Structure and bonding · GCSE Chemistry
Covalent bonding
GCSE Chemistry revision on covalent bonding: shared electron pairs in H₂, Cl₂, H₂O, CH₄ and CO₂, and why simple molecules have low melting points and do not conduct.
Share pairs to fill the outer shell. The covalent bonds inside the molecule are strong; the forces between simple molecules are weak. That is why they are gases or low-boiling liquids and do not conduct.
The important bits
What you need to know
- 1
Covalent bonding is the sharing of one or more pairs of electrons between non-metal atoms, so that each atom achieves a full outer shell.
- 2
A single covalent bond is one shared pair (H–H in H₂, H–Cl in HCl). A double bond is two shared pairs (O=C=O in CO₂). A triple bond is three shared pairs (N≡N in N₂).
- 3
Learn the displayed formulae: H₂, Cl₂, O₂, N₂, HCl, H₂O (oxygen has two lone pairs), NH₃, CH₄ (tetrahedral), CO₂ (linear).
- 4
Simple molecular substances have strong covalent bonds within each molecule but weak intermolecular forces between molecules, so they have low melting and boiling points.
- 5
They do not conduct electricity: there are no delocalised electrons and no mobile ions. The molecules are neutral.
- 6
Many simple molecules are gases or liquids at room temperature (O₂, CO₂, H₂O, Cl₂). Iodine is a simple molecular solid: I₂ molecules with weak forces between them, so it sublimes easily.
- 7
Dot-and-cross diagrams show only outer electrons. Shared pairs sit in the overlap; lone pairs stay on one atom. Carbon needs four bonds; oxygen needs two; hydrogen needs one.
- 8
Contrast with ionic compounds: covalent simple molecules melt low and never conduct; ionic lattices melt high and conduct when molten or aqueous. Do not say covalent bonds are weak — intermolecular forces are weak.
Quotations worth analysing
Short evidence. Real method.
“A covalent bond is a shared pair of electrons between atoms.”
Sharing happens between non-metals. Do not transfer electrons here. Each shared pair counts towards a full outer shell for both atoms.
“H₂O CH₄ CO₂ N₂ Cl₂”
Water is bent with two O–H bonds. Methane is four C–H bonds. Carbon dioxide has two C=O double bonds. Nitrogen has a triple bond, which is why N₂ is unreactive at room temperature.
“H₂(g) + Cl₂(g) → 2HCl(g)”
Hydrogen chloride is still a simple molecule. It dissolves in water to make HCl(aq), hydrochloric acid, which does contain H⁺(aq) ions — that is a reaction with water, not the bonding in HCl gas.
Go deeper
Strong inside, weak between — never mix those forces
This is the sentence that separates grade 4 from grade 7. To boil water you do not break O–H covalent bonds. You overcome the weak forces between H₂O molecules. The molecules stay H₂O in steam: H₂O(l) → H₂O(g). If you did break covalent bonds you would decompose water into hydrogen and oxygen, which needs electrolysis or a very high temperature. Exam answers that say “water has weak covalent bonds” lose the mark. Say weak intermolecular forces. Diamond is the contrast: every carbon is covalently bonded to four others in a giant structure, so you cannot melt it without breaking covalent bonds — hence the very high melting point.
Go deeper
Ionic versus covalent is a properties table, not a slogan
Ionic: giant lattice, high melting point, conducts when molten or in solution, often soluble in water, brittle crystals. Simple covalent: small molecules, low melting point, do not conduct, often insoluble in water (though some polar molecules dissolve), gases, liquids or soft solids. Metallic: delocalised electrons, conducts as a solid, malleable. Giant covalent: very high melting point, usually do not conduct (graphite is the exception). If a six-mark asks you to compare NaCl and Cl₂, work through melting point and conductivity with the particle reason for each. Cl₂ is Cl–Cl molecules with weak forces; NaCl is Na⁺ and Cl⁻ with strong ionic bonds.
Go deeper
Dot-and-cross diagrams are counting exercises
Hydrogen has 1 outer electron, so H₂ shows two dots or crosses in the overlap and nothing else. Oxygen has 6 outer electrons: in H₂O, two are in O–H bonds and four sit as two lone pairs. Carbon has 4: in CH₄ all four are in bonds, no lone pairs. In CO₂, carbon shares two pairs with each oxygen (double bonds) so carbon has 8 in its outer shell and each oxygen has 8. Students draw five bonds on carbon or forget the lone pairs on oxygen and then cannot explain why water is H₂O not H₄O. Count to eight (or two for hydrogen) for every atom before you move on.
See the idea in action
Explain why chlorine, Cl₂, is a gas at room temperature but sodium chloride is a solid with a high melting point. Chlorine is simple molecular: each molecule is two chlorine atoms sharing a pair (Cl–Cl). Forces between Cl₂ molecules are weak, so little energy is needed to separate them and chlorine boils at −34 °C. Sodium chloride is a giant ionic lattice of Na⁺ and Cl⁻. Strong electrostatic attractions act in all directions, so a lot of energy is needed to melt it. Cl₂ does not conduct; molten NaCl does, because ions can move.
Exam technique
Turn knowledge into marks
If the substance is a simple molecule, say covalent bonds stay intact on melting and intermolecular forces are overcome. Never call covalent bonds weak. Contrast ionic lattices whenever the question compares a salt with a gas.
Common mistakes
Do not give these marks away
- 01
Saying simple molecules have weak covalent bonds, or that they conduct because electrons are shared.
- 02
Drawing the wrong number of bonds on carbon, oxygen or nitrogen in a dot-and-cross diagram.
- 03
Treating HCl(g) as ionic, or NaCl as a covalent molecule.
Why does CO₂ have a low boiling point?
AThe C=O covalent bonds are weak
BThere are weak forces between simple CO₂ molecules
CCarbon dioxide is ionic
DCO₂ has delocalised electrons
Show the answer
There are weak forces between simple CO₂ molecules. Covalent bonds inside each CO₂ molecule are strong. Boiling only separates molecules, which takes little energy because intermolecular forces are weak.
Quick questions
If this is the bit you searched
What is covalent bonding GCSE Chemistry?
The sharing of electron pairs between non-metal atoms so that each atom has a full outer shell. A single bond is one shared pair; a double bond is two.
Why do simple covalent molecules have low melting points?
The covalent bonds inside each molecule are strong, but the intermolecular forces between molecules are weak, so little energy is needed to separate the molecules.
Why do covalent molecules not conduct electricity?
The molecules are neutral. There are no delocalised electrons and no mobile ions to carry charge.
What is the difference between ionic and covalent properties?
Ionic compounds form giant lattices with high melting points and conduct when molten or dissolved. Simple covalent substances form small molecules with low melting points and do not conduct.