AQA Chemistry (8462) 4.2.1.4 · Process
Covalent bonding
When two atoms both want electrons, neither will give any away. How do they still end up with full shells?
Triple Science · Higher tier
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Two hydrogen atoms. One electron each. Both need two.
Hydrogen gas is not made of single atoms. It is made of H₂ molecules, and pulling one apart takes a lot of energy. Yet each hydrogen atom has only one electron, and a full first shell needs two. Neither atom is a metal, so neither will hand its electron over.
Commit first
How can both hydrogen atoms have a full shell at the same time?
Two non-metal atoms overlap their outer shells and share a pair of electrons, one from each. Both atoms count the shared pair as their own. The pair sits between the two nuclei and is attracted to both of them, and that attraction holds the atoms together: one shared pair is one covalent bond. Covalent bonds are strong. An atom forms as many bonds as it needs electrons: hydrogen one, oxygen two, nitrogen three, carbon four. Call the next one before you see it.
Worked · water, H₂O
Step 1 · count the outer electrons
Step 1 of 3
Oxygen is in Group 6: six outer electrons, two short of eight. Each hydrogen has one electron, one short of two.
Your turn · the shared-pair builder
Choose how many pairs each bond shares.
0 of 7 built
Most covalent substances are small molecules like these: a fixed number of atoms, strong bonds inside, and separate molecules that are only weakly attracted to each other. Some are very large molecules, such as the polymers in plastic. Some are giant covalent structures, like diamond and silicon dioxide (sand), where the bonds never stop. The strong bond is always the one between atoms. Sort out which statements believe that.
Think again · strong bond or weak force?
“Water boils at only 100 °C, so its covalent bonds must be weak.”
Sort the claims
Accurate, or the weak-bond mistake?
Six statements a student wrote about water. Decide which are accurate chemistry and which carry the mistake above.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- State
- A number or a fact. No explanation asked for.
- Draw
- Outer shells only. Shared pairs in the overlap, lone pairs on the atom.
- Explain
- Count electrons: how many each atom has, needs, and shares.
Key fact
A covalent bond is a shared pair of electrons between two non-metal atoms. One pair is a single bond, two a double, three a triple, and every covalent bond is strong.
Examiner tip
Key note · AQA 4.2.1.4 (8462)
Covalent bonding
- Covalent bonds form between non-metal atoms.
- A covalent bond is a shared pair of electrons, attracted to both nuclei.
- Single bond: 1 shared pair. Double: 2. Triple: 3.
- An atom forms as many bonds as it is short of a full shell: H 1, O 2, N 3, C 4.
- Draw dot-and-cross with outer shells only: H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄.
- Covalent bonds are strong. Weak forces act between molecules, not inside them.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask Mr Badmus AI
Not sure how many pairs a molecule shares?
Dot-and-cross diagrams show outer shells only, the AQA convention for covalent molecules; one atom's electrons are dots and the other's crosses. The builder's line drawing is a displayed formula: each line is one shared pair. Real molecules have shapes in three dimensions that neither drawing shows.