O-Level and SEC G3 Chemistry K324
C3: Chemical Bonding and Structure
Connect bonding and structure to formulae, diagrams, physical properties, and expected conductivity.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Bonding questions ask for a link from particles and forces to observable properties. K324 covers ionic, covalent, metallic, simple molecular, macromolecular, and giant covalent structures, so identify the structure first and then explain melting, conductivity, hardness, malleability, or solubility from that structure.
Ionic, covalent and metallic bonding
Ionic bonding forms when electrons transfer from metal atoms to non-metal atoms. The resulting oppositely charged ions usually have noble-gas electron arrangements and are held in a giant lattice by strong electrostatic attractions. Dot-and-cross diagrams must show transferred outer electrons and the correct ion charges.
A covalent bond is a shared pair of electrons between non-metal atoms. Metallic bonding is the attraction between a lattice of positive ions and delocalised electrons. Do not describe a covalent molecule as ions or a metal as neutral atoms loosely surrounded by electrons.
Structure controls physical properties
Ionic compounds have high melting points because many strong attractions must be overcome. They conduct only when molten or aqueous because ions can then move. Simple molecular substances have low melting and boiling points because weak forces act between molecules, even though covalent bonds within each molecule are strong.
Metals conduct through mobile delocalised electrons and are malleable because layers of ions can shift while attraction remains. Giant covalent structures have many strong bonds. Diamond is hard and non-conducting; graphite has layers that slide and delocalised electrons that conduct. Silicon dioxide is a giant covalent network, while poly(ethene) is a macromolecule made of long chains.
Elements, compounds, mixtures and alloys
An element contains one type of atom. A compound contains elements chemically combined in a fixed ratio, while a mixture contains substances not chemically bonded and can have variable composition. Use particle diagrams to distinguish these categories and do not use the word molecule for every substance.
An alloy is a mixture of a metal with another element. Different-sized atoms disrupt regular layers, so the layers do not slide as easily and the alloy may be harder than the pure metal. Brass and stainless steel are examples. Deduce properties from the structure shown rather than memorising one property list for every material.
Formulae and relationships
| Relationship | Use |
|---|---|
| Represent electron loss when a sodium ion forms. | |
| Represent electron gain when a chloride ion forms. |
Worked examples
Example 1: Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.
- Identify sodium chloride as a giant ionic lattice.
- In the solid, ions are fixed in lattice positions and cannot carry charge through the sample.
- When molten, ions are mobile and move to carry charge.
Answer: Only molten sodium chloride conducts because its charged ions are free to move.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Determine whether electron transfer, electron sharing, or metallic bonding fits the substances involved.
- Connect structure and bonding to melting point, conductivity, solubility, and mechanical properties.
- Draw formulae and diagrams with correct charges, ratios, outer electrons, and extent of structure.
Official outcome coverage
K324 C3: 4 mapped outcomes, references C3(a), C3(b), C3(c), C3(d). Check the official K324 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use conductivity, solubility, or melting evidence to compare structures while recognising that one property alone may not identify a substance.
Exam traps and retrieval check
Avoid these traps
- Saying covalent bonds are weak when the weak forces are between simple molecules.
- Using free electrons to explain conduction in molten ionic compounds instead of mobile ions.
- Calling an alloy a compound with a fixed formula.
Check from memory
What holds an ionic lattice together?
Strong electrostatic attraction between oppositely charged ions.
Why can graphite conduct?
It has delocalised electrons that can move through the structure.
Why are alloys often harder than pure metals?
Different-sized atoms disrupt regular layers and make sliding more difficult.
Pure versus Combined scope
Combined Chemistry uses the same top-level topic label but a narrower outcome set. Use the Combined component checklist before removing or adding detail.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K324. Open the related IP explanation.