Question

3 inadequacies in octet rule

Answer 1

The octet rule states that atoms can combine either by transferring 
valence electrons from one atom to another or by sharing their valence electrons in order to attain an octet of electrons in their valence shell.
 
Some limitations of the octet rule are:
 
It cannot be applied to the nonmetals after silicon in the Periodic Table. These elements can “expand their octet” and have more than eight valence electrons around the central atom. 
 
Molecules with an odd number of electrons such as NO and NO₂ cannot satisfy the octet rule. At least one atom must have an odd number of electrons.
 
In some molecules the central atom cannot possibly have eight valence electrons. For example, LiCl, BeCl₂ and BCl₃ do not obey the octet rule.

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Answer 2

Answer:

Octet rule fails to explain the following:

(1) The stability of incomplete octet molecules, i.e., the molecules with the central atom containing less than eight electrons.

For example, $\sf{BeCl_2}, {BF_3}$.

(2) The stability of expanded octet molecules, i.e., the molecules with the central atom containing more than eight electrons.

For example, $\sf{PC_5}, {SF_6}$.

(3) Observed shape and geometry of the molecules.

(4) Difference in energies and reactivities of different molecules.