Cyanide is drawn inside brackets as [C≡N]⁻. Carbon and nitrogen share a triple bond, each atom keeps one lone pair, and the negative formal charge belongs to carbon in this contributor.
Carbon contributes four electrons, nitrogen contributes five and the negative charge adds one, giving ten. A triple bond uses six electrons. The remaining four electrons form one lone pair on each atom, completing both octets.
How to draw CN−
Count 10 valence electrons.
4 from carbon + 5 from nitrogen + 1 for the negative charge.
Connect C and N.
A single bond is only a starting skeleton.
Form a triple bond.
Three shared pairs let both atoms approach an octet.
Add one lone pair to each atom.
The total is now exactly ten electrons.
Add brackets and the − charge.
Formal charge is −1 on carbon and 0 on nitrogen.
Why is the charge on carbon?
Formal charge on carbon is 4 − 2 nonbonding electrons − 3 shared pairs = −1. Nitrogen is 5 − 2 − 3 = 0. Formal charge bookkeeping does not mean the electron density is completely localized.
Common mistakes
Forgetting the extra electron from the −1 charge.
Drawing only a double bond and leaving an incomplete octet.
Omitting brackets or the overall charge.
Placing two lone pairs on one atom and none on the other.