Which of the following molecules or ions does not exhibit resonance?

a. [NH4]+

b. [NO3]-

c. No right choice.

Option a. [NH₄]⁺

Explanation:

In order to a molecule or ion have resonance structures (named resonance contributors) it must have a π bond.

π bonds are in double or triple bonds. Single bonds are formed by sigma bonds.

Resonance structures are alternate representations for the same molecule or ion with different structures. Every structure will have the same number of electrons, but the electrons can move from one atom to another, which can only be done between π bonds; never between sigma bonds.

In the ion [NH₄]⁺ there are only sigma bonds: the N atom is at the centre of the iona, bonded to four H atoms; each N - H bond is a sigma bond. So, this structure does not exhibit any resonance.

On the other hand, [NO₃] ⁻ has a double bond between one of the oxygen atoms and the N atom at the centre. So, ion has a π bond, which indicates that there are resonance structures.