Question

Explain the structure of ammonia.

Answer 1

VSEPR theory:-

1. VSEPR stand for, Valence Shell Electron Pair Repulsion theory.
2. In 1940, Sidgwick and Powell proposed this theory on the basis of the repulsion interaction of the electron pairs in the valence shell of the atoms.
3. Basically from this theory, we can calculate the number of lone pairs and bond pairs present in a molecule with the help of this formula : $\frac{1}{2}\times [V+N-C+A]$.
4. Here, V= the number of valence electrons present in the central atom, N= total number of monovalent atoms, C= cationic charge, and A= anionic charge.
5. Also, we can predict the shape of the molecules of the electron pairs that surround the central atom of the molecules.

Structure of ammonia using VSEPR theory:-

1. In ammonia$\left({\mathrm{NH}}_3\right)$, there are 3 hydrogen atoms that are linked with 1 nitrogen atom.
2. The electronic configuration of nitrogen is$1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^3$, hence it has 5 valence electrons in its outermost shell.
3. Hydrogen here is a monovalent atom as its atomicity is 1.
4. So according to the formula, for calculating electron pair, the values of V= 5 ($\because$Nitrogen has 5 valence electrons), N= 3($\because$3 monovalent hydrogen atom is present), and C= 0 and A=0 ($\because$No cations or anions are present in Ammonia, hence charge is 0).
5. Putting the values in the formula we get the number of electron pair

$$\begin{gathered} =\frac{1}{2}\times [\mathrm{V}+\mathrm{N}-\mathrm{C}+\mathrm{A}] \\ =\frac{1}{2}\times [5+3-0+0] \\ =\frac{1}{2}\times 8 \\ =4 \end{gathered}$$

6. Here total of 4 electron pairs are present, out of them 3 are bond pairs ($\mathrm{N}-\mathrm{H}$) and 1 is a lone pair present in nitrogen.

7. Now since there are 4 electron pairs present, the hybridization will be ${\mathrm{sp}}^3$, so its shape will be trigonal pyramidal:

Therefore, the structure of ammonia is trigonal pyramidal.