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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.

Where it leads

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

VValence electrons

From the periodic table, the group number for a main-group atom.

NNonbonding electrons

Every lone-pair dot on the atom, counted individually.

BNumber of bonds

Every bond line on the atom (a double bond counts as 2).

Choosing the best structure

  1. 1Prefer the structure whose atoms are all as close to zero as possible.
  2. 2When charges can't be avoided, put the negative charge on the more electronegative atom.
  3. 3The formal charges must add up to the overall charge of the molecule or ion.

Before and after: CO₂

Higher energy
COOFormal charge on O: +1. A formal charge is what an atom's charge would be if every bond were split exactly down the middle, so it shows where charge sits in the drawing. Formal charge = valence electrons − nonbonding electrons − half the bonding electrons. Click for this atom's full working.+Formal charge on O: −1. A formal charge is what an atom's charge would be if every bond were split exactly down the middle, so it shows where charge sits in the drawing. Formal charge = valence electrons − nonbonding electrons − half the bonding electrons. Click for this atom's full working.−

O≡C−O leaves one O at +1 and one at −1. Real charges, so this loses.

Preferred
COO

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

The circle method: draw a ring around the atom, count every dot inside it, then add one for each bond line the ring crosses. Subtract that total from the atom's valence electrons, and whatever is left is the formal charge, no separate N and B bookkeeping needed.

Formal-charge-zero patterns

ElementBondsLone pairs
Carbon40
Nitrogen31
Oxygen22
Halogens13

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

Do not forget the overall ion charge. On a polyatomic ion the formal charges must sum to that charge, not to zero. If your sum comes out to zero on NO₃⁻, you have missed a charge somewhere.

Practice

In the hydroxide ion, OH−, oxygen has one bond to hydrogen and three lone pairs. What is the formal charge on that oxygen?
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