A covalent bond is a shared pair of electrons between two non-metal atoms. A dot-and-cross diagram shows the outer electrons of each atom, with the shared pairs placed where the atoms overlap. The aim is for each atom to end up with a full outer shell, which is 2 electrons for hydrogen and 8 for most other non-metals.
This lesson follows explaining ion formation using electrons in bonding and structure. There electrons moved from one atom to another. Here they are shared.
How does sharing give a full outer shell?
Take two chlorine atoms, each with 7 outer electrons. If each atom shares one electron, the shared pair counts for both atoms.
Each now has 7 + 1 = 8 outer electrons. The result is a Cl₂ molecule.
The number of bonds an atom makes is the number of electrons it is short of a full shell. Hydrogen makes 1 bond, oxygen makes 2, nitrogen makes 3 and carbon makes 4.
How to draw a simple covalent molecule, step by step
- Find the outer electrons for each atom from its group number or electron arrangement.
- Work out how many more each atom needs for a full shell.
- Place shared pairs between the atoms until every atom has what it needs.
- Add lone pairs to any atom that has outer electrons left over.
- Check the count: total electrons in the diagram must equal the total outer electrons of all the atoms.
Worked example
Draw the dot-and-cross diagram for water, H₂O, and state the number of bonding pairs and lone pairs.
Oxygen (2,6) has 6 outer electrons and needs 2 more. Each hydrogen (1) has 1 outer electron and needs 1 more.
Oxygen shares one pair with each hydrogen, which gives 2 bonding pairs. Oxygen’s other 4 electrons stay as 2 lone pairs.
..
H : O : H ":" = shared pair, ".." = lone pair
..In words: O has 2 shared pairs and 2 lone pairs, so 4 pairs or 8 electrons in its outer shell. Each H has 1 shared pair, so 2 electrons. In an exam diagram, use dots for one atom’s electrons and crosses for the other’s.
Count check: total outer electrons = 6 + 1 + 1 = 8. In the diagram there are 2 bonding pairs (4 electrons) and 2 lone pairs (4 electrons) = 8. The count matches.
The mistake to watch for
A common error is to leave out the lone pairs, or to draw ammonia, NH₃, with only the three bonds.
Mistaken answer: ammonia drawn as N with three shared pairs and nothing else.
Nitrogen has 5 outer electrons. Three are shared, but the other two must still be shown.
The correction is to run the count check in step 5.
For NH₃: total outer electrons = 5 + 3 × 1 = 8, which is 4 pairs. Three are bonding pairs, so the fourth pair is a lone pair on nitrogen. Without it, the diagram shows only 6.
Check yourself
Try these without looking back, then open each answer.
1. Hydrogen chloride, HCl. How many shared pairs are there, and how many lone pairs on chlorine?
Show answer
Hydrogen needs 1 more electron and chlorine (7 outer electrons) needs 1 more. They share 1 pair. Chlorine’s remaining 6 electrons are 3 lone pairs.
Count check: 1 + 7 = 8 outer electrons. In the diagram, 1 bonding pair (2) + 3 lone pairs (6) = 8.
2. Carbon dioxide, CO₂. How many pairs does each C to O bond share, and what is the total number of outer electrons?
Show answer
Carbon has 4 outer electrons and needs 4 more. Each oxygen (6 outer electrons) needs 2 more. So each C to O link shares 2 pairs, a double bond. Each oxygen also has 2 lone pairs.
Total outer electrons = 4 + 6 + 6 = 16. In the diagram: 4 bonding pairs (8) + 4 lone pairs (8) = 16.
3. Nitrogen, N₂. How many pairs are shared between the two atoms, and how many lone pairs does each atom have?
Show answer
Each nitrogen atom has 5 outer electrons and needs 3 more, so they share 3 pairs, a triple bond. Each atom has 5 − 3 = 2 electrons left, which is 1 lone pair each.
Count check: 10 outer electrons. In the diagram: 3 bonding pairs (6) + 2 lone pairs (4) = 10.
Where this leads next
Next, see how ionic and covalent bonding lead to large-scale structures in relating a giant structure to a property. Keep the bonding and structure practice set for a timed test of dot-and-cross diagrams and lone-pair counting.
Some students draw neat molecules but skip the count check, so the same lost electron keeps returning. That is the kind of habit that shows up quickly in online one-to-one Chemistry tuition.