Question

what are the limitation of octet rule with example and reason?

Answer 1

Limitations of octet rule:
a) Two Electrons: Hydrogen and helium have only one electron shell. The first shell has only one s orbital and no p orbital, so it holds only two electrons. Therefore, these elements are most stable when they have two electrons
b) Expanded Octet: The covalently bonded compounds like PCl₅, ClF₃, ICl₃, SF₆, IF₇, OsF₈ and OsO₄ have expanded octets in them. The central atoms in these molecules have more than eight electrons in their outermost shell.
c) Less Than an Octet: In BeCl₂, the central atom Be has only 4 electrons and in BF₃, boron shares its three electrons with three fluorine atoms. The fluorines follow the octet rule, but boron has only six electrons.
d) Odd Numbers: Nitrogen have an odd number of electrons and will form somewhat stable compounds. Nitric oxide has the formula NO. No matter how electrons are shared between the nitrogen and oxygen atoms, there is no way for nitrogen to have an octet. It will have seven electrons instead.

Reason for expanded octet is presence of d-orbitals, which are empty and thus the inner paired electron can excite to empty orbitals and form bond with incoming atoms (ligands).
There is no other specific rule for other elements that do not obey octet rule. By looking at particular electronic configuration of an element, its bonding, geometry and shape is decided.