Atoms and Molecules
Molecules and Chemical Bonds
How are formal charges different?
Formal charges are given for individual atoms within molecules. These are determined by dividing the electrons in every bond equally between the atoms that share them, regardless of the elements involved. Textbooks typically follow this somewhat obtuse statement with an equation (which always helps, right?) like this:
Formal Charge = Group Number – Nonbonding Electrons – ½ Bonding Electrons
Carbon is in Group 4 of the periodic table; it has two nonbonding electrons (the two dots shown), and since there are three bonds to oxygen, there are six bonding electrons. So the formal charge is 4 – 2 – ½ (6), or –1. Oxygen is in Group 6 and has the same number of nonbonding and bonding electrons as carbon does in this example. The formal charge on oxygen is therefore 6 – 2 – ½ (6), or +1. Carbon monoxide has no net (or total) charge (because 1 + – 1 = 0), but the individual atoms do have formal charges.