Describe the Chemistry of the Halogens | 描述卤素的化学性质

📚 Describe the Chemistry of the Halogens | 描述卤素的化学性质

The halogens are the elements of Group 17 (Group VII) of the periodic table: fluorine, chlorine, bromine, iodine, and astatine. This article describes their trends in physical properties, reactivity, and typical reactions, with emphasis on the AQA A-level specification.

卤素是元素周期表第 17 族（ⅦA 族）的元素，包括氟、氯、溴、碘和砹。本文将围绕 AQA A-level 考纲，系统描述它们的物理性质递变、反应活性以及典型化学反应。

1. Electronic Configuration | 电子构型

All halogens have a general outer-shell configuration of ns² np⁵, meaning they require one more electron to achieve a stable noble-gas configuration. This accounts for their high reactivity and strong tendency to form halide ions, X⁻.

所有卤素原子的最外层电子构型均为 ns² np⁵，即只需再获得一个电子就能达到稀有气体的稳定构型。这解释了它们的高反应活性以及易形成卤离子 X⁻ 的强烈倾向。

For example, chlorine has the electronic configuration [Ne] 3s² 3p⁵. The single unpaired electron is readily gained in redox reactions, allowing a full octet to be achieved.

例如，氯的电子构型为 [Ne] 3s² 3p⁵。氧化还原反应中容易获得一个电子，从而形成完整的八隅体。

2. Physical Properties | 物理性质

Down the group, the halogens show a regular progression in colour and state at room temperature. Fluorine is a pale yellow gas, chlorine is a green-yellow gas, bromine is a reddish-brown volatile liquid, and iodine is a grey-black crystalline solid that sublimes to a purple vapour.

在卤素单质中，从氟到碘，颜色和常温状态呈现规律性变化：氟为淡黄色气体，氯为黄绿色气体，溴为红棕色易挥发液体，碘为紫黑色晶体，受热升华产生紫色蒸气。

Melting and boiling points increase down the group.

熔点和沸点随原子序数增大而升高。
The increase is due to stronger London dispersion forces between larger molecules.

这是因为分子体积增大，伦敦色散力增强。
The halogens are non-polar and are soluble in non-polar solvents; iodine is slightly soluble in water but readily dissolves in cyclohexane.

卤素单质是非极性分子，易溶于非极性溶剂；碘在水中溶解度很小，但在环己烷中溶解度很大。

3. Atomic and Ionic Radii | 原子半径与离子半径

Atomic radius increases down the group because each successive element has an additional electron shell, making the outer electrons further from the nucleus.

原子半径自上而下增大，因为每个新元素都比前一个多一个电子层，最外层电子离核更远。

Ionic radius also increases down the group, as the X⁻ ions have the same charge but larger electron clouds. For example, the ionic radius of F⁻ is 133 pm, while that of Cl⁻ is 181 pm.

离子半径同样自上而下增大，因为 X⁻ 离子带有相同电荷，但电子云更大。例如，F⁻ 的离子半径为 133 pm，而 Cl⁻ 为 181 pm。

4. First Ionisation Energy | 第一电离能

First ionisation energy decreases down the group. Although the nuclear charge increases, the additional shielding from inner electrons and the greater atomic radius outweigh the increased nuclear attraction.

卤素的第一电离能自上而下逐渐降低。尽管核电荷增大，但内层电子的屏蔽效应增强以及原子半径增大，抵消了核吸引的影响。

The trend is consistent with the halogen atoms being successively less able to attract an additional electron into their outer shell, though their electron affinity is nonetheless negative.

这一趋势与卤素原子吸引额外电子的能力逐渐减弱一致，但其电子亲和能仍然为负值。

5. Electronegativity | 电负性

Electronegativity decreases down the group. Fluorine is the most electronegative element, with a Pauling value of 3.98. Chlorine is 3.16, bromine 2.96, and iodine 2.66.

电负性自上而下减小。氟是电负性最大的元素，鲍林标度为 3.98；氯为 3.16，溴为 2.96，碘为 2.66。

This decrease explains why the oxidising power of the halogens also decreases down the group: the more electronegative an element, the more readily it gains an electron from another species.

这种下降趋势解释了卤素氧化能力自上而下减弱的原因：电负性越强，越容易从其他物质中夺取电子。

6. Oxidising Ability | 氧化能力

Each halogen is an oxidising agent and is itself reduced to the X⁻ ion. The ability to act as an oxidising agent decreases down the group because the tendency to gain an electron becomes less favourable as atomic radius increases.

每种卤素都是氧化剂，自身被还原为 X⁻ 离子。由于原子半径增大，获得电子的趋势减弱，因此卤素的氧化能力自上而下降低。

The standard electrode potentials illustrate this:

标准电极电势可以说明这一点：

F₂ + 2e⁻ ⇌ 2F⁻ E° = +2.87 V
Cl₂ + 2e⁻ ⇌ 2Cl⁻ E° = +1.36 V
Br₂ + 2e⁻ ⇌ 2Br⁻ E° = +1.09 V
I₂ + 2e⁻ ⇌ 2I⁻ E° = +0.54 V

Therefore, fluorine is the strongest oxidising agent, and iodine is the weakest.

因此，氟是最强的氧化剂，碘最弱。

7. Reaction with Hydrogen | 与氢气的反应

All halogens combine with hydrogen to form hydrogen halides, HX. The vigour of the reaction decreases down the group: fluorine reacts explosively in the dark, chlorine reacts rapidly in bright sunlight, bromine reacts slowly when heated, and iodine reacts reversibly and incompletely.

所有卤素都能与氢化合生成卤化氢 HX。反应的剧烈程度自上而下减弱：氟在暗处即可发生爆炸性反应，氯在强光下快速反应，溴在加热时缓慢反应，碘则发生可逆的不完全反应。

Hydrogen halides dissolve in water to form acidic solutions. The acid strength of HX in aqueous solution increases down the group because H–X bond enthalpy decreases, making the bond easier to break.

卤化氢溶于水形成酸性溶液。在水中，HX 的酸性强度自上而下增强，因为 H–X 键焓减小，键更容易断裂。

8. Reaction with Water | 与水的反应

Chlorine, bromine, and iodine undergo disproportionation reactions with water, forming the corresponding hydrohalic acid and hypohalous acid. Fluorine reacts differently because it oxidises water to oxygen.

氯、溴、碘与水发生歧化反应，生成相应的氢卤酸和次卤酸。氟的反应则不同，它会将水氧化为氧气。

The general equation for chlorine is:

氯的典型方程式为：

Cl₂ + H₂O ⇌ HCl + HOCl

Chlorine is used in water treatment because the HOCl produced is a weak acid that can dissociate to give OCl⁻, both of which are effective disinfectants.

氯用于水处理，因为生成的 HOCl 是弱酸，可解离产生 OCl⁻，二者都是有效的消毒剂。

9. Displacement Reactions | 置换反应

In aqueous solution, a more reactive halogen will displace a less reactive halogen from its halide salt. This is a redox reaction in which the more reactive halogen is reduced and the halide ion is oxidised.

在水溶液中，较活泼的卤素可将较不活泼的卤素从其卤化物盐中置换出来。这是一个氧化还原反应：活泼卤素被还原，卤离子被氧化。

For example, chlorine water reacts with potassium bromide:

例如，氯水与溴化钾反应：

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

If a clear solution of potassium iodide is added to chlorine water, a brown colour appears due to the formation of iodine. In the presence of an organic solvent, a purple layer confirms the presence of iodine.

若将碘化钾溶液加入氯水中，因生成碘而出现棕色；若有有机溶剂，则出现紫色层，可确证碘的存在。

10. Reactions of Halide Ions with Silver Nitrate | 卤离子与硝酸银的反应

Silver nitrate solution is used to identify halide ions in aqueous solution. The precipitate is a silver halide, and the colour helps to distinguish the halide.

硝酸银溶液可用于鉴别溶液中的卤离子。生成的银盐沉淀颜色各异，可用于区分卤离子。

Ag⁺ + Cl⁻ → AgCl, a white precipitate, soluble in dilute ammonia.

Ag⁺ + Cl⁻ → AgCl，白色沉淀，溶于稀氨水。
Ag⁺ + Br⁻ → AgBr, a cream precipitate, soluble in concentrated ammonia.

Ag⁺ + Br⁻ → AgBr，淡黄色（乳白色）沉淀，溶于浓氨水。
Ag⁺ + I⁻ → AgI, a yellow precipitate, insoluble in ammonia.

Ag⁺ + I⁻ → AgI，黄色沉淀，不溶于氨水。

This test is commonly forced in AQA exam questions and illustrates the decreasing solubility of silver halides down the group.

该实验是 AQA 考试中的常见考点，也体现了卤化银溶解度自上而下递减的规律。

11. Industrial Production and Uses | 工业制备与用途

Chlorine is manufactured on an industrial scale by the electrolysis of concentrated sodium chloride brine. The half-equations at the electrodes are:

氯气在工业上通过电解饱和食盐水制备。电极反应为：

Anode: 2Cl⁻ → Cl₂ + 2e⁻
Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻

Fluorine is produced by electrolysing molten potassium fluoride in hydrogen fluoride. Bromine is obtained by oxidising bromide ions in sea water with chlorine, and iodine is extracted from seaweed or brine.

氟通过电解熔融的氟氢化钾（在氟化氢介质中）制得。溴由海水中的溴离子经氯气氧化得到，碘可从海藻或盐卤中提取。

Halogens and their compounds are widely used: chlorine for water purification and PVC production, iodine as an antiseptic, and bromine compounds as flame retardants and in silver bromide photography.

卤素及其化合物用途广泛：氯用于水净化和生产聚氯乙烯（PVC），碘用作消毒剂，溴化合物用作阻燃剂，溴化银用于摄影。

12. Summary | 总结

In summary, the halogens show smooth trends down the group: increasing atomic radius and melting point, decreasing ionisation energy, electronegativity, and oxidising power. Their reactions with hydrogen, water, and halide ions all reflect this decreasing reactivity.

总之，卤素在元素周期表中呈现规律性递变：原子半径和熔点增大，电离能、电负性和氧化能力降低。它们与氢气、水和卤离子的反应都体现了这种反应活性的下降趋势。

Mastery of these trends and equations is essential for success in AQA A-level chemistry questions about the halogens.

掌握这些递变规律和反应方程式，是应对 AQA A-level 化学有关卤素考题的关键。

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Describe the Chemistry of the Halogens | 描述卤素的化学性质