Chapter 3
-
Upload
azfa-teeniey-nawi -
Category
Documents
-
view
88 -
download
1
Transcript of Chapter 3
Uniqueness principleDiagonal effectInert-pair effect
TKT2023 noorshida mohd ali
PERIODIC TRENDS OF THE ELEMENTS
The way certain properties (chemical and physical) of elements vary according to their location on the periodic table
Some variations in:
Effective nuclear charge increases
Eff
ecti
ve n
ucl
ear
char
ge
decr
ease
s
Atomic radius decreases
Ion
ic r
adiu
s in
crea
se
Ionic radius decreases
Ato
mic
rad
ius
incr
ease
Ion
izat
ion
en
erg
y de
crea
se
Ionization energy increases
Electron affinity increases
Ele
ctro
n a
ffin
ity
decr
ease
s
Electronegativity increases
Ele
ctro
neg
ativ
ity
decr
ease
s
TKT2023 noorshida mohd ali
Uniqueness principle
The chemistry of the second-period elements (Li, Be, B, C, N, O, F, Ne) are significantly different from
other elements in their respective groups
Li Be B C N O F Ne
TKT2023 noorshida mohd ali
Li Be B C N O F Ne
Uniqueness principle
Second element in each group (Na, Mg, Al, Si, P, S, Cl, Ar) are more representative
Na Mg Al Si P S Cl Ar
TKT2023 noorshida mohd ali
Li Be B C N O F Ne
Uniqueness principle
Why the first elements of the groups are different from their congeners (elements of the same group)?
a) The small size of the elements leading to a high polarizing power and a high degree of covalent character in their compoundsb) The greater probability of bonds (p-p)c) The lack of availability of the d orbitals
TKT2023 noorshida mohd ali
small size of the first elements
• Size effect leads to :
1) Smaller electron affinities - ability to accept one or more electrons
Electrons added to these small atom, experience more electron repulsions
2) Larger charge densities
3) Enhanced degrees of covalent character in their compounds
Electron affinity is the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion
X (g) + ē X-(g)
TKT2023 noorshida mohd ali
The first electron affinities of the halogens
TKT2023 noorshida mohd ali
What about fluorine?
> It is a very small atom.
> Incoming electron quite close to the nucleus.
> The existing electron density is very high.
> The extra repulsion is particularly great and
lessens the attraction from the nucleus
enough to lower the electron affinity below that
of chlorine.
TKT2023 noorshida mohd ali
lithium behaves differently than others
small size
high charge density of cation allow it to polarize nearby anion – allows a large degree of covalency in its bond and less ionic
less soluble in water and more soluble in polar organic solvents
GROUP 1 (ALKALINE METALS) : Li, Na, K, Rb, Cs, Fr
TKT2023 noorshida mohd ali
1) Small lithium cation get very close to larger, more diffuse or filled electron cloud of chloride ion.
2) Electron cloud of chloride ion is distorted or polarized by lithium ion
3) This distortion makes overlap between two ions. Orbital overlap and sharing electron between two species – characteristic of a covalent bond overlap between the valence orbitals in Li+ (empty 2s) and Cl- (filled 3p) is increased
TKT2023 noorshida mohd ali
Weakness of the fluorine – fluorine bond
-owing to the small atomic size – closeness/ nearness in F2 compared to Cl2
-lone pairs of electrons on adjacent fluorine atoms repel each other (increase repulsion )
-weakening the F-F bond in F2 molecule
Bond energies increase TKT2023 noorshida mohd ali
increased bonding in the first element• Due to small size, increase bond formation among themselves and with other elements (capable of forming strong double and triple bond)
bonds involve parallel overlap between for example two p orbitals ( bonding can occur using d orbitals and antibonding molecular orbitals of some molecules) • Utilizing bond than bond :
• Parallel orbital overlap or bonding is more effective in smaller first elements for example carbon than larger congeners, silicon
C C C O N N C C O O C O C C
TKT2023 noorshida mohd ali
lack of availability of d orbitals in the first elements
• The lighter elements lack availability of d orbitals and therefore cannot form compounds with expanded octets
CF4ClF3
SiF62-
TKT2023 noorshida mohd ali
• Similarities between pairs of elements in different groups and periods of the periodic table
Phenomenon happens :
1) small ionic size
2) closeness of the charge densities of their cations (charge density = charge of an ion divided by its radius : charge nm-1)
3) electronegativity
Diagonal effect
Li Be
Mg
B
Al Si
1 2 13 14
DIAGONAL RELATIONSHIPS exists between the chemistry of the first member of a group and the second member of the next group
TKT2023 noorshida mohd ali
Li Be
Mg
B
Al Si
1 2 13 14
Charge of ionIonic radius, ÅCharge densityElectronegativity
Charge of ionIonic radius, ÅCharge densityElectronegativity
+10.731.41.0
+20.414.91.5
+11.130.880.9
Na
C
+20.712.81.2
+30.25122.0
+30.535.71.5
+40.29142.5
+40.40101.8
Diagonal effect TKT2023 noorshida mohd ali
•High charge density of Al3+ ion (5.7) and Be2+ ion (4.9)
•Same electronegativity (1.5)
•Be-X and Al-X : covalent character
•Its small size & high charge density of cation Be2+ and Al3+ allow them to polarize the electron cloud of anion (X atom in M-X bond) to give additional covalent character
AlH3 resembles BeH2 in its properties (example of the diagonal relationship)
Li Be
Mg
B
Al Si
1 2 13 14
4.9
2.8 5.7
Diagonal effect •Ionic radius of Be2+ (0.41Å) is more similar to Al3+ (0.53Å) than Mg2+ (0.71Å)
0.41Å
0.71Å 0.53Å
TKT2023 noorshida mohd ali
Oxidation state : +4 CCl4, CO2, SiCl4, SiO2, SnO2
CO easily oxidized to CO2 (+2 to +4)
Convert tin(II) to tin(IV) – more stable. Sn2+ ions in solution suitable as reducing agents
Reversed situation for lead
C Si Ge Sn Pb
+4
Inert-pair effect
C Si Ge Sn Pb
+2Moving down a group, there are more and more examples oxidation state +2 : CO, SnCl2, PbO, Pb2+
TKT2023 noorshida mohd ali
Ion
izat
ion
en
erg
y de
crea
se Ionization energy increases
ATOMIC RADIUSThe increase in size of atom is accompanied by a decrease of ionization energy
-going down a group, size of the atom increases -the outer electrons lie farther away from the nucleus -attractive pull from the nucleus on the outer electrons decreases -easier to pull out an electron from the outer shell of the atom
Inert-pair effect
TKT2023 noorshida mohd ali
IONIZATION ENERGY
Slight increase between tin and lead
Large increase between tin and lead
TKT2023 noorshida mohd ali
Inert-pair effect
• All four elements (Al, Ga, In, Tl) give trivalent compounds but the univalent state becomes increasingly important for Ga, In and Tl
• Valence ns2 electrons of metallic elements : In, Tl, Sn, Pb, Sb, Bi and Po
are less reactive than expected.
• Inert ns2 pairs mean oxidation state is 2 less than the expected group valence for the heavier elements of groups 13, 14, 15 and 16.
As an example in group 13 the +1 oxidation state of Tl is the most stable and TlIII compounds comparatively rare. The stability increases in the following sequence:
AlI < GaI < InI < TlI
TKT2023 noorshida mohd ali
Inert-pair effect
• Valence electrons in an s orbital are more tightly bound are of higher energy than electrons in p orbitals and therefore less likely to be involved in bonding.
• Get closer to the bottom of the group (heavier elements – increasing tendency s2 pair not to be used in the bonding (left unchanged / inert pair)
• Electrons closer to the nucleus – difficult to remove the heavier the element the greater this effect
TKT2023 noorshida mohd ali
Inert-pair effect
• Two major reasons for this effect:
a) larger than normal effective nuclear charges (higher than expected ionization energies for Ga, In, Tl)
4s, 5s and 6s electrons experience larger effective nuclear charge than expected – they are more difficult to ionize
b) lower bond energies (as expected)
due to increase in atomic size and bond distance
TKT2023 noorshida mohd ali