Lewis Structures and Bonding

HFHF
Oxidation State
Oxidation State
Formal Charge
Formal Charge
Assume all bonding electrons go to the more electronegative atom (heterolytic cleavage).
Assume all bonding electrons go to the more electronegative atom (heterolytic cleavage).
Split bonding electrons equally between atoms (homolytic cleavage).
Split bonding electrons equally between atoms (homolytic cleavage).
HF−HF
Oxidation State
Formal Charge
Assume all bonding electrons go to the more electronegative atom (heterolytic cleavage).
Split bonding electrons equally between atoms (homolytic cleavage).
This gives each atom a hypothetical charge called its oxidation state.
Count the electrons each atom owns, then compare to its free-atom count: more → negative; fewer → positive.
–1
+1
0
0

Formal Charge

What It Is

Formal charge is a bookkeeping system — it assigns “ownership” of electrons to each atom in a Lewis structure by assuming all bonds are shared equally (50/50), then checks whether each atom has more or fewer electrons than it brought to the table.

\[\mathrm{FC} = \mathrm{valence~electrons} - \mathrm{dots} - \mathrm{lines}\]

  • Valence electrons — what the free atom starts with
  • Dots — lone pair electrons (fully “owned”)
  • Lines — bonds (each bond = 2 electrons, atom “owns” half = 1 per line)

A formal charge of zero means the atom owns exactly as many electrons as it has in its ground state. A nonzero formal charge means the Lewis structure assigns it more or fewer than expected.

Note

Formal charge is not actual charge. It is an artifact of how we draw Lewis structures — real electron density is distributed according to electronegativity, not split 50/50.

Why We Minimize Formal Charge

The core principle: charge separation costs energy.

A Lewis structure with +1 and −1 on adjacent atoms implies electron density has been pulled away from one atom and piled onto another. That is electrostatically unfavorable — you’re fighting Coulomb’s law to keep opposite charges separated rather than letting them neutralize through bonding.

Minimizing formal charge selects the structure that:

  • Maximizes covalent sharing — more bonding pairs, fewer lone pairs sitting idle
  • Minimizes charge separation — lower electrostatic energy
  • Best approximates real electron density — the actual molecule distributes electrons smoothly; the Lewis structure with minimal formal charges is the closest bookkeeping match

When Formal Charges Are Unavoidable

Some molecules cannot be drawn with all formal charges at zero. When nonzero charges are required:

  • Place the negative formal charge on the more electronegative atom. That atom is better at stabilizing extra electron density — it is lower energy for electrons to sit there.
  • Avoid like charges on adjacent atoms. Two adjacent positives (or two adjacent negatives) is energetically costly.

Summary of Guidelines

  1. Formal charges must sum to the overall charge of the species (requirement, not preference).
  2. Minimize formal charges — structures with all zeros are preferred.
  3. Negative charge on the more electronegative atom if charges are unavoidable.
  4. Avoid like charges on adjacent atoms.

These are all heuristics pointing at the same physical reality: electrons settle into the lowest-energy arrangement, and electronegativity tells you which atoms hold extra electron density most comfortably.