Simple Molecules and Covalent Bonds
Covalent bonding for IGCSE Chemistry 0620: shared electron pairs, dot-and-cross diagrams for H2 to CO2, and why simple molecules melt low and don't conduct.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
The single most-reported error in 0620 bonding questions, series after series: “methane has a low boiling point because its covalent bonds are weak.” Wrong, and worth zero. Covalent bonds are strong; it is the forces between molecules that are weak. This page fixes that distinction permanently and covers the dot-and-cross diagrams, definitions and property explanations that Papers 1-4 build from it.
What a covalent bond is (Core)
When two non-metal atoms react, neither can take electrons from the other, so they share. A covalent bond is a shared pair of electrons between two atoms; sharing lets each atom count the shared electrons toward a full outer shell (noble-gas configuration). Supplement candidates should add the deeper description: the bond is the electrostatic attraction between the positive nuclei and the shared pair of electrons between them.
One shared pair is a single bond (H–H, C–H). Two shared pairs make a double bond (O=O, C=O); three make a triple bond (N≡N). Double and triple bonds are Supplement.
Dot-and-cross diagrams, in words (Core and Supplement)
Each diagram shows overlapping outer shells with shared electrons sitting in the overlap, dots for one atom’s electrons and crosses for the other’s. The Core set: hydrogen, H2, one shared pair, nothing else. Chlorine, Cl2: one shared pair, each Cl keeping three non-bonding (lone) pairs. Hydrogen chloride, HCl: one shared pair, three lone pairs on Cl. Water, H2O: oxygen shares one pair with each of two hydrogens and keeps two lone pairs. Ammonia, NH3: nitrogen shares with three hydrogens, one lone pair left. Methane, CH4: carbon shares with four hydrogens, no lone pairs.
The Supplement set adds: oxygen, O2, two shared pairs (a double bond) plus two lone pairs per atom. Carbon dioxide, CO2: two double bonds, carbon between two oxygens. Nitrogen, N2: three shared pairs and one lone pair each. Ethene, C2H4: a C=C double bond plus four C–H bonds. Methanol, CH3OH: all single bonds, two lone pairs on oxygen. The marker counts electrons: every atom must end with a full outer shell (2 for H, 8 otherwise), and lone pairs must be shown.
Why simple molecular substances behave as they do (Core)
| Property | Explanation |
|---|---|
| Low melting and boiling points | Weak intermolecular forces (forces between molecules) need little energy to overcome |
| Gases or liquids, or low-melting solids, at room temperature | Same reason |
| Do not conduct electricity | Molecules have no overall charge, so no free ions or electrons to carry charge |
| Mostly insoluble in water (with exceptions like HCl, NH3) | Weak attraction between molecules and water |
Melting or boiling a simple molecular substance separates the molecules from each other; the strong covalent bonds within each molecule stay intact. Boiling water gives H2O gas, not hydrogen and oxygen. Compare this top-to-bottom with ions and ionic bonds: different particles, different forces, opposite properties. Then see what happens when covalent bonding goes giant in giant covalent structures.
Worked exam question
Chlorine exists as Cl2 molecules. (a) Describe the bonding in a chlorine molecule. You may refer to a dot-and-cross diagram. (2) (b) Chlorine boils at −35 °C. Explain why its boiling point is low. (2) (c) Explain why liquid chlorine does not conduct electricity. (1)
Model answer: (a) The two chlorine atoms share one pair of electrons (1); each atom then has eight electrons in its outer shell / a noble-gas configuration, with three lone pairs each (1). (b) The forces of attraction between the Cl2 molecules are weak (1), so only a small amount of energy is needed to separate them (1). (c) There are no ions or free electrons: the molecules are uncharged, so nothing can carry the charge (1).
Mark-by-mark: (a) the shared-pair statement is the bond mark; full outer shells the second. (b) the word “between molecules” is the difference between 2 marks and 0; “weak bonds” unqualified is read as weak covalent bonds and rejected. (c) must name the missing charge carriers.
The mistakes that cost marks
- “Weak covalent bonds” as the reason for low boiling points. Covalent bonds are strong; weak intermolecular forces are overcome on boiling. Examiner reports flag this every single series.
- Dot-and-cross diagrams missing lone pairs, or with the wrong electron count. Chlorine’s three lone pairs per atom are part of the answer.
- Saying covalent substances don’t conduct “because they have no electrons”. They have plenty of electrons, but none are free to move, and there are no ions.
- Drawing ionic-style brackets and charges around molecules. Covalent diagrams show overlapping shells and no charges.
How examiners want it phrased
| Typical student wording | Accepted mark-scheme wording |
|---|---|
| ”The atoms stick together by sharing" | "A covalent bond is a shared pair of electrons between two atoms" |
| "Methane’s bonds are weak so it boils easily" | "The intermolecular forces between methane molecules are weak and need little energy to overcome" |
| "It can’t conduct because it’s covalent" | "There are no free ions or electrons to carry charge" |
| "Oxygen has a strong double bond" | "The two oxygen atoms share two pairs of electrons, forming a double covalent bond” |
The Malaysia note
Paper 2 MCQs on bonding hinge on speed-reading diagrams, and Malaysian students juggling 8-10 IGCSE subjects in the May/June series rarely give bonding diagrams the rehearsal they need. The concept feels understood, so the drawing never gets timed practice. Set a five-minute drill: all six Core molecules from memory. If the intermolecular-forces distinction in this page is news to you or your child, that alone justifies booking the free 1-hour trial lesson: it is the most profitable correction in topic 2.
Test yourself
Work through all three without looking back. Click each answer to check.
Q1 (1 mark). Define a covalent bond.
Show answer
• A shared pair of electrons between two atoms [1]
Q2 (2 marks). A dot-and-cross diagram of a water molecule is drawn. State the number of shared pairs of electrons in the molecule and the number of lone pairs on the oxygen atom.
Show answer
• Two shared pairs, one between oxygen and each hydrogen atom [1] • Two lone (non-bonding) pairs on the oxygen atom [1]
Q3 (3 marks). Carbon dioxide is a simple molecular compound. (a) Explain why it has a low boiling point. (2) (b) State what happens to the covalent bonds within the CO2 molecules when the liquid boils. (1)
Show answer
• (a) The forces of attraction between the CO2 molecules (intermolecular forces) are weak [1] • Only a small amount of energy is needed to overcome them [1] • (b) The covalent bonds within each molecule are strong and are not broken; the molecules stay intact [1]
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Frequently asked questions
What is a covalent bond in mark-scheme language?
A shared pair of electrons between two (non-metal) atoms. At Supplement level, add that the bond is the electrostatic attraction between the nuclei and the shared pair of electrons.
Why do simple covalent substances have low melting points?
The forces between molecules (intermolecular forces) are weak and need little energy to overcome. The covalent bonds inside the molecules are strong and do not break on melting. Confusing the two is the most-penalised error in the bonding topic.
Which dot-and-cross diagrams do I need to know?
Core: H2, Cl2, H2O, CH4, NH3, HCl. Supplement adds molecules with double and triple bonds and unshared pairs: O2, CO2, N2, C2H4, CH3OH. Practise drawing each from memory.