# Lewis structure

A Lewis structure shows every valence electron as bonds (shared pairs) or lone pairs. Count total valence electrons, join atoms with single bonds, then add lone pairs so each atom nears an octet. Hydrogen needs only two.

Chemistry (university) · Part 02: Chemical bonding and molecular shape · Card 3 of 9 · Last updated 2026-10-19

## Analogy

Like a seating plan that places every person (electron) exactly once, with no empty or extra seats.

## Example

**Water, H₂O**

Valence electrons: 6 + 1 + 1 = 8
O in the centre, two O–H bonds (4 e⁻)
Two lone pairs on O (4 e⁻)
Total = 8

## In practice

Drawing the Lewis structure is the first step to predicting a molecule's shape, polarity and reactions.

## Practice

### Flashcards

- **How do you count valence electrons for a Lewis structure of an ion?** Add up the valence electrons of all atoms, then add one per negative charge or subtract one per positive charge.

### Interview questions

**Draw the Lewis structure of the nitrate ion and explain resonance and formal charge.**

- Total valence electrons: 24
- Three equivalent resonance forms
- Formal charges: N +1, one O −1 in each form
- Real bond order 4/3

## Questions people ask

### What is the octet rule and when does it fail?

The octet rule says main-group atoms tend to end up with eight valence electrons. It fails for molecules with an odd number of electrons such as NO, for atoms with fewer than eight like BF₃, and for larger atoms like sulfur in SF₆, which can hold more than eight.

### How do I choose the central atom?

The central atom is usually the one that makes the most bonds and is the least electronegative, not counting hydrogen. Carbon is central in CO₂ and sulfur in SO₂. Hydrogen and fluorine are never central because they form only one bond.

## Related concepts

- [Formal charge](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-2/chemistry-university-part-2-concept-4)
- [Resonance](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-2/chemistry-university-part-2-concept-5)

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