2.5 Lewis Diagrams

What Lewis diagrams show

A Lewis diagram (electron dot structure) shows how valence electrons are arranged among atoms in a molecule or ion. Bonding pairs are shown as lines (or dot pairs) between atoms, and lone pairs are shown as dots on individual atoms. The goal is usually to give each atom (except H) an octet of electrons.

Drawing steps & formal charge

To draw a Lewis structure: count total valence electrons, arrange atoms with the least electronegative (usually central) atom in the middle, connect atoms with single bonds, then distribute remaining electrons as lone pairs to satisfy octets, adding multiple bonds if needed. Formal charge = (valence e⁻) − (nonbonding e⁻) − (bonding e⁻/2); the best structure minimizes formal charges, placing negative charge on more electronegative atoms.

Resonance and exceptions

When more than one valid Lewis structure exists, the true structure is a resonance hybrid, an average of all forms with equal or nearly equal energy. Some molecules break the octet rule: odd-electron species (like NO), electron-deficient atoms (like B or Be), and expanded octets (for elements in period 3+, like S or P) can have fewer or more than 8 electrons.

Key terms

Lewis diagram

A drawing showing valence electrons as bonds (lines) and lone pairs (dots) around atoms in a molecule or ion.

Octet rule

Atoms tend to gain, lose, or share electrons to have 8 valence electrons (except H, which needs 2).

Formal charge

Valence electrons − nonbonding electrons − (bonding electrons/2); used to evaluate the best Lewis structure.

Resonance structure

One of two or more valid Lewis structures for a molecule; the real structure is an average (hybrid) of all of them.

Octet rule exceptions

Odd-electron species (NO), electron-deficient atoms (B, Be), and expanded octets (period 3+ elements like S, P).

Try an AP-style question

How many valence electrons are in the Lewis structure of CO₂?

1. 10
2. 16
3. 12
4. 22