Question

Consider the Lewis structure below. What are the formal charges of each atom?

C - ?
N - ?

Answer 1

We can write a formal Lewis structure of `""^(-):C-=N:`

Explanation:

Around nitrogen there are 7 electrons, and thus given `Z=7`, nitrogen is NEUTRAL. On the other hand, around the carbon there are also 7 electrons: 3 from the triple bond; 2 from the lone pair; and 2 inner core. Since for `C`, `Z=6`, carbon bears a FORMAL NEGATIVE CHARGE. When cyanide binds to a metal centre, it usually binds thru the carbon, so this supports this assignment. And of course there is hydrogen cyanide, i.e. `H-C-=N`; hydrogen is bound to carbon in the parent acid.

For both carbon, and nitrogen, we conceive of the presence of 2 inner core electrons, i.e. the `1s^2`, which are not proposed to be bonding, and are along for the ride, and are thus NOT represented in the given Lewis structure.

Note that when we assign charge complex ions, and molecules, the two electrons of a covalent bond are conceived to be shared between the bound atoms, and lone pairs are conceived to be associated ONLY with the atom that bears the lone pair.