Atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells. This is known as octet rule.
Significance:
(i) It is useful for understanding the structures of most of the organic compounds.
(ii) It applies mainly to the second period elements of the periodic table.
Limitations:
(i) The incomplete octet of the central atom: In some compounds, the number of electrons surrounding the central atom is less than eight. This is especially the case with elements having less than four valence electrons. Examples are LiCl, BeH2 and BCl3
Li, Be and B have 1, 2 and 3 valence electrons.
(ii) Odd-electrons molecules: In molecules with an odd number of electrons like nitric oxides, NO and nitrogen dioxide NO2, the octet rule is not satisfied for all the atoms.
(iii) The expanded octet: Elements in and beyond the third period of the periodic table have, apart from 3s and 3p orbitals, 3d orbitals also available for bonding. In a number of compounds of these elements there are more than eight valence electrons around the central atom. This is termed as the expanded octet. The octet rule does not apply in such cases.
Some of the examples of such compounds are: PF5, SF6, H2SO4 and a number of co-ordination compounds.
