Octet rule : The tendency of atoms in molecules to have eight electrons in their valence shells (two for hydrogen atom) is known as the octet rule.
Significance :
(i) It explains the formation of most of the compounds.
(ii) It is quite useful for understanding the structures of most of the organic compounds.
Limitations :
(i) In some compounds, the number of electrons surrounding the central atom is less than eight e.g., LiCl,BeH2 and BCl3.
(ii) In molecules with an odd number of electron like nitric oxide (NO) and nitrogen dioxide (NO2), the octet rule is not satisfied for all the atoms.
:N

...

=

O:;

N

|| :O:

...

−

O
(iii) In a number of compounds there are more than eight valence electrons around the central atom. This is termed as the expanded octet. Some of the examples of such compounds are PF5,SF6,H2SO4 and a number of coordination compounds.