Chemical Bonding: Calculating Formal Charges

Sometimes we're not sure if we've drawn the Lewis structure correctly. Other times there may be more than one way to draw a Lewis structure. Formal Charges let us check to be sure we have the best structure*.

Formal Charges and Lewis Structure for XeO3

A few things to note:

  • Calculate the formal charge for each atom.
  • If there are two Lewis structures the one with formal charges closer to zero is the best structure (unless they are ions).
  • For positive or negative ions (like SO42- or NH4+) your formal charges should add up to match the ion charge.
  • Watch for compounds with Sulfur and Phosphorus. Check formal charges when you see these elements.

Video: Calculating Formal Charges

*The best structure is the most common form of the molecule. In a given sample the majority of the molecules will have a Lewis structure where the formal charges are closer to zero (with the exception of ions).

Formal Charges:

  • SO3
  • I3-
  • XeO3
  • SOCl2
  • PO43-