# Compounds containing the ammonium ion are often used as fertilizers. What is the geometry of an ammonium ion (NH_4^+)?

This one is easy............$\text{ammonium ion}$ assumes the shape of a $\text{tetrahedron}$, with nitrogen as the central atom.
Around nitrogen in $\text{ammonia ion}$ there are four regions of electron density, and $\text{VESPER}$ assures us that these 4 electron pairs assume the shape of a tetrahedron, i.e. with $\angle H - N - H = {109.5}^{\circ}$. Why is the geometry of the parent species, $N {H}_{3}$, different?