Theories of chemical bonding are based on the knowledge that:
1. Metallic elements from Groups I, II, III tend to lose electrons and form positive ions that have a noble gas configuration.
2. Non-metallic elements in Groups VI and VII gain electrons top form negative ions with a noble gas configuration.
3. Elements in groups IV and V do not form charged ions.
4. Noble gases do not form chemical bonds. A full outer shell of electrons has extra stability.
This involves the transfer of electrons from metal atoms to a non-metal atom to form charged ions.
The oppositely charged ions are held together by electrostatic attractions.
Formation of cations is governed by ionisation energies, with Group I elements forming ions most readily and Group III elements forming ions with difficulty. Group IV elements never form ions because the ionisation energy is too great.
Formation of anions is governed by electron affinities. This is the energy change involved when a mole of uni-negative ions from a mole of gaseous atoms. Group VII and Group VI both form negative ions. Group VII form ions more readily due to greater electron affinity.
A covalent bond is made when atoms share one or more electrons to form a molecule. A single covalent bond is made when each atom donates one electron to the bond, It is also possible to form double and triple bonds where two and three electrons are donated.
The two atoms come close together so that their outer orbitals overlap. Both nuclei are attracted to the shared pair of electrons and this attraction binds the atoms together.
Each atom has been stabilised as it gains a full outer shell (2, 8, 8).
Co-ordinate or dative covalent bonding:
In a normal covalent bond, each atom donates one electron to the shared pair. In a co-ordinate bond electrons come from the same atom.