Question

State octet rule. 

Explain its inadequecies with respect to :

a. Incomplete octet 

b. Expanded octet

Answer 1

Statement : 

During the formation of chemical bond, atom loses, gains or shares electrons so that its outermost orbit (valence shell) contains eight electrons. 

Therefore,

The atom attains the nearest inert gas electronic configuration.

a. Molecules with incomplete octet :

e.g. BF₃, BeCl₂, LiCl

In these covalent molecules, the atoms B, Be and Li have less than eight electrons in their valence shell but these molecules are stable. Li in LiCl has only two electrons, Be in BeCl₂ has four electrons while B in BF₃ has six electrons in the valence shell.

b. Molecules with expanded octet : 

Some molecules like SF₆, PCl₅, H₂SO₄ have more than eight electrons around the central atom.