In the cyanide ion (CN^–), the carbon (C) atom shares a triple covalent bond with the nitrogen (N) atom. Its Lewis structure is shown below.

Let us calculate the formal charge of the CN^– molecule by determining the formal charges on C and N.

Carbon

V = 4, N = 2, B = 6

Therefore, formal charge on carbon is given by,

q_f = 4 – 2 – 6/2 = -1

The formal charge on C in CN^–is -1.

Nitrogen

V = 5, N = 2, B = 6

Therefore, formal charge on nitrogen is given by,

q_f = 5 – 2 – 6/2 = 0

The formal charge on N in CN^– is zero.

The net formal charge is: -1 + 0 = -1

The formal charge of cyanide ion is -1.