Question

Which one of the following is expected to exhibit resonance? (A) `CO_2` (B) `NH_4^(+)` (C) `HCN` (D) `NO_2""^(-)`

Answer 1

The expected answer is probably D) `"NO"_2^-`.

Explanation:

Both `"CO"_2` and `"HCN"` also exhibit resonance, but they have only one major contributor. `"CO"_2` has two major contributors.

To determine if resonance is possible, you draw the Lewis structure and then see if you can put the electrons in different locations.

A) `"CO"_2`

The Lewis structure for `"CO"_2` is

There are two other resonance contributors, but they involve charge separation, so they are minor contributors.

B) `"NH"_4^+`

The Lewis structure of `"NH"_4^+`is

This is the only Lewis structure — no resonance.

C) `"HCN"`

The Lewis structure of `"HCN"` is

You can draw a structure with a `"C=N"` bond, but it puts formal charges on `"C"` and `"N"`, so it is a minor contributor.

D) `"NO"_2^-`

`"NO"_2^-` has two equivalent Lewis structures. They are both major resonance contributors.