Give the significance of octet rule. Explain why this rule is not valid for H and Li atoms.

i. Significance : 

Octet rule is found to be very useful :

• in explaining the normal valence of elements
• in the study of the chemical combination of atoms leading to the formation of molecule.

ii. Octet rule is not valid for H and Li atoms. 

According to octet rule, during the formation of a chemical bond, each atom loses, gains or shares electrons so that it achieves stable octet (eight electrons in the valence shell). 

However,

H and Li atoms tend to have only two electrons in their valence shell similar to that of Helium (1s²), which called duplet.

Hence,

Octet rule is not valid for H and Li atoms.