What Is a Lewis Structure? Dots, Bonds, and Octets

Chemistry throws molecular formulas at you like H₂O and CO₂, but a formula doesn't show how the atoms actually connect. A Lewis structure is the little dot-and-line drawing that fills in that gap — and once you can draw one, molecules stop being mysterious.

The short answer: a Lewis structure (or electron-dot structure) is a diagram that shows how the valence electrons of atoms are arranged in a molecule. Shared pairs (the bonds) are drawn as lines, and unshared pairs (lone pairs) are drawn as dots, so you can see at a glance which atoms are bonded and where the leftover electrons sit.

What a Lewis structure actually shows

A Lewis structure tracks only the valence electrons — the outer-shell electrons that do the bonding. It uses two symbols:

  • A line = a bonding pair (two shared electrons). A double line is two shared pairs; a triple line is three.
  • A pair of dots = a lone pair (two electrons that belong to one atom and aren't shared).

The goal is usually to give every atom a full outer shell. For most main-group atoms that means the octet rule: eight electrons around each atom. Hydrogen is the big exception — it's happy with just two.

How to draw one, step by step

Here's the reliable recipe:

  1. Count all valence electrons. Add up the valence electrons from every atom (use the group number for main-group elements). Add electrons for negative charges, subtract for positive ones.
  2. Pick the central atom. It's usually the least electronegative atom (except hydrogen, which is always on the outside). Carbon is almost always central.
  3. Connect atoms with single bonds. Draw one line from the central atom to each outer atom. Each line uses two electrons.
  4. Distribute the remaining electrons as lone pairs, starting with the outer atoms, to complete their octets (hydrogen needs none extra).
  5. Make multiple bonds if needed. If the central atom still lacks an octet, turn a nearby lone pair into a double or triple bond.

Worked example 1: water (H₂O)

  • Count: O has 6 valence electrons, each H has 1 → 6 + 1 + 1 = 8 total.
  • Arrange: O in the centre, an H on each side.
  • Bond: two O–H single bonds use 4 electrons.
  • Finish: 4 electrons remain → place them on oxygen as two lone pairs.

Result: H–O–H with two lone pairs on the oxygen. Oxygen sees 8 electrons (2 bonds + 2 lone pairs), each hydrogen sees 2. Everyone's shell is full. ✓

Worked example 2: carbon dioxide (CO₂)

  • Count: C has 4, each O has 6 → 4 + 6 + 6 = 16 total.
  • Arrange: C in the centre, an O on each side.
  • Bond & finish: single bonds alone leave carbon short of an octet, so form a double bond to each oxygen: O=C=O.

Now carbon has 8 electrons (two double bonds), and each oxygen has 8 (one double bond + two lone pairs). All 16 electrons are placed. ✓

Common mistakes to avoid

  • Counting core electrons. Lewis structures use valence electrons only. Oxygen brings 6 to the table, not its full 8.
  • Forgetting hydrogen's duet. Hydrogen never takes a lone pair and never wants eight — two electrons (one bond) is a full shell for it.
  • Skipping the electron count. If your finished drawing doesn't use exactly the number of valence electrons you counted at the start, something's wrong. The count is your built-in check.

FAQ

What is a Lewis structure used for?
To show how atoms are bonded and where lone pairs sit. It's the starting point for predicting a molecule's shape, its polarity, and how it will react.

What's the difference between a bonding pair and a lone pair?
A bonding pair is shared between two atoms (drawn as a line); a lone pair belongs to a single atom and isn't shared (drawn as two dots).

What is the octet rule?
The tendency of main-group atoms to end up with eight valence electrons — a full, stable outer shell. Hydrogen is the exception, aiming for two.

Do Lewis structures show the real 3D shape?
Not directly. They show connectivity and electron pairs; you then use those pairs (with VSEPR theory) to work out the actual 3D geometry.

The takeaway

A Lewis structure is a valence-electron map: lines for shared bonding pairs, dots for lone pairs, drawn so each atom reaches a full shell (eight electrons for most, two for hydrogen). Count your electrons, place the central atom, add bonds, then fill in lone pairs — and you can diagram almost any small molecule.

Next up → [What Is a Polar Molecule?] — using the shape a Lewis structure implies to judge polarity. See also [What Is a Valence Electron?] and [Ionic vs Covalent Bonds].

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