A covalent bond is a shared pair of electrons. To draw a molecule, count the outer electrons first, then share pairs until every atom reaches its octet or duplet.
This lesson is part of SPM Chemistry chemical bonding. What happens between finished molecules is in distinguishing intermolecular forces from chemical bonds.
How do I count before drawing?
Write the outer electrons, then the electrons still needed, then the shared pairs.
| Molecule | Atom | Outer electrons | Needs | Shared pairs | Lone pairs |
|---|---|---|---|---|---|
| H₂O | O | 6 | 2 | 2 (one with each H) | 2 |
| NH₃ | N | 5 | 3 | 3 (one with each H) | 1 |
| CO₂ | C | 4 | 4 | 4 (two with each O) | 0 |
| N₂ | N | 5 | 3 | 3 (with the other N) | 1 |
Hydrogen has one outer electron and needs one more, so it forms one bond and has no lone pair.
Worked example 1: water
Oxygen has six outer electrons and needs two. Each hydrogen provides one electron, so oxygen forms one single bond with each hydrogen.
The diagram has an oxygen atom in the centre, two shared pairs to two hydrogens, and two lone pairs on the oxygen. The oxygen then has 2 + 2 + 2 + 2 = 8 electrons around it, and each hydrogen has 2.
Worked example 2: carbon dioxide
Carbon has four outer electrons, and each oxygen has six and needs two. Each oxygen shares two pairs with carbon, forming a double bond.
Carbon ends up with 4 shared pairs (8 electrons). Each oxygen has 2 shared pairs and 2 lone pairs, which also gives 8. The molecule is written O=C=O.
The mistake that loses marks
A student draws nitrogen, N₂, with a double bond and leaves each nitrogen with only six electrons around it. A triple bond gives each nitrogen three shared pairs and one lone pair, which makes eight.
The fix is to count each atom after drawing. Every atom must reach 8, or 2 for hydrogen, and the total electron count must match the atoms supplied.
Check yourself
Draw ammonia, NH₃, and state the number of shared pairs and lone pairs on the nitrogen.
Answer
Nitrogen has five outer electrons and needs three more. It shares one pair with each of three hydrogen atoms, so it has three shared pairs and one lone pair.
Check: 3 shared pairs (6 electrons) + 1 lone pair (2 electrons) = 8 around nitrogen, and each hydrogen has 2.
What to study next
Put covalent structures to use in explaining properties from bonding and structure, then see the practice set.
The mistake log and paper-error review tracks the diagram slips you repeat. For a teacher to watch you draw, see online one-to-one Chemistry tuition.