BNG_Formal Charge Lesson

Formal Charge

Sometimes there are multiple ways to draw the Lewis structure for a molecule. In situations like this, chemists need a way to compare these structures. Formal charge is a bookkeeping method of showing electron distribution in a Lewis structure. Formal charge also shows exactly where the charge is located. For example, nitrate has a net charge of -1. This is the overall charge on the ion, but it does not tell us how the electrons are distributed in order to achieve this charge. And, calculating formal charge will give us an idea of how reasonable a particular structure is.

Formal charge is the difference between the original number of valence electrons and the actual number of electrons used in filling the octet. The actual number of electrons is the addition of the lone pair electrons and one electron from each bond.  Only one electron is counted in a bond because there are two electrons being shared in the bond but only one is being possessed by each atom. The formula is as follows:

FORMAL CHARGE Formal Charge = # of valence electrons of the atom - ( # of bonds + # of unbonded electrons)

If two or more Lewis dot structures can be drawn which satisfy the octet rule, the most stable one will be the structure where:

  • The formal charges are as small as possible.
  • Any negative charges are located on the more electronegative atoms.

Click through the following slideshow to see how to assign formal charge and interpret the results.

Remember to work on the module practice problems as you complete each section of content.

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