Covalent bond — one shared pair of electrons
A covalent bond forms when a pair of electrons is shared between two atoms, each atom putting one electron into that pair. Sharing lets both atoms reach a full outer shell — a noble gas electronic configuration — with no electron transferred. It occurs between non-metal atoms only (the right of the Periodic Table, hydrogen included). Core molecules to know: H2, Cl2, H2O, CH4, NH3 and HCl.
(Extended) Bond order — 1, 2 or 3 shared pairs
The number of shared pairs between the same two atoms sets the bond order: 1 pair = a single bond (H2, HCl), 2 pairs = a double bond (O2, the C=C in C2H4, and each C=O in CO2), 3 pairs = a triple bond (N2). More pairs are needed whenever one pair would leave an atom short of a full outer shell. The Extended molecules are CH3OH, C2H4, O2, CO2 and N2.
Simple molecular substances — two properties
Simple molecular compounds are small, separate molecules. (a) Low melting and boiling points: melting or boiling only overcomes the weak forces of attraction between molecules, not the strong covalent bonds within them, so little energy is needed. (b) Poor electrical conductivity as solid, liquid or dissolved: molecules are electrically neutral overall, so there are no free ions and no delocalised electrons to carry charge.
Drawn from real examiner reports.
(Extended) Covalent bonds are not weak
"The covalent bonds are weak so it melts easily" is factually wrong and scores nothing — covalent bonds are strong. Two different attractions exist: strong covalent bonds within each molecule, which stay intact, and weak intermolecular forces between separate molecules, which are the only thing broken on melting or boiling.
Nov 2022 P41 Q5bii — bonds within molecules are not broken on boiling; Jun 2023 P42 Q8e — candidates must distinguish covalent bonds from intermolecular forces.
"Sharing electrons" is not the definition
Answers like "atoms share" or "the atoms are joined together" score zero because they never say what is shared. The mark scheme needs both halves: a pair of electrons (not just "electrons") shared between two atoms, and the result — each atom gains a full outer shell / noble gas configuration.
H2, O2, N2 and Cl2 are diatomic
These elements exist as two-atom molecules, not lone atoms. Candidates write "N" or "2N" for nitrogen gas, or draw a single free atom where a molecule was asked for. Always write the subscript 2 in the formula, and draw both atoms in a dot-and-cross diagram — the electron count only works out when both are present.
Jun 2022 P16 Q16, P42 Q8aii; Nov 2023 P11 Q15 — O2 and N2 are diatomic (candidates wrote N or 2N for the product of NO reduction).
(Extended) One shared pair is not always enough
Drawing a single bond where a double or triple is needed leaves atoms short of an octet. O2 needs 2 shared pairs, N2 needs 3, and CO2 needs two C=O double bonds so that carbon and both oxygens reach 8 outer electrons. With only one shared pair each oxygen in O2 has 7 and each nitrogen in N2 has 6 — the diagram is chemically wrong however neat it looks.
Jun 2023 P42 Q8d — N2 has a triple bond and CO2 needs full octets.
Sharing (covalent) ≠ transfer (ionic)
Covalent bonding shares a pair of electrons between two non-metals. Ionic bonding (C2.4) transfers electrons between a metal and a non-metal, giving oppositely charged ions. A halogen bonded to a metal — sodium chloride — is ionic, even though the halogen itself forms a covalent diatomic molecule such as Cl2.
Nov 2023 P32 Q2a — covalent bonds form between non-metals; halogens form ionic compounds with metals.
CH3OH hides a fourth hydrogen
Methanol is written CH3OH, but that is CH4O — 1 carbon, 4 hydrogens (3 on the carbon, 1 on the oxygen) and 1 oxygen, giving Mr = 32. Counting only the 3 hydrogens of the CH3 group is the standard slip. The same care applies to the bond count: methanol has 5 covalent bonds, not 4.
Outer-shell electrons only
Take each atom's outer-shell electron count from its group number, then draw nothing else — inner shells never appear in a dot-and-cross diagram. Use dots for one atom's electrons and crosses for the other's, consistently, so the origin of every electron is visible.
Shared pairs go in the overlap
Draw each shared pair inside the overlap of the two atoms' circles — one dot plus one cross. A pair drawn outside the overlap reads as a lone pair. Lone (non-bonding) pairs sit outside, on the correct atom: oxygen in H2O has 2 shared pairs and 2 lone pairs.
Finish by counting to 8 (2 for H)
Before moving on, count the outer electrons around every atom: 8 for everything in this syllabus, 2 for hydrogen. An atom left showing 6 or 7 means a bond or a lone pair is missing — that is the check that catches a missed double or triple bond.
Describe (Core) vs explain (Extended)
"Describe" the properties means state the fact — low melting point, does not conduct. "Explain" (Extended) means give the structure-and-bonding chain: strong covalent bonds within molecules, weak forces between them, so little energy is needed. Answer the command word asked.
Cambridge 0654 spec reference: Section C2 "Atoms, elements and compounds", sub-topic C2.5. Core covers the definition of a covalent bond, dot-and-cross diagrams for , , , , and , and describing the properties of simple molecular compounds. Extended (Supplement) adds , , , and , plus explaining those properties in terms of structure and bonding.
| Term | Mark-scheme-precise meaning |
|---|---|
| Covalent bond | A shared pair of electrons between two atoms, giving each atom a full outer shell (noble gas electronic configuration) |
| Simple molecular compound | A substance made of small, individual, covalently bonded molecules held together (between molecules) only by weak forces of attraction |
| Intermolecular force | A weak force of attraction between separate molecules (the specific type is not required in this syllabus) -- distinct from the strong covalent bonds within a molecule |
Molecules to know (dot-and-cross diagrams -- outer-shell electrons only):
| Tier | Molecule | Bonding detail |
|---|---|---|
| Core | 1 shared pair; each H has a full shell of 2 | |
| Core | 1 shared pair; each Cl has a full outer shell of 8 (plus 3 lone pairs each) | |
| Core | 2 shared pairs (O--H ); O has 2 lone pairs | |
| Core | 4 shared pairs (C--H ); C has a full outer shell of 8, no lone pairs | |
| Core | 3 shared pairs (N--H ); N has 1 lone pair | |
| Core | 1 shared pair; H has 2, Cl has 8 (3 lone pairs) | |
| Extended | C--H , C--O single bond, O--H single bond; O has 2 lone pairs | |
| Extended | C=C double bond (2 shared pairs) + 4 C--H single bonds | |
| Extended | O=O double bond (2 shared pairs); each O has 2 lone pairs | |
| Extended | 2 C=O double bonds (2 shared pairs each); C has no lone pairs, each O has 2 | |
| Extended | NN triple bond (3 shared pairs); each N has 1 lone pair |
Properties of simple molecular compounds:
(Extended) The full explanation of (a) requires the structure-and-bonding reasoning: strong covalent bonds hold atoms together within a molecule (these do not break on melting/boiling); only the weak intermolecular forces between molecules break, needing little energy -- hence low melting and boiling points compared with ionic or giant covalent substances.
Define a covalent bond.
Hydrogen chloride, , is a simple molecular compound formed by one covalent bond between hydrogen and chlorine. Calculate its relative formula mass, . (: H = 1, Cl = 35.5) (2 marks)
