Post on 28-Dec-2015
Group 1
Li
Na
K
Rb
Cs
Fr
Group 2
Be
Mg
Ca
Sr
Ba
Ra
Group 7
F
Cl
Br
I
At
All elements in the same group have the same number of electrons in their outer shell.
Group 1 = 1
Group 2 = 2
Group 3 = 3
Group 4 = 4
Group 5 = 5
Group 6 = 6
Group 7= 7
Group 8 = 8
Group 3
B
Al
Ga
In
Tl
Group 4
C
Si
Ge
Sn
Pb
Group 5
N
P
As
Sb
Bi
Group 6
O
S
Se
Te
Po
Group 8
He
Ne
Ar
Kr
Xe
Rn
.Group 1
Li 2s1
Na 3s1
K 4s1
Rb 5s1
Cs 6s1
Group 7
F 2s2 2p5 = 7
Cl 3s2 3p5 = 7
Br 4s2 4p5 = 7(ignoring d
orbitals)
I 5s2 5p5 = 7(ignoring d
orbitals)
Group 8 all have a complete outer shell, this makes them very stable and unreactive, hence they are called “inert” or “noble” gases.
He2
Helium has two electrons, 1s2, so the first shell is full.
Ne2 + 8 =10
Neon has 8 electrons, 2s2 2p6, so the second shell is also full.
Ar2 + 8 + 8 = 18
Argon has 18 electrons, only the first part of the third shell is full, 3s2 3p6, but this is still very stable.
Kr2 + 8 + 8 + 10 + 8 = 36
Krypton has 36 electrons, the third shell is full, as is the first part of the fourth shell, 4s2 4p6.
Inert gases and bonding
• Most elements are not as stable as the inert gases.
• They react, forming chemical bonds, so that they will have a complete outer shell and hence be as stable as an inert gas.
• Two main types of bonds are formed in the process;
1) Ionic 2) Covalent
Ionic bonding
• In ionic bonding one atom loses an electron(s), so that it has a complete outer shell (ie has the same number of electrons as an inert gas) , forming a positive ion.
M → M+ + e
The other element accepts an electron(s), to complete its outer shell, forming a negative ion.
X + e → X-
Metals generally form positive ions.
Non metals generally form negative ions.
Group 1
Li
Na
K
Rb
Cs
Fr
Group 2
Be
Mg
Ca
Sr
Ba
Ra
Group 3
Al
To obtain a complete outer shell metals loose electrons to form positive ions.
Group 1 lose 1 electron
Group 2 lose 2
Aluminium lose three.
Group 8
He2
Ne10
Ar18
Kr36
M+ ion
Li+2
Na+10
K+18
Rb+36
M atom
Li3
Na11
K19
Rb37
They will then have the same number of electrons (isoelectronic) as an atom of a stable inert gas in group 8. Eg Group 1.
Group 8
He2
Ne10
Ar18
Kr36
Xe54
X Atom
F9
Cl17
Br35
I53
X- ion
F-10
Cl-18
Br-36
I-54
To get a complete outer shell non metals gain electrons to form negative ions. Eg Group 7
• Generally opposite charges attract.
• This means that the positive ion and the negative ion will attract each other.
• This attraction is an ionic bond.• The ions attract each other
strongly, so the ionic bond is a very strong bond.
Dot and cross diagrams;
• The outermost electrons of one atom are represented by a dot, those of the other by a cross.
• Eg; Lithium Hydride Li H• Li = 1s2 2s1 = Li.
• H = 1s1 = HX
• Li H = (Li)+ (.xH)-
Covalent bonding
• In covalent bonding atoms share electrons to get a complete outer shell of electrons (ie the electronic configuration of an inert gas.)
•Eg; H2
•H = 1s1 = H.
•H2 = H:H
Group 8
He2
Ne10
Ar18
Kr36
Xe54
X Atom
F9
Cl17
Br35
I53
X2 molecule
F : F8-2-8 = 10 each
Cl : Cl17-2-17 = 18 each
Br : Br35-2-35 = 36 each
I : I52-2-52 = 54 each
To get a complete outer shell non metals share electrons to form covalent molecules. Eg Group 7
Group 6 elements need a share of two additional electrons to achieve a complete outer shell.
O atom
O6
Group 8
He2
Ne8
O2 molecule
::O :: O::4-2-2-4
Group 5 elements need a share of three additional electrons to achieve a complete outer shell.
N5
:N .::. N:2-3-3-2
Ne8
Group 4 elements need a share of four additional electrons to achieve a complete outer shell.
C4
11 C 1
1
H1
H1
H1
H1
Ne8
Valency and covalent bonding• The number of extra electrons that an
atom needs to share is also known as its valency or combining power.
• Hydrogen needs 1 electron and has a valency of 1.
• Chlorine also has a valency of 1.• Oxygen needs 2 and has a valency of 2.• Nitrogen has a valency of 3.• Carbon has a valency of 4
Multiple bonds
• Each covalent bond involves sharing a pair of electrons.
• So in a double bond two pairs of electrons are shared.
• Eg O2
• O = 1s2 2s2 2p4
• To complete its shell each oxygen must gain two electrons.
• O::O
Electron deficient compounds
• Beryllium, aluminium and boron, all form compounds in which they do not have a full outer shell.
• Eg Beryllium chloride• Be = 1s2 2s2 • Cl = 1s2 2s2 2p6 3s2 3p5• Be uses its two outer electrons to make
two covalent bonds with Cl.• Giving it 2 + 2 = 4, not 8, electrons in
its outer shell.
Electron rich compounds
• Some elements, such as sulphur, can form covalent compounds in which they have a share of more than 8 electrons in their outer shell.
• Eg; Sulphur hexaflouride (SF6)
• S = 1s2 2s2 2p6 3s2 3p4• F = 1s2 2s2 2p5• Sulphur uses each of its six outer electrons
to form a covalent bond with chlorine, giving it a share of 12, not 8, electrons.
Dative bonds
A dative bond is a covalent bond in which one atom provides both electrons to be shared.
:NH3 H+Ammonia has a lone pair on N that is not involved in bonding as N has a full shell.
Protons have no electrons.
:NH3HAn ammonium ion, NH4
+
forms by N sharing both the lone pair electrons with the proton.
+