How can I calculate formal charge?

1 Answer
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 #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/