Formal charge

 Formal charge- defined as the difference between the no of valence electrons of that atom in an isolated of free state and the no of electron assigned to that atom in the lewis structure

= no. of valence electron in the free atom – no of non-bonding electrons (lone pair)- (no of bonding or shared electrons)/2

Charge on the molecule or ion = sum of all the formal charges

Significance-

·       It do not indicate real charge separation within the molecules. Indicating the charge on the atoms in the lewis structure helps in keeping track of the valence electrons in the molecule

·       Helpful in selection in lowest energy structure from a no of possible lewis structure for a given species