Topic 2 of 6
Melting, conduction and volatility
Use the actual particles and bonding in the elements.
A-Level 9476 (2026-2027)
Melting-point patterns reveal changes of structure
The sharp fall after silicon is a change from a giant network to molecular substances.
| Elements | Structure and melting explanation | Electrical behaviour |
|---|---|---|
| Na, Mg, Al | Metallic. Across these examples, greater positive-ion charge density and more delocalised electrons generally strengthen metallic bonding. Mg and Al have similar melting temperatures; crystal details also matter. | Conduct as solids and liquids through mobile electrons. |
| Si | Giant covalent network; melting requires extensive disruption of strong covalent bonds, giving a very high melting point. | Semiconductor; conductivity is much lower than an ordinary metal and depends strongly on temperature and impurities. |
| White P4, S8, Cl2 | Simple molecular forms. Melting separates molecules against intermolecular attractions, not the covalent bonds inside them. | Poor conductors because they lack mobile charged particles. |
Among the specified molecular forms, S8 has a larger, more polarizable electron cloud than P4 or Cl2 and generally stronger instantaneous dipole-induced dipole attractions. Hence its melting point is higher. State the allotrope: red or black phosphorus cannot be explained by pretending it consists of white-phosphorus P4 molecules.
- Electron clouds become larger
Cl2 → Br2 → I2 contains progressively more electrons and more readily distorted clouds.
- Intermolecular attractions strengthen
Instantaneous dipoles induce larger dipoles in neighbouring molecules.
- Boiling points increase and volatility decreases
At room conditions chlorine is a gas, bromine a liquid and iodine a solid.