Lesson 2 of 6
Melting, conduction and volatility
Use the actual particles and bonding in the elements.
A-Level 9476 (2026-2027)
I can interpret melting and conductivity across the period.Syllabus
Syllabus 9476, 5(c). Interpret melting and conductivity across the period.
I can explain halogen volatility.Syllabus
Syllabus 9476, 5(d). Explain halogen volatility.
Melting-point patterns reveal changes of structure
The sharp fall after silicon is a change from a giant network to molecular substances.
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Silicon and phosphorus are neighbours in Period 3. How do their melting points compare?
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Silicon is far higher, as it is giant covalent
Silicon is a giant covalent network, so melting breaks strong covalent bonds. Phosphorus is simple molecular, P4, held by weak dispersion forces.
| 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.
Watch out for this
A student says: "Iodine is less volatile than chlorine because the I-I bond is stronger than the Cl-Cl bond." What is wrong with this?
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Do not explain iodine's lower volatility by a stronger I-I covalent bond. Vaporisation preserves I2 molecules; the attractions overcome are between molecules.
Exam-style question
Silicon melts at 1410 °C but white phosphorus melts at 44 °C. Explain this difference in terms of structure and bonding. [3]
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Check your understanding
Why is chlorine a gas at room temperature but iodine a solid?
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I2 has more electrons, so stronger instantaneous dipole attractions between molecules.
A larger, more polarisable electron cloud gives stronger forces between I2 molecules, so more energy is needed to separate them. The I-I bond does not break when iodine melts.