Two non-metal atoms both want electrons, so neither can simply take them. Instead they share: each puts an electron into a pair that belongs to both atoms, and each counts the shared pair towards its full outer shell.
A covalent bond is formed when a pair of electrons is shared between two atoms, usually non-metals. By sharing, each atom reaches a noble-gas configuration. Simple molecular substances have low melting and boiling points and are poor conductors of electricity. (Extended) Their melting points are low because only the weak intermolecular forces between molecules are overcome; the strong covalent bonds inside the molecules do not break.
Fig. 1: dot-and-cross diagram of a water molecule
Section 01
Sharing a pair.
Build each molecule in the simulator. Count the electrons around every atom once the bonds are made: hydrogen should reach 2, every other atom 8.
Molecule
Covalent bonds
Lone (non-bonding) pairs
hydrogen, H2
1 (H-H)
0
chlorine, Cl2
1 (Cl-Cl)
3 on each Cl
hydrogen chloride, HCl
1 (H-Cl)
3 on Cl
water, H2O
2 (O-H)
2 on O
ammonia, NH3
3 (N-H)
1 on N
methane, CH4
4 (C-H)
0
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
A covalent bond is:
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How many covalent bonds are in a molecule of methane, CH4?
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How many lone (non-bonding) pairs of electrons are on the nitrogen atom in ammonia, NH3?
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In a chlorine molecule, Cl2, how many electrons does each chlorine atom have in its outer shell, counting the shared pair?
Section 02
Double and triple bonds.
Extended. Some atoms must share two or three pairs with the same partner to reach eight outer electrons.
Oxygen, O2: each O (2,6) shares two electrons, making a double bond, O=O. Each O keeps two lone pairs.
Carbon dioxide, CO2: carbon (2,4) shares all four outer electrons, two with each oxygen: O=C=O, two double bonds, and each O has two lone pairs.
Nitrogen, N2: each N (2,5) shares three electrons: a triple bond with six shared electrons, and one lone pair on each N.
Ethene, C2H4: a C=C double bond plus two C-H bonds on each carbon.
Methanol, CH3OH: all single bonds: three C-H, one C-O and one O-H, with two lone pairs on the oxygen.
Examiner trap
Each bond line stands for one shared pair, two electrons. A double bond is two pairs, four electrons; a triple bond three pairs, six electrons. Drawing CO2 with single bonds leaves each oxygen with only seven outer electrons, so always check that every atom other than hydrogen ends with eight.
Section 03
Properties of simple molecules.
Methane, water, ammonia and chlorine are all made of small separate molecules, and that single fact explains their properties.
Low melting and boiling points: many are gases or liquids at room temperature.
Why (Extended): the forces between molecules, intermolecular forces, are weak, so little energy is needed to separate the molecules. The covalent bonds within each molecule are strong and are not broken on melting or boiling.
Poor electrical conductors: the molecules are neutral, and there are no ions or delocalised electrons free to move.
Examiner trap
When water boils, the O-H covalent bonds do not break; the molecules simply move apart and the steam is still H2O. An answer saying 'covalent bonds are weak' or 'covalent bonds break on boiling' loses the mark: it is the intermolecular forces that are weak.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
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(Extended) Why does methane have a low boiling point?
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(Extended) How many electrons are shared between the two atoms in a nitrogen molecule, N2?
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(Extended) Which describes the bonding in carbon dioxide, CO2?
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Why does liquid bromine not conduct electricity?
Skill unlocked
Covalent bonding, mastered.
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