Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- A Lewis diagram shows all the valence electrons in a molecule or ion. Shared pairs are lines; lone pairs are pairs of dots. Core electrons are not drawn.
- Single bond = 2 shared electrons; double = 4; triple = 6.
- C, N, O and F almost always end with 8 electrons around them (octet rule). H ends with 2.
- B and Be can end with fewer than 8 (BF₃: B has 6).
- Atoms in period 3 or beyond can have more than 8 (SF₄: S has 10).
- A Lewis diagram shows connectivity, not shape.
The method
| Step | What to do |
|---|---|
| 1 | Total valence electrons. Negative ion: add the charge. Positive ion: subtract it. |
| 2 | Skeleton: least electronegative atom in the centre; H and F outside. |
| 3 | Single bonds to every outer atom (2 electrons each). |
| 4 | Lone pairs to complete each outer atom’s octet. |
| 5 | Leftover electrons on the central atom. |
| 6 | Central atom short of 8? Turn outer lone pairs into double or triple bonds. |
| 7 | Recount; brackets and charge for an ion. |
Shortcut for octet molecules: bonds = (needed − available) ÷ 2, where needed = 8 per non-H atom + 2 per H.
Usual bonding patterns (neutral atoms)
| H | C | N | O | Halogen (outer) |
|---|---|---|---|---|
| 1 bond, 0 lone pairs | 4 bonds, 0 | 3 bonds, 1 | 2 bonds, 2 | 1 bond, 3 |
Assumptions behind the model
- Electrons are either shared in pairs or held as lone pairs; the model ignores how they are spread in space.
- The skeleton is known or can be chosen from electronegativity.
Mistakes to avoid
- Ignoring the charge. NH₄⁺ has 8 valence electrons; CN⁻ has 10.
- H in the centre. It forms only one bond.
- Leftover electrons not drawn. In SF₄ the last 2 electrons are a lone pair on S.
- Expanded octet on a period 2 atom. N, C, O and F never exceed 8.
- Double bond to an outer halogen. Use O, N or C lone pairs to make multiple bonds.
- No brackets on an ion. Brackets plus the charge show the charge belongs to the whole ion.
Quick self-check
- How many valence electrons are in SO₃²⁻? (6 + 18 + 2 = 26)
- How many lone pairs are on the central atom of PCl₃? (One: 3 bonds + 1 lone pair = 8)
- What is the bond between the two carbon atoms in C₂H₂? (A triple bond: 10 electrons; needed 20, so 5 bonds = 2 C–H + 1 C≡C)
- How many electrons surround Xe in XeF₂? (10: 2 bonds and 3 lone pairs)
Next: practice questions.