Question

What are the major and minor resonance contributor(s) for the azide anion, `N_3^–`?

Answer 1

The steps for drawing the Lewis structures are:

1. Write all possible connections for the nitrogen atoms.

N-N-N

2. Calculate`V`, the number of valence electrons you actually have available.

`V` =3 N +1 = 3×5 + 1 = 16.

3. Calculate `P`, the number of π electrons there must be in the molecule:

`P = 6n + 2 – V`

where `n` is the number of non-hydrogen atoms in the molecule.

`P` = 6×3 + 2 - 16 = 4 π electrons. So there are either two double bonds or one triple bond.

4. Draw new structures.

This time insert the double and triple bonds in all possible combinations.

5. Add valence electrons to give each atom an octet.

6. Calculate the formal charge on each atom.

We end up with 3 possibilities.

7. Identify the major contributor.

All three structures have separation of charge.

The first two structures have a charge of 2- on one N atom. The third structure has a charge of -1 on two separate N atoms.

The most stable contributor will have the greatest delocalization or spreading out of charge.

So the first two structures are minor contributors, and the third structure is the major contributor.