Question

What are the limitations of the octet rule?​

Answer 1

★ Limitations of the octet rule:-

→ It is based upon the inert nature of noble gases. However, some noble xenon and krypton form compounds such as XeF2, KrF2. Etc.

→ The octet rule cannot be applied to the elements in and beyond the the periodic table. The elements present in these periods have more valence electrons around the central atom.

For example:- PF5, SF6, etc.

→ The octet rule is not satisfied for all atoms in a molecules having an odd number of electrons. For example, NO and NO2 do not satisfy the octet rule.

→ This rule cannot be applied to those compounds in which the number of electrons surrounding the central atom is less than eight. For example:- LiCl, BeH2, AlCl3 etc, do not obey the octet rule.

→ The rule failed to predict the shape and relative stability of molecules.

→ This theory does not account for the shape of molecules.

Answer 2

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.