Question #c93e4

Oct 4, 2016

We use simple $\text{VSEPR}$, and we use approximation, not calculation.
The geometry around nitrogen is tetrahedral to a 1st approximation. But we describe geometry on the basis of atoms, not on the basis of lone pairs. $\angle H - N - H$ $\cong$ ${107.5}^{\circ}$, reduced from the ideal tetrahedral angle of ${109.5}^{\circ}$, due to the influence of the nitrogen centred lone pair. As a non-bonding pair, it is more closely associated with the nitrogen centre, and thus a strong influence on the geometry around nitrogen, acting to compress $\angle H - N - H$.
This geometry is precisely analogous to the water molecule, yet here there are TWO oxygen-centred lone pairs, and $\angle H - O - H$ is compressed to approx. ${104.5}^{\circ}$