A covalent bond is a shared pair of electrons
A covalent bond is formed between atoms by the sharing of a pair of electrons. In terms of forces it is the electrostatic attraction between the positive nuclei and the negative shared pair. Two mark-scheme elements: a shared pair of electrons, and attraction between the nuclei (plural — both atoms) and that pair. A single shared pair is a single bond; a double bond (, ) is two shared pairs, a triple () three.
Simple molecular vs giant covalent
Simple molecular substances (e.g. ) are small molecules: strong covalent bonds inside each, but weak intermolecular forces between them. Only these weak forces are overcome on melting, so melting points are low; as relative molecular mass rises the forces strengthen and points rise. Giant covalent structures (e.g. ) are networks of strong covalent bonds throughout, so melting needs a huge amount of energy — very high melting points.
Diamond, graphite and C60
All three are allotropes of carbon. Diamond: each carbon forms 4 covalent bonds in a rigid 3-D network — very hard and non-conducting. Graphite: each carbon forms 3 covalent bonds in layers held by weak forces that slide, so it is soft; the spare electron per atom is delocalised and free to move, so graphite conducts. C60 fullerene: separate molecules of 60 carbons with only weak intermolecular forces, so it is soft and low-melting.
Drawn from real examiner reports.
Covalent definition: "pair" and "nuclei"
The definition needs a shared pair of electrons — "sharing electrons" or "atoms joined" without "a pair" does not score. In terms of forces it is the attraction between the nuclei (plural — both atoms) and the shared pair; "the nucleus" implies one atom and loses the mark. Confusing it with metallic bonding (cations, a sea of electrons) voids the answer.
June 2024 Paper 1CR Q11(b): fewer than half scored both marks; marks lost for not giving a shared pair of electrons and for writing "nucleus" instead of "nuclei", and some confused it with metallic bonding by mentioning cations.
Melting overcomes forces, not bonds
When a simple molecular substance melts, the strong covalent bonds inside the molecules do NOT break — only the weak intermolecular forces between molecules are overcome. "The covalent bonds break" scores zero. "Simple covalent structure" is acceptable; "simple covalent bond" is not.
June 2024 Paper 1CR Q9(b)(iii): many broke the bonds in hydrogen chloride on melting rather than overcoming the intermolecular forces; "a simple covalent bond" was not creditworthy.
No intermolecular forces in diamond
Diamond and graphite are giant covalent: diamond has no intermolecular forces, and graphite's weak forces act only between layers. Why diamond is hard (two points): each carbon bonds to 4 others in a rigid network of strong covalent bonds. Why graphite is soft: layers held by weak forces that slide over each other.
June 2024 Paper 1CR Q11(c)(ii): very few gained all four marks; some lost marks for mentioning intermolecular forces in diamond, and many failed to give two structural points for hardness and two for softness.
Graphite conducts: delocalised + move
"Giant covalent" alone does not distinguish diamond from graphite — true of both, so it scores nothing on a comparison. Graphite conducts because each carbon uses only 3 of its 4 outer electrons for bonding, leaving one delocalised electron per atom that is free to move. "Free electrons" without "delocalised", or without "free to move", does not score.
June 2024 Paper 1CR Q11(c)(i): graphite conducts because it has delocalised electrons free to move; "free electrons" without "delocalised" or without "move" did not score.
Dot-and-cross: pairs, outer shell only
Show only the outer-shell electrons and draw each bond as a shared pair (one dot + one cross) in the overlap. Common errors: an extra electron on hydrogen (only 2 fit its shell); a single bond where a double bond is needed — a double bond is 4 electrons, 2 from each atom; and unpaired electrons. A correct diagram shows the electrons in pairs.
June 2024 Paper 1CR Q9(b)(i): a double bond contains 4 electrons, 2 from each atom; an extra electron drawn on hydrogen (only 2 fit its shell) lost the mark.
Weak forces are between molecules
"Weak forces between the bonds" is meaningless and scores zero — the weak forces act between the molecules, so say weak intermolecular forces between molecules. Also compare energies correctly: use "little" or "low" energy, never "less energy". These precise phrases earn the marks on simple-molecular melting-point answers.
November 2024 Paper 1C: "weak forces between the bonds" scored zero; answers needed "weak intermolecular forces between molecules" and "little energy" rather than "less energy".
Structure then bonding then energy
For "explain the property" questions follow a fixed chain: (1) Structure — simple molecular, giant covalent or giant ionic; (2) Bonding — what is overcome on melting; (3) Energy — little or a large amount; (4) Conclusion — low or high melting point.
Distinguish diamond from graphite
On a diamond-vs-graphite comparison, "giant covalent" alone earns nothing — it is true of both. Give two points each: diamond — each carbon bonded to 4 others in a rigid network; graphite — layers held by weak forces that slide, plus a delocalised electron per atom.
Conductivity needs free charge carriers
For conductivity, apply one rule: a substance conducts only if it has charged particles free to move. Covalent substances usually have no free electrons or ions, so they do not conduct. The exception is graphite, where a delocalised electron per carbon is free to move.
Define a covalent bond.
State what is meant by a covalent bond. (2 marks)
