Formal charge is the charge of an individual atom. Use the formal charge formula below to calculate the formal charge of an individual atom.

The sum of the formal charges gives the total charge on the entire molecule. For example, after using the formal charge formula, the charge of the entire molecule below is -2, but the formal charge on each oxygen is -1.

Use the formal charge formula to calculate that the formal charge of this molecule is -2.

To find the formal charge of an atom, you:

  1. Obtain the ideal number of valence electrons from the periodic table. A period table figure is at the beginning of this chapter that depicts how to find the number of valence electrons based on the element’s column number.
  2. Count the number of valence electrons the atom is actually using in the molecule.
    1. A single bond counts as 1, a double bond counts as 2, and a triple bond counts as 3.
    2. A single electron counts as 1 (so an electron pair would count as 2)
  3. Use the following formal charge formula to calculate the formal charge of the atom:

 

Formal charge formula:

Ideal number of valence electrons (step 1) -number of valence electrons being used (step 2) = Formal Charge.

 

Example

What is the formal charge on the nitrogen atom below?

Picture41

Answer

The formal charge on the nitrogen is -1.

Here’s the step-by-step process to find the formal charge by using the formal charge formula.

  1. Looking at the periodic table, nitrogen is in the 5th column, meaning it ideally has 5 valence electrons.
  2. The nitrogen atom has 2 lone pairs of electrons (4 electrons) and 2 single bonds. We add up the total number of electrons (4) plus the number of bonds (2), and we obtain 6, so the nitrogen is using 6 valence electrons.
  3. We now use the formal charge formula: (step 1 – step 2 = formal charge). Plugging in numbers: (5 – 6 = -1). Therefore, the formal charge on the nitrogen is -1. The negative formal charge should be drawn on the nitrogen.

Use the formal charge formula to see that this molecule has a -1 formal charge on the nitrogen atom.