In the thiocyanate ion (SCN^–), the central carbon (C) atom shares double bonds with both the sulfur (S) and nitrogen (N) atoms. Its stable Lewis structure is shown below.

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

Sulfur

V = 6, N = 4, B = 4

Therefore, formal charge on sulfur is given by,

q_f = 6 – 4 – 4/2 = 0

The formal charge on S in SCN^– is zero.

Carbon

V = 4, N = 0, B = 8

Therefore, formal charge on carbon is given by,

q_f = 4 – 0 – 8/2 = 0

The formal charge on C in SCN^–is zero.

Nitrogen

V = 5, N = 4, B = 4

Therefore, formal charge on nitrogen is given by,

q_f = 5 – 4 – 4/2 = -1

The formal charge of N in SCN^–is -1.

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

The formal charge of thiocyanate ion is -1.