Instead of transferring electrons (as in ionic bonding), non-metals SHARE pairs of electrons.
Each shared pair of electrons counts as ONE COVALENT BOND.
Why sharing works:
Each atom counts the shared electrons as part of its OWN outer shell.
Both atoms achieve full outer shells — without either losing electrons entirely.
The shared electrons are attracted to BOTH nuclei — this attraction holds the atoms together.
Covalent bonds are STRONG — a lot of energy is needed to break an individual covalent bond within a molecule.
Examples of Covalent Molecules
HYDROGEN (H₂):
Each H has 1 electron — needs 1 more for a full shell (2 electrons).
Two H atoms share 1 pair of electrons → single covalent bond → each H now 'has' 2 electrons. ✓
WATER (H₂O):
O has 6 outer electrons — needs 2 more. H has 1 — needs 1 more.
O forms 2 covalent bonds, one with each H atom.
O now has 8 outer electrons (full). Each H has 2 (full). ✓
AMMONIA (NH₃):
N has 5 outer electrons — needs 3 more.
N forms 3 single covalent bonds with 3 H atoms.
METHANE (CH₄):
C has 4 outer electrons — needs 4 more.
C forms 4 single covalent bonds with 4 H atoms.
OXYGEN (O₂):
Each O has 6 outer electrons — needs 2 more.
Two O atoms share 2 pairs of electrons → DOUBLE covalent bond.
NITROGEN (N₂):
Each N has 5 outer electrons — needs 3 more.
Two N atoms share 3 pairs → TRIPLE covalent bond. N₂ is very stable.
Dot-and-Cross Diagrams for Covalent Molecules
DOT-AND-CROSS diagrams show the shared electrons in covalent bonds.
Convention: each pair of electrons in a bond is shown between the two atoms. Lone pairs (non-bonding) are shown on the relevant atom.
H₂: H × H (one shared pair between the two H atoms — single bond)
H₂O:
H—O—H with 2 lone pairs on O.
The 2 bonding pairs give O its full 8, and each H its full 2.
CH₄ (methane):
4 H atoms arranged around central C.
4 single bonds — C has 8 electrons in outer shell. Each H has 2.
CO₂:
O=C=O — two double bonds.
C has 8 outer electrons (two sets of 2 shared). Each O also has 8.
The key rule: every atom in a correctly drawn dot-and-cross diagram should have a FULL OUTER SHELL.
⚠️ Common Mistake
Covalent bonds are STRONG — but simple molecular substances have LOW melting points because the FORCES BETWEEN molecules (intermolecular forces) are WEAK. The covalent bond is within the molecule; melting involves separating molecules from each other (overcoming intermolecular forces), not breaking the bonds within them.
📌 Key Note
Covalent bonding: non-metals share electron pairs. Each shared pair = one covalent bond. Both atoms achieve full outer shells. Single bond: 1 shared pair. Double bond: 2 shared pairs. Covalent bonds within molecules are STRONG. Intermolecular forces between molecules are WEAK → low melting points.
🎯 Matching Activity — Match the Molecule to its Bonding
Match each molecule to how many covalent bonds it contains. — drag the symbols on the right to match the component names on the left.
H₂
Drop here
H₂O
Drop here
CH₄
Drop here
O₂
Drop here
N₂
Drop here
2 single covalent bonds — O forms 2 bonds with 2 H atoms
1 double covalent bond — 2 shared pairs between 2 O atoms
1 single covalent bond — 1 shared pair between 2 H atoms
4 single covalent bonds — C forms 4 bonds with 4 H atoms
1 triple covalent bond — 3 shared pairs between 2 N atoms
⭐ Higher Tier Only
Draw dot-and-cross diagrams for: H₂, Cl₂, HCl, O₂ (double bond), N₂ (triple bond), CH₄, H₂O, NH₃, CO₂. Show lone pairs on relevant atoms. Understand bond polarity in terms of electronegativity differences. The shared electrons are attracted to both nuclei — electrostatic attraction is the covalent bond.
🎯 Test Yourself
Question 1 of 2
1. Carbon (Group 4) bonds with hydrogen (Group 1) to form methane (CH₄). How many covalent bonds does carbon form?
2. Why does oxygen exist as O₂ rather than individual O atoms?
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