Calculation of Formal Charge

Organic compounds are characterized by the presence of covalent bonds. The structures of organic compounds can be drawn by using Lewis method. Atoms of different elements may carry some charge, known as the formal charge. A neutral molecule may also have charged atoms, but in that case the number of positive charges must be exactly equal to negative charges.

Let’s see how we can calculate the formal charge of an atom in the molecule. A very simple formula for calculating the formal charge is:

Formal charge = No. of electrons in valence shell – (no. of lone pair electrons + ½ bonded electron pairs)

Examples:

^-OH

The formal charge on oxygen in hydroxyl ion is:

6 – (6 + ½ x 2)

= -1

H₂SO₄

Formal Charge on sulphur would be

= 6 – (0 + ½ 12)

= 6- (6)

= 0