Formal Charge
The formula, the circle-method shortcut, and a before-and-after that shows why lowest charge wins.
Why it matters
When two both look legal, is the tiebreaker that reveals which one nature actually prefers: the one keeping charges closest to zero.
When you use it
After you have drawn a structure, to check it, or to choose between competing forms.
What it unlocks
Picking the best resonance structure, and understanding why some ions are drawn the way they are (negative charge lands on the most atom).
Formal charge
FC = V − N − B
valence electrons, minus nonbonding electrons, minus the number of bonds
What the letters mean
From the periodic table, the group number for a main-group atom.
Every lone-pair dot on the atom, counted individually.
Every bond line on the atom (a double bond counts as 2).
Choosing the best structure
- 1Prefer the structure whose atoms are all as close to zero as possible.
- 2When charges can't be avoided, put the negative charge on the more electronegative atom.
- 3The formal charges must add up to the overall charge of the molecule or ion.
Before and after: CO₂
O≡C−O leaves one O at +1 and one at −1. Real charges, so this loses.
O=C=O puts every atom at 0. Closest to zero wins.
Both drawings obey the octet and use the same 16 electrons. The difference is the formal charges labelled on each atom: the structure that keeps them nearest zero is the one that best describes the real molecule.
Memory trick
Formal-charge-zero patterns
| Element | Bonds | Lone pairs |
|---|---|---|
| Carbon | 4 | 0 |
| Nitrogen | 3 | 1 |
| Oxygen | 2 | 2 |
| Halogens | 1 | 3 |
Memorize these and you can spot a zero-formal-charge atom on sight: carbon with four bonds, nitrogen with three bonds and a lone pair, oxygen with two and two, a halogen with one bond and three lone pairs.
Common mistake
Practice