Question

\(\mathrm{H{-}N\cdots N\cdots N}\) (the first N-N bond is marked (I) and the second N-N bond is marked (II))
In hydrogen azide (above) the bond orders of bond (I) and (II) are :

• A. (I) \(\lt\) 2, (II) \(\gt\) 2
• B. (I) \(\gt\) 2, (II) \(\lt\) 2
• C. (I) \(\gt\) 2, (II) \(\gt\) 2
• D. (I) \(\lt\) 2, (II) \(\lt\) 2

Hint:

Draw the resonance structures of HN3 and average the two N-N bond orders.

Answer 1

The Correct Option is A

Hydrogen azide has two main resonance structures:
\(\mathrm{H{-}\ddot N^-{-}N^+{\equiv}N}\) and \(\mathrm{H{-}N{=}N^+{=}\ddot N^-}\).
Bond (I) (N attached to H to the central N): single in one form, double in the other, so the bond order is about 1.5, i.e. \(\lt 2\).
Bond (II) (central N to terminal N): triple in one form, double in the other, so the bond order is about 2.5, i.e. \(\gt 2\).
Hence (I) \(\lt 2\) and (II) \(\gt 2\): option (1).
Answer: (1) (I) \(\lt\) 2, (II) \(\gt\) 2