How do you calculate formal charge?

Formal charge is found by taking the number of valence electrons an atom normally has, subtracting the number of nonbonding electrons it actually has in the structure, and then subtracting half of the bonding electrons shared in bonds around that atom.

Think of an atom like a house that should have a certain number of people living in it, which are its valence electrons. First count how many people the house should have normally. Then count how many people are sleeping alone in rooms — those are the nonbonding electrons. Next count all the people who are sharing rooms with someone else, but only give half of those to the house because they share them with a neighbor — those are half the bonding electrons. Subtract the nonsharing sleepers and the half-shared roommates from the normal number and whatever number you get is the formal charge, positive if you have fewer than normal and negative if you have more.

Students often forget to split bonding electrons in half and instead subtract the full number, which gives the wrong charge; always divide bonding pairs evenly between the two atoms. Another common mistake is using the wrong number for valence electrons, especially for atoms in main group five and six where people think of different shell counts; check the periodic table group number to be sure. Also counting lone pair electrons as pairs instead of individual electrons can lead to errors, so count each lone electron and then total them before subtracting.

For more examples and step-by-step practice problems visit www.gumballdegree.com to find videos and resources that walk through formal charge calculations.

How to Calculate Formal Charge

1. Count valence electrons
2. Count nonbonding electrons
3. Count bonding electrons
4. Give half of bonding electrons
5. Subtract to find formal charge

Common Formal Charge Mistakes

1. Subtracting full bonding electrons
2. Using wrong valence electron count
3. Miscounting lone pair electrons

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