Define octet rule. Write its significance and limitations.

Explanation

The tendency or urge of elements to attain stable configuration of eight electrons in their valence shell is called octet rule.
Significance
(i) It is useful for understanding the structure of the organic compounds.
(ii) it mainly applies to the elements of second period of periodic table.
Limitations
(i) The incomplete octet of the central atom
Some compounds do not follow the octet rule. Their central atom contain less than eight electrons. for example, `Li:Cl,H:Be:H`
(ii) Odd - electron molecules
Octet rule is not followed by the molecules having odd number of electrons like-nitric oxide, NO and nitrogen dioxide, `NO_2`.
`underset(cdot)overset(..)(N)=underset(..)overset(..)(O)" "underset(..)overset(..)(O)=overset(cdot +)(N)-underset(..)overset(..-)(O):`
(iii) The expanded octet
Some compounds have than eight valence electrons around the atom. this is known as the expanded octet. For examole, `PF_(2), SF_(6), H_(2)SO_(4)` and a number of co-ordination compounds.
(iv) Octct rule is based upon chemical inertness of noble gases. However noble gases also combine with oxygen and fluorine to from a number of compounds like `XeF_(2), KrF_(2), XeOF_(2)` etc.
(v) Octet theory does not account for the shape of molecules.
(vi) it does not explain the relative stability of the molecules.