Formal charge is an accounting procedure. It allows chemists to determine the location of charge in a molecule as well as compare how good a Lewis structure might be. The formula for calculating formal charge is shown below:

Consider the molecule H₂CO₂. There are two possible Lewis structures for this molecule. Each has the same number of bonds. We can determine which is better by determining which has the least formal charge. It takes energy to get a separation of charge in the molecule (as indicated by the formal charge) so the structure with the least formal charge should be lower in energy and thereby be the better Lewis structure.

The two possible Lewis structures are shown below. They are connected by a double headed arrow and placed in brackets. The non-zero formal charge on any atoms in the molecule have been written near the atom.

The two structures differ only in the arrangement of the valence electrons in the molecule. No atoms have been moved. These are called resonance structures. The better Lewis structure or resonance structure is that which has the least amount of formal charge.

1. Lewis Structures
2. Formal Charge
3. Resonance Structures
4. Valence Shell Electron Pair Repulsion Theory
5. Bond and Molecular Polarity
6. Intermolecular Forces
7. VSEPR Tutorial
8. VSEPR Quiz

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