📚 Period 3 Chlorides: Structure, Bonding and Reactions | 第三周期氯化物：结构、键合与反应
Period 3 chlorides are a favourite topic in Cambridge A-Level Chemistry because they reveal how bonding changes from ionic to covalent as you move from sodium to sulfur. In this article, we will compare the formulae, structures, melting points and reactions with water of the chlorides formed by Period 3 elements.
第三周期氯化物是剑桥 A-Level 化学中的经典考点，因为它们展示了从钠到硫，化学键如何从离子键逐渐转变为共价键。本文将比较第三周期元素所形成的氯化物的化学式、结构、熔点以及与水的反应。
1. Overview and the chlorides you need to know | 概述及你需要掌握的氯化物
The Period 3 chlorides you are expected to know are NaCl, MgCl₂, AlCl₃ (or Al₂Cl₆), SiCl₄, PCl₃, PCl₅ and S₂Cl₂. Their formulae already hint at the change in bonding: metals form chlorides with ionic formulae, while non-metals form simple molecular chlorides.
你需要掌握的第三周期氯化物包括 NaCl、MgCl₂、AlCl₃（或 Al₂Cl₆）、SiCl₄、PCl₃、PCl₅ 和 S₂Cl₂。它们的化学式已经暗示了键合的变化：金属形成离子型氯化物，而非金属形成简单分子型氯化物。
| Chloride | Formula | Structure type | Bonding | pH of aqueous solution |
|---|---|---|---|---|
| Sodium chloride | NaCl | Giant ionic lattice | Ionic | ≈ 7 |
| Magnesium chloride | MgCl₂ | Giant ionic lattice | Ionic with some covalent character | ≈ 6 |
| Aluminium chloride | AlCl₃ / Al₂Cl₆ | Covalent dimer | Covalent with dative bonds | ≈ 2-3 |
| Silicon tetrachloride | SiCl₄ | Simple molecular | Covalent | Strongly acidic |
| Phosphorus trichloride | PCl₃ | Simple molecular | Covalent | Strongly acidic |
| Phosphorus pentachloride | PCl₅ | Ionic in solid state | Covalent / ionic | Strongly acidic |
| Disulfur dichloride | S₂Cl₂ | Simple molecular | Covalent | Acidic |
The table summarises the key chlorides you must be able to compare. Notice how the structure changes from giant ionic to simple covalent as you cross the period.
上表总结了你必须能够比较的关键氯化物。注意沿周期从左到右，结构如何从巨型离子变为简单共价。
2. Sodium chloride: giant ionic lattice | 氯化钠：巨型离子晶格
Sodium chloride, NaCl, is a giant ionic lattice held together by strong electrostatic attractions between Na⁺ and Cl⁻ ions. Each Na⁺ ion is surrounded by six Cl⁻ ions and each Cl⁻ by six Na⁺ ions, giving a 6:6 coordination.
氯化钠（NaCl）是一种巨型离子晶格，由 Na⁺ 和 Cl⁻ 之间的强静电引力结合在一起。每个 Na⁺ 周围有 6 个 Cl⁻，每个 Cl⁻ 周围有 6 个 Na⁺，配位数为 6:6。
NaCl has a high melting point of about 801 °C because a large amount of energy is needed to break the strong ionic bonds throughout the lattice. It does not conduct electricity when solid, but conducts when molten or dissolved in water because the ions become mobile.
NaCl 熔点很高，约为 801 °C，因为破坏整个晶格中强大的离子键需要大量能量。固态时不导电，但熔融或溶于水后可以导电，因为离子可以自由移动。
NaCl dissolves in water to give a neutral solution with pH ≈ 7. The ions are simply hydrated: NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq). Neither ion hydrolyses to a significant extent.
NaCl 溶于水形成中性溶液，pH ≈ 7。离子只是被水合：NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq)。两种离子都几乎不发生水解。
3. Magnesium chloride: ionic with some covalent character | 氯化镁：离子型但带有部分共价性
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