Question

What is the Lewis structure of HCN?

Answer 1

Cyanide anion, `5` `+` `4` valence electrons: `[:C-=N:]^-`. There is a formal negative charge associated with this anion. Where does it reside?

Explanation:

The nitrogen nucleus has 3 electrons from the triple bond, and 2 electrons from its lone pair, and 2 inner core electrons; the associated charge balances the 7 protons in the nitrogen nucleus, so the nitrogen is formally neutral.

The carbon atom has (or shares) 3 electrons from the triple bond, and a lone pair of electrons, which it owns. With 2 inner core electrons, this makes 7 electrons with which it is associated. Since, the atomic number of carbon is `6`, the carbon atom is formally negatively charged.

When I write `H-C-=N:`, the carbon atom has a share of 4 electrons, and with 2 inner core electrons it is formally neutral, as is the nitrogen and the hydrogen.