# How can I calculate formal charge?

Oct 28, 2014

The formula for calculating the formal charge on an atom is simple.

Formal charge = [ of valence electrons] – [electrons in lone pairs + 1/2 the number of bonding electrons]

Since the number of bonding electrons divided by 2 is equal to the number of bonds surrounding the atom, this formula can be shortened to:
Formal Charge = [ of valence electrons on atom] – [non-bonded electrons + number of bonds].

Let's look at an example. Take the compound $B {H}^{4}$, or tetrahydrdoborate. Boron, $\left(B\right)$ has 3 valence electrons, zero non-bonded electrons, and 4 bonds around it. This means that the formula becomes $3 - \left(0 + 4\right)$, giving an answer of $- 1$.

Next, let's look at the hydrogen atoms in $B {H}^{4}$. Hydrogen has one valence electron, zero non bonded electrons, and one bond. So the formal charge of hydrogen in $B {H}^{4}$ is $1 - \left(0 + 1\right)$, which gives a formal charge of 0.