Linking properties to structure and bonding
Every GCSE chemistry exam asks you to predict properties from structure or deduce structure from properties. This topic ties together everything from C2.
The four structural types — at a glance
| Structure | m.p./b.p. | Conducts (solid) | Conducts (molten/aq) | Soluble in water? | Examples |
|---|---|---|---|---|---|
| Simple molecular | Low | No | No | Sometimes | H₂O, O₂, CO₂, I₂ |
| Giant ionic | High | No | Yes | Often | NaCl, MgO |
| Giant covalent | Very high | No (except graphite) | No | No | Diamond, graphite, SiO₂ |
| Metallic | High (mostly) | Yes | Yes | No | Cu, Fe, Na |
Simple molecular substances
Built from a fixed number of atoms covalently bonded into a discrete molecule (e.g. CO₂, H₂O).
- Low m.p./b.p. — only weak intermolecular forces between molecules need to be overcome. The covalent bonds inside molecules are strong but unbroken on melting.
- No conduction — molecules are neutral; no free charged particles.
- Soluble in water if molecule can H-bond (e.g. sugar, ammonia); insoluble otherwise (e.g. methane).
Polymers
Very long covalent chain molecules. Solid at room temperature with higher m.p. than small molecules of similar atoms because chains are long and intermolecular forces along their length sum to a much larger total.
Giant ionic structures
Lattice of alternating positive and negative ions held by strong electrostatic attractions in all directions.
- High m.p./b.p. — lots of energy needed to break many strong ionic bonds.
- Solid: don't conduct — ions are fixed in place.
- Molten or in solution: conduct — ions are free to move and carry charge.
- Often soluble in water (ions become surrounded by water molecules).
Giant covalent structures
Networks of atoms covalently bonded in continuous 3D structures. Examples: diamond, graphite, silicon dioxide.
- Very high m.p. — many strong covalent bonds throughout.
- Don't conduct (except graphite, which has delocalised electrons in layers).
- Insoluble in water.
Metallic structures
Lattice of positive ions in a sea of delocalised electrons.
- High m.p./b.p. (mostly).
- Conduct in solid and liquid states — delocalised electrons free to move.
- Malleable, ductile — layers slide over each other.
- Insoluble in water (typically).
How to deduce structure from properties
Use a flow chart approach:
- Conducts when solid? → If yes, metal.
- High m.p. and conducts only when molten/aqueous? → giant ionic.
- Very high m.p. and doesn't conduct? → giant covalent.
- Low m.p./b.p.? → simple molecular.
- Solid at room temp but flexible chains? → polymer.
✦Worked example
A substance has m.p. 801 °C, conducts when molten but not when solid, dissolves in water.
- High m.p. — eliminates simple molecular.
- Conducts when molten only — eliminates metal (which conducts solid).
- → giant ionic (it is sodium chloride).
⚠Common mistakes
- Saying simple molecules have weak bonds. It's the intermolecular forces that are weak, not the covalent bonds within the molecule.
- Saying ionic compounds always dissolve. Many do, but some (e.g. silver chloride) do not.
- Forgetting graphite is the giant covalent exception that conducts.
- Confusing "atoms" with "ions" in metals. Metals are positive ions in a sea of electrons.
Links
Pulls together C2.2 (ionic), C2.3 (covalent), C2.4 (metallic). Property reasoning is heavily tested in 6-mark questions.
AI-generated · claude-opus-4-7 · v3-deep-chemistry