Question

Azide ion (${{N}_{3}}^{-}$) exhibits an N-N bond order of 2 and may be represented by resonance structures I, II and III given below. Select correct statements.

(A) Structures I and II make greater contributions than III
(B) Structures II and III make greater contributions than I
(C) Structures I and III make greater contributions than II
(D) All the above three structures make equal contributions.

Answer 1

Hint: Here, contributions they are talking about is towards the reactivity of the azide ion. The resonance structure that can easily remove nitrogen gas from its structure will be comparatively less stable.

Complete step by step solution:
Here, we are being asked which of the given resonance structures of azide ion give major contributions. Here, they are talking about contributions towards reactivity of the azide ion.
-We know that stable compounds have less energy compared to unstable ones and so they have comparatively less reactivity. Unstable compounds have high reactivity and they react with other compounds easily than the stable compounds.
-So, we can say that the more unstable resonance structures of azide will react more often and will give more contributions towards reactivity of azides.
-The resonance structures II and III have an N-N triple bond while structure I have two N-N double bonds.
-The structure II and III are more unstable because it contains a nitrogen atom with -2 charge on it which is also called nitrene. So, both II and III will give $N\equiv N$ (Nitrogen gas) upon reaction and so they are more reactive than structure I. Structure I does not have nitrogen with -2 charge and two nitrogen there has a charge of -1.
-So, we can say that structure II and III are less stable and more reactive. So, they will give more contributions than structure I.

So, the correct answer is (B).

Note: Do not assume that as two nitrogen atoms are charged in structure I so it will be more unstable. Nitrogens with -2 charge are more unstable than the nitrogen atoms with -1 charge on it.