# What is formal charge?

Aug 8, 2015

Formal charge is the charge left on the central atom when all the bonding pairs (of electrons) are removed sequentially.

#### Explanation:

Let's consider the simple case of ammonia, $N {H}_{3}$, versus its ammonium salt, $N {H}_{4}^{+}$. Now ammonia is a neutral molecule, and there is a non-bonding pair, a lone pair, of electrons localized to the nitrogen centre. Its reaction with ${H}^{+}$ is very simply represented:

$N {H}_{3} + {H}^{+} \rightarrow N {H}_{4}^{+}$

The ammonium is now quaternized. Why? Well, any chemical reaction conserves mass and charge, and this one does as well. But why do we write ammonium as $N {H}_{4}^{+}$, with a formal positive charge on the nitrogen nucleus?

The nitrogen in ammonia, $N {H}_{3}$ is neutral because it shares 3 electrons from the 6 electrons that comprise the $N - H$ bonds, and gets a full contribution from its lone pair, i.e. 5 electrons + 2 inner electrons electrostatically balance the 7 protons in the nucleus of a nitrogen atom. When $N$ is quaternized as ammonium, $N {H}_{4}^{+}$, it is conceived to have a $\frac{1}{2}$ share only of the 8 electrons of the $N - H$ bonds, $4 e$ in total, and is therefore written as $N {H}_{4}^{+}$, with the positive charge formally associated with the nitrogen atom.