Question

How can I calculate formal charge?

Answer 1

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 `BH^4`, or tetrahydrdoborate. Boron, `(B)` has 3 valence electrons, zero non-bonded electrons, and 4 bonds around it. This means that the formula becomes `3-(0+4)`, giving an answer of `-1`.

Next, let's look at the hydrogen atoms in `BH^4`. Hydrogen has one valence electron, zero non bonded electrons, and one bond. So the formal charge of hydrogen in `BH^4` is `1-(0+1)`, which gives a formal charge of 0.

Sources: http://www.masterorganicchemistry.com/2010/09/24/how-to-calculate-formal-charge/