the cations increases
the polarizing power of the cations decreases
the thermal stability increases
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
• The carbonates of all Group I metals (except lithium carbonate)
withstand up to a temperature around 800C
1. The Carbonates1. The Carbonates
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
• Lithium carbonate
decomposes at around 700C
form lithium oxide and carbon dioxide
1. The Carbonates1. The Carbonates
Li2CO3(s) Li2O(s) + CO2
(g)
C700
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
• Lithium carbonate
relatively unstable
lithium ion has the smallest size
the polarizing power is the highest
1. The Carbonates1. The Carbonates
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
• The electron cloud of any large anion
distort to a great extent
decompose more readily on heating
1. The Carbonates1. The Carbonates
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
• Group II metal ions
higher charges
smaller sizes
higher polarizing power
1. The Carbonates1. The Carbonates
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
• The electron cloud of the carbonate ion is much distorted
more readily to undergo thermal decomposition
1. The Carbonates1. The Carbonates
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.62)
• Example:
1. The Carbonates1. The Carbonates
C100BeCO3(s) BeO(s) + CO2(g)
MgCO3(s) MgO(s) + CO2(g) C540
CaCO3(s) CaO(s) + CO2(g) C900
SrCO3(s) SrO(s) + CO2(g) C1290
BaCO3(s) BaO(s) + CO2(g) C1360
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• The hydroxides of all Group I metals (Except lithium hydroxide)
stable to heating with a Bunsen flame
2. The Hydroxides2. The Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• Lithium hydroxide
the least stable on heating
2. The Hydroxides2. The Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• Lithium ion
extremely small
high polarizing power
distorts the electron cloud of the hydroxide ion
decomposition occurs
2. The Hydroxides2. The Hydroxides
2LiOH(s) Li2O(s) + H2O(g)
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• The hydroxides of Group II metals
less stable to heat
greater the polarizing power
2. The Hydroxides2. The Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• Going down the group
cationic sizes increases
thermal stability increases
2. The Hydroxides2. The Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• The enthalpy changes
provide evidence for the trend of increasing thermal stability
2. The Hydroxides2. The Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• Example:
2. The Hydroxides2. The Hydroxides
Be(OH)2(s) BeO(s) + H2O(g) H = +54 kJ m
ol–1 Mg(OH)2(s) MgO(s) + H2O(g)
H = +81 kJ mol–1
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• Example:
2. The Hydroxides2. The Hydroxides
Ca(OH)2(s) CaO(s) + H2O(g) H = +109 kJ m
ol–1 Sr(OH)2(s) SrO(s) + H2O(g)
H = +127 kJ mol–1
Ba(OH)2(s) BaO(s) + H2O(g) H = +146 kJ m
ol–1
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
Relative Solubility of the SulphatesRelative Solubility of the Sulphates(VI) and Hydroxides of(VI) and Hydroxides ofs-Block Elementss-Block Elements
1. Processes involved in Dissolution a1. Processes involved in Dissolution and their Energeticsnd their Energetics
• When an ionic solid dissolves in water
two processes are taking place
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• Two processes are
1. the breakdown of the ionic lattice
2. the subsequent stabilization of the ions by water molecules (this
process is called hydration)
1. Processes involved in Dissolution a1. Processes involved in Dissolution and their Energeticsnd their Energetics
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• When an ionic solid dissolves in water
there must be energetically favourable interactions between t
he water molecules and the dissolved ions
1. Processes involved in Dissolution a1. Processes involved in Dissolution and their Energeticsnd their Energetics
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• These interactons
compensate for the breaking of ionic bonds present in the ionic lattice
considered from the point of view of energetics
1. Processes involved in Dissolution a1. Processes involved in Dissolution and their Energeticsnd their Energetics
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• The first process
involves an absorption of energy
break down the ionic lattice
1. Processes involved in Dissolution a1. Processes involved in Dissolution and their Energeticsnd their Energetics
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• The second process
involves a release of energy
the ions are hydrated
1. Processes involved in Dissolution a1. Processes involved in Dissolution and their Energeticsnd their Energetics
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• Example:
The dissolution of sodium chloride in water
1. Processes involved in Dissolution a1. Processes involved in Dissolution and their Energeticsnd their Energetics
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• The enthalpy change of solution of sodium chloride = +4 kJ mol–1
when one mole of sodium chloride is completely dissolved in a sufficien
tly large volume of solvent to form an infinitely dilute solution
1. Processes involved in Dissolution a1. Processes involved in Dissolution and their Energeticsnd their Energetics
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.63)
• NaCl(s) Na+(g) + Cl–(g)
1.1. The sodium chloride solid lattice is bThe sodium chloride solid lattice is broken down to give its constituent ioroken down to give its constituent ions in the gaseous statens in the gaseous state
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• The enthalpy change
accompanies this process is the reverse of the lattice enthalpy of sodium
1.1. The sodium chloride solid lattice is bThe sodium chloride solid lattice is broken down to give its constituent ioroken down to give its constituent ions in the gaseous statens in the gaseous state
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• The lattice enthalpy of sodium chloride= –776 kJ mol–1
1.1. The sodium chloride solid lattice is bThe sodium chloride solid lattice is broken down to give its constituent ioroken down to give its constituent ions in the gaseous statens in the gaseous state
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• The enthalpy change of the above process = +776 kJ mol–1
NaCl(s) Na+(g) + Cl–(g) H = +776 kJ m
ol–1
1.1. The sodium chloride solid lattice is bThe sodium chloride solid lattice is broken down to give its constituent ioroken down to give its constituent ions in the gaseous statens in the gaseous state
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
2.2. The hydration of the resulting ionsThe hydration of the resulting ions
The enthalpy change of hydration is the enthalpy change accompanies the hydration of one mole of both of these gaseous ions.
Na+(g) + Cl–(g) Na+(aq) + Cl–(aq) H = –772 kJ mol–1
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• According to Hess’s law
2.2. The hydration of the resulting ionsThe hydration of the resulting ions
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• The enthalpy change of solution:
Hsoln = Hhyd – Hlattice
2.2. The hydration of the resulting ionsThe hydration of the resulting ions
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• For a salt to be soluble in water
its enthalpy change of solution has to be a negative or a small positive
value
2.2. The hydration of the resulting ionsThe hydration of the resulting ions
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• The sulphates(VI) and hydroxides of Group I metals
more soluble in water than those of Group II metals
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• Group I metal ions
larger sizes
smaller charge
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
• The Hlattice of their compounds
smaller in magnitude than those of Group II compounds
the dissolution of Group I compounds is more exothermic
the enthalpy changes of solution are more negative
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• For the sulphates(VI) of Group II metals
the cations are much smaller than theanions
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• The Hlattice is mainly determined by
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
the reciprocal of the sum of cationic
and anionic radii (i.e. ) rr1
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• The large ionic radius of the anion
the much smaller sizes of cations
relatively insignifiant in contributing to the sum of r + and r –
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• Going down the group
the increase in size of the cations does not cause a significant chan
ge in the Hlattice
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• The increase in size of the cations
cause the Hhyd to become less negative
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• That is to say
the decrease in Hhyd is more significant than the decrease in Hlattice
the Hsoln becomes less negative
the solubility of the sulphates(VI) of Group II metals decreases
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• For the hydroxides of Group II metals
the sizes of anions and cations are of the same order of magnitude
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• Again
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
rr1
the Hlattice is proportional to
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• Going down the group
cationic size increases
the change in Hhyd is comparatively small
less energy is required to break down the ionic lattice (i.e. the Hlattice
becomes less negative)
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• That is to say
the decrease in Hlattice is more significant than the decrease in
Hhyd
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
• The Hsoln
becomes more negative
the solubility of the hydroxides of Group II metals increases
2. Relative Solubility of the Sulphates2. Relative Solubility of the Sulphates(VI) and Hydroxides(VI) and Hydroxides
Check Point 40-3Check Point 40-3
The END
Metals are sometimes referred to as electropositive elements. Why?
AnswerThey have low electronegativity values.
Back
40.1 Characteristic Properties of the s-Block Elements (SB p.40)
s-Block compounds give a characteristic flame colour in the flame test. Based on this, can you give one use of
s-block compounds?Answer
s-Block compounds can be used in fireworks.
Back
40.1 Characteristic Properties of the s-Block Elements (SB p.46)
(a) Which ion has a greater ionic radius, potassium ion or calcium ion? Explain your answer.
Answer(a) Potassium ion (0.133 nm) has a greater ionic radius than calcium i
on (0.099 nm) . In fact, potassium ion and calcium ion are isoelect
ronic and have the same number of electron shells. However, calc
ium ion has one more proton than potassium ion, the electron clou
d of calcium ion will experience greater attractive forces from the n
ucleus. This leads to a smaller ionic radius of calcium ion.
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
(b) Explain why Group I elements show a fixed oxidation state of +1 in their compounds in terms of ionization enthalpies. Answer
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
(b) Group I elements form ions with an oxidation state of +1 only. It is
because they have only one outermost shell electron. Once this o
utermost shell electron is removed, a stable fully-filled electronic c
onfiguration is obtained. Therefore, the first ionization enthalpies
of Group I elements are low. The second ionization involves the r
emoval of an electron from an inner electron shell. Once this elect
ron is removed, the stable electronic configuration will be disrupte
d. Therefore, their second ionization enthalpies are very high. As
a result, Group I elements form predominantly ionic compounds
with non-metals by losing their single outermost shell electron, an
d they form ions having a fixed oxidation state of +1.
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
(c) Ions of Group I and Group II elements have a very low tendency to form complexes. Give one reason to explain your answer. Answer
(c) As ions of Group I and Group II elements do not have low-lying vaca
nt orbitals available for forming dative covalent bonds with the lone p
air electrons of surrounding ligands, they rarely form complexes.
40.1 Characteristic Properties of the s-Block Elements (SB p.48)
(d) Give one test which would enable you to distinguish a sodium compound from a potassium compound.
Answer(d) Sodium compounds and potassium compounds can be distinguishe
d by conducting a flame test. In the flame test, sodium compounds
give a golden yellow flame, while potassium compounds give a lilac
flame.
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40.1 Characteristic Properties of the s-Block Elements (SB p.48)
What is a dative covalent bond? How is it formed?
AnswerA dative covalent bond is a covalent bond in which the shared pair o
f electrons is supplied by only one of the bonded atoms. A dative co
valent bond is formed by the overlapping of an empty orbital of an at
om with an orbital occupied by a lone pair of electrons of another ato
m.
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40.1 Characteristic Properties of the s-Block Elements (SB p.48)
(a) (i) List the factors that affect the value of the ionization enthalpy of an atom.
Answer(a) (i) There are four main factors affecting the magnitude of the
ionization enthalpy of an atom. They are the electronic configu
ration of an atom, the nuclear charge, the screening effect, an
d the atomic radius.
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(a) (ii) Why is ionization enthalpy of an atom always positive?
Answer(a) (ii) Ionization enthalpy of an atom always has a positive value
because energy is required to overcome the attractive forces b
etween the nucleus and the electron to be removed.
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(a) (iii) Describe the general trend of the first and second ionization enthalpies down Group I of the Periodic Table. Answer
40.2 Variation in Properties of the s-Block Elements (SB p.56)
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(a) (iii) The first ionization enthalpies of Group I elements are rela
tively low. The outermost s electron is located in a new electro
n shell. The attractive force between this s electron and the nu
cleus is relatively weak. Also, this s electron is effectively shiel
ded from the attraction of the nucleus by the fully-filled inner el
ectron shells. Once this electron is removed, a stable octet or
duplet electronic configuration is obtained. Consequently, this
s electron is relatively easy to be removed, and hence the first
ionization enthalpies of Group I elements are relatively low. Ho
wever, the second ionization of Group I elements involves the l
oss of an inner shell electron which is closer to the nucleus. Th
e removal of this electron disrupts the stable electronic configu
ration. Therefore, the second ionization enthalpies of Group I e
lements are extremely high.
(b) (i) List the factors that affect the value of the hydration enthalpy of an ion.
Answer(b) (i) The value of the hydration enthalpy of an ion depends on
the size and the charge of the ion.
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(b) (ii) Why does hydration enthalpy of an ion always have a negative value?
Answer
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(b) (ii) Hydration enthalpy of an ion always has a negative value
because it is the amount of energy released resulting from the
attraction between the ion and water molecules.
(b) (iii) Describe the general trend of the hydration enthalpy down Group II of the Periodic Table.
Answer
40.2 Variation in Properties of the s-Block Elements (SB p.56)
(b) (iii) Going down Group II, the hydration enthalpy of the ions d
ecreases (becomes less negative). Since the ions get larger i
n size on moving down the group, the charge density of the io
ns falls. As a result, the electrostatic attraction between the io
ns and water molecules becomes weaker, and the hydration
enthalpy becomes less negative down the group.
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The burning of lithium, sodium and potassium in oxygen gives different types of oxides. Why do the metals beh
ave differently?Answer
40.2 Variation in Properties of the s-Block Elements (SB p.57)
On burning in air, lithium forms only lithium oxide, and it does not form the
peroxide or superoxide. This is because the size of lithium ion is very small,
leading to its high polarizing power. When a peroxide ion or superoxide ion
approaches a lithium ion, the electron cloud of the peroxide ion or superoxi
de ion (large in size) would be greatly distorted by the lithium ion. The great
er the distortion of the electron cloud, the lower the stability of the compoun
d. That is why lithium peroxide and lithium superoxide do not exist. Sodium
ion has a larger size than lithium ion. Its lower polarizing power allows it to f
orm the peroxide when sodium is burnt in air. Potassium ion has a much lar
ger size, so it has relatively low polarizing power. The electron cloud of the
peroxide ion or superoxide ion would not be seriously distorted by potassiu
m ion. This allows the peroxide ions or superoxide ions to pack around pota
ssium ion with a higher stability. As a result, potassium is able to form stabl
e peroxide or superoxide on burning in air.
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40.2 Variation in Properties of the s-Block Elements (SB p.57)
(a) Suggest a reason why the reaction of lithium with water is less vigorous than those of sodium and potassium.
Answer(a) The reactivity of Group I metals with water is related to the relative
ease of the metal atoms to lose the outermost shell electron. Going
down the group, as the atomic size increases, the outermost shell e
lectron becomes easier to be removed. Therefore, the reactivity of
Group I metals towards water increases down the group. Lithium re
acts with water vigorously. Sodium reacts with water violently and
moves on the water surface with a hissing sound.
40.2 Variation in Properties of the s-Block Elements (SB p.58)
(b) Which element is the strongest reducing agent, calcium, strontium or barium?
Answer(b) Barium is the strongest reducing agent. It is because the reducing p
ower of an element is related to the ease of the atom to lose the out
ermost shell electron. Since barium has larger atomic sizes, its oute
rmost shell electrons are less firmly held by the nucleus. Therefore,
barium has a higher tendency to lose its outermost shell electrons t
han both calcium and strontium.
40.2 Variation in Properties of the s-Block Elements (SB p.58)
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The value of Hsoln of a solid does not indicate whether the solid is soluble in water or not. So how can we predic
t the solubility of a solid in water?Answer
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40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.64)
Generally speaking, for a solid to be soluble in water, its enthalpy chan
ge of solution has to be a negative or a small positive value.
(a) Give balanced chemical equations for the following reactions:
(i) Thermal decomposition of barium carbonate
(ii) Reaction between sodium peroxide and water
(iii) Reaction between calcium oxide and dilute hydrochloric acid
Answer
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
(a) (i) BaCO3(s) BaO(s) + CO2(g)
(ii) Na2O2(s) + 2H2O(l) 2NaOH(aq) + H2O2(aq)
(iii) CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l)
(b) Suggest a reason why barium sulphate(VI) is insoluble in water, while potassium sulphate(VI) is soluble inwater although they have cations of similar sizes and the same anion.
(The ionic radii of potassium ion and barium ion are 0.133 nm and 0.135 nm respectively.) Answer
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
(b) When an ionic solid dissolves in water, two processes are taking pla
ce. They are the breakdown of the ionic lattice and the subsequent s
tabilization of the ions by water molecules. The enthalpy change inv
olved in the whole dissolution process is known as the enthalpy cha
nge of solution, Hsoln, which is equal to Hsoln = Hhyd – Hlattice. For
an ionic compound to be soluble in water, the enthalpy change of so
lution has to be a negative or a small positive value. The reason why
barium sulphate(VI) is insoluble in water while potassium sulphate(V
I) is soluble in water is that potassium ion has a smaller charge than
barium ion. The Hlattice of potassium sulphate(VI) is smaller in magni
tude (less negative) than that of barium sulphate(VI). As a result, the
enthalpy change of solution of potassium sulphate(VI) is more negat
ive, and hence it is soluble in water while barium sulphate(VI) is not.
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
(c) Compare the solubility of calcium sulphate(VI) and barium sulphate(VI) in water. Explain your answer.
Answer(c) Calcium sulphate(VI) is expected to be more soluble than barium sul
phate(VI). It is because calcium ion has a smaller size than barium i
on. This causes the Hhyd of calcium sulphate(VI) to be more negativ
e than that of barium sulphate(VI). As a result, the Hsoln of calcium s
ulphate(VI) becomes more negative than that of barium sulphate(VI),
and hence calcium sulphate(VI) is more soluble in water than bariu
m sulphate(VI).
40.3 Variation in Properties of the Compounds of the s-Block Elements (SB p.65)
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