Question

What are the major and minor resonance contributor(s) for the azide anion $N_{3}^{-}$ ?

Answer 1

Hint : We know that the \[N\] is having an atomic number $7$ and hence the number of valence electrons in \[N\]is \[7.\] There is a negative charge which adds one electron to the total number of valence electrons present in $N_{3}^{-}$. The collective structures are called the resonance structures. Let’s discuss the steps for drawing the resonance structures by solving this question. The given ion in the question is the azide ion, which consists of three \[N\] atoms.

Complete Step By Step Answer:
So in the question we are asked how many resonance structures are possible for $N_{3}^{-}$ . We have heard the term resonance structures many times in organic chemistry classes where it is defined as the delocalization of the electrons. Here also the resonance structures are possible due to the delocalization of the electrons between the atoms.
The steps for drawing the Lewis structures are:
Write all possible connections for the nitrogen atoms. $\,N-N-NN-N-N$
Calculate V, the number of valence electrons you actually have available. \[V~=3N+1\text{ }=\text{ }3\times 5+1\text{ }=16.\]
Calculate P, the number of π electrons there must be in the molecule: \[P=6n+2V\] where n is the number of non-hydrogen atoms in the molecule. \[P~=6\times 3+2-16=4\pi \]electrons. So there are either two double bonds or one triple bond.
Draw new structures. This time insert the double and triple bonds in all possible combinations and add valence electrons to give each atom an octet.
Calculate the formal charge on each atom.
We end up with 3 possibilities.
\[^{2-}:\underset{\centerdot \centerdot }{\overset{\centerdot \centerdot }{\mathop{N}}}\,-\underset{+}{\overset{\centerdot \centerdot }{\mathop{N}}}\,\equiv \underset{\centerdot \centerdot }{\overset{\centerdot \centerdot }{\mathop{N:}}}\,\underset{{}}{\longleftrightarrow}:\underset{{}}{\overset{{}}{\mathop{N}}}\,\equiv \underset{+}{\overset{\centerdot \centerdot }{\mathop{N}}}\,\equiv {{\underset{\centerdot \centerdot }{\overset{\centerdot \centerdot }{\mathop{N:}}}\,}^{2-}}\underset{{}}{\longleftrightarrow}:\underset{{}}{\overset{\centerdot \centerdot }{\mathop{N}}}\,=\underset{{}}{\overset{+}{\mathop{N}}}\,\equiv {{\underset{{}}{\overset{\centerdot \centerdot }{\mathop{N:}}}\,}^{-}}\]
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.

Note :
Remember that resonance structures are generally when we cannot describe all the properties of the molecule with the help of a single structure. In certain polyatomic ions and in molecules we need different Lewis structures to explain the bonding in the structure and to explain the various properties of the molecule and these different structures collectively comprise a hybrid structure.