Two types of bonding:
1) ionic bonding → transfer of an electron from one nucleus to another
2) covalent bonding → sharing of electrons between nuclei
1.1 Ionic bonding:
Ionic bonding involves an exchange of electrons between nuclei and attraction of ions
E.g., Na + F → NaF
Characteristics of nuclei involved in an ionic bond: one nucleus has a low ionisation energy other nucleus has a high electron affinity
Consideration of electron configuration:
Na = 1s
2
2s
2
2p
6
3s
1
F = 1s
2
2s
2
2p
5
Electrons are transferred thus:
Na → Na +
+ e
-
F + e
-
→ F
-
In terms of Lewis structures:
We now have Na
+
and F
-
– two oppositely charged ions which interact via Coulombic attraction:
F

4
Q
1
Q

0

2 r
2
 z
1 z
2 r
2 e
2
4

0
 where F is the force [N = newtons]
Q
1
and Q
2
are the charges on the ions [C = coulombs]
ε
0
is the vacuum permittivity of space [ ε
0
= 8.854×10 -12 C 2 J -1 m -1 ]
ε is the relative permittivity or dielectric constant of the medium [unitless] r is the distance between the centre of the ions z
1
and z
2
are the ionic valencies of ions 1 and 2 e is the elementary charge [ e = 1.602×10
-19
C] e and
ε
0
are fundamental constants and are given on your exam papers.
The dielectric constant,
ε
, is equal to unity for vacuum; other media have
ε
> 1.

Notes on ionic bonding:
1) Coulombic interaction is a strong force
2) The force is long-ranged and increases in strength at small separations
3) The force acts along the line separating ions
4) The force is isotropic
5)
Ionic bonds are “unsaturated”
6) Ionic bonds are usually associated with ionic solids which have high melting points and are brittle
7) The nuclei change significantly in size upon forming ions: cations are smaller than neutral atoms, while anions are larger than the neutral atoms
E.g. Change in size upon forming ions of several elements
Element Atom (Å) Cation (Å) Element Atom (Å) Anion (Å)
Li
Na
Cs
3.0
3.7
5.2
F
Cl
Br
1.3
2.0
2.3

1.2
Covalent bonding
Why should two hydrogen atoms bond together to form H
2
?
Compare two unbonded atoms at close separation compared to a molecule made of these nuclei:
Characteristics of covalent bonds:
1) Covalent bonds are strong (200 – 800 kJ mol-1)
2) Covalent bonds are short-ranged
3) Covalent bonds are “saturated”
4) Covalent bonds act in specific directions, but not necessarily along the axis between the two nuclei
5) Nuclei can participate in a number of covalent bonds, depending on the element. This is the valency of the nucleus.
6) Multiple covalent bonds are possible – double and triple bonds
7) There is no simple analytical expression for the interaction potential