📚 Group 17: The Halogens | 第17族：卤素
Group 17 of the Periodic Table contains fluorine, chlorine, bromine, iodine, astatine and tennessine. The first four members are stable enough for laboratory study, while astatine is radioactive and tennessine is synthetic. These elements are called halogens, meaning 'salt formers', because they react readily with metals to produce ionic salts such as sodium chloride.
元素周期表第17族包含氟、氯、溴、碘、砹和鿬。前四种元素足够稳定，可用于实验室研究；砹具有放射性，鿬为人工合成元素。这些元素被称为卤素，意为“成盐者”，因为它们容易与金属反应生成离子盐，例如氯化钠。
1. Introduction to the Halogens | 卤素简介
All halogens have seven valence electrons in their outer shell with the general configuration ns²np⁵. They are one electron short of a noble gas configuration, so they typically gain one electron to form halide ions with a −1 charge. In their elemental state, halogens exist as diatomic molecules such as F₂, Cl₂, Br₂ and I₂.
所有卤素原子的最外层都有7个价电子，其价电子构型通式为 ns²np⁵。它们比稀有气体构型少一个电子，因此通常获得一个电子形成带 −1 电荷的卤离子。在单质状态下，卤素以双原子分子存在，例如 F₂、Cl₂、Br₂ 和 I₂。
The reactivity of halogens is closely linked to their ability to attract an additional electron. Down the group, atomic size increases and the outer electron shell becomes further from the nucleus, which weakens the attraction for an incoming electron. This explains many of the periodic trends discussed below.
卤素的反应性与它们吸引额外电子的能力密切相关。沿族向下，原子半径增大，最外层电子离原子核更远，这削弱了对进入电子的吸引力。这解释了下文讨论的许多周期性变化规律。
2. Appearance and Physical Trends | 外观与物理变化趋势
At room temperature, fluorine is a pale yellow gas, chlorine is a greenish-yellow gas, bromine is a dark red liquid and iodine is a grey-black solid. The colour deepens down the group as the molecules become larger and absorb light at longer wavelengths.
在室温下，氟是淡黄色气体，氯是黄绿色气体，溴是深红色液体，碘是灰黑色固体。沿族向下，颜色逐渐加深，因为分子变大，吸收波长更长的光。
| Halogen | Formula | State at 25°C | Colour |
|---|---|---|---|
| Fluorine | F₂ | Gas | Pale yellow |
| Chlorine | Cl₂ | Gas | Greenish-yellow |
| Bromine | Br₂ | Liquid | Dark red |
| Iodine | I₂ | Solid | Grey-black |
Density also increases down the group. Fluorine and chlorine are gases at room temperature, so their densities are low, while bromine is a liquid and iodine is a solid with a much higher density per unit volume.
密度沿族向下也增大。氟和氯在室温下是气体，因此密度较低；溴是液体，碘是固体，单位体积密度要高得多。
3. Atomic Radius and Ionisation Energy | 原子半径与电离能
Down Group 17, atomic radius increases steadily from fluorine to iodine. Each element has one more occupied electron shell than the one above it. Although nuclear charge also increases, the additional inner shells provide shielding that reduces the effective nuclear attraction on the outer electrons.
沿第17族向下，原子半径从氟到碘稳步增大。每种元素都比上一周期的元素多一个已填满的电子层。虽然核电荷也在增加，但新增的内层电子提供了屏蔽作用，降低了原子核对外层电子的有效吸引力。
Because the outer electrons are further from the nucleus and more shielded, the first ionisation energy decreases down the group. Less energy is needed to remove an electron from a larger halogen atom, even though halogens generally have high ionisation energies compared with metals.
由于外层电子离核更远且屏蔽作用更强，第一电离能沿族向下递减。从较大的卤素原子中移走一个电子所需能量更少，尽管与金属相比，卤素的电离能总体较高。
4. Electronegativity and Electron Affinity | 电负性与电子亲和能
Electronegativity is a measure of the ability of an atom to attract the bonding pair of electrons in a covalent bond. Fluorine is the most electronegative element in the Periodic Table with a Pauling value of 4.0. Down the group, electronegativity decreases: Cl 3.0, Br 2.8, I 2.5.
电负性是衡量原子在共价键中吸引成键电子对能力的物理量。氟是元素周期表中电负性最大的元素，鲍林标度值为4.0。沿族向下，电负性递减：Cl 3.0、Br 2.8、I 2.5。
Electron affinity is the energy change when one mole of gaseous atoms gains one mole of electrons to form gaseous anions. The process is exothermic for all halogens, but the most negative value is usually for chlorine rather than fluorine. Fluorine's unexpectedly less exothermic value is due to strong electron–electron repulsion in its very small 2p orbital.
电子亲和能是指一摩尔气态原子获得一摩尔电子形成气态阴离子时的能量变化。对所有卤素而言，该过程都是放热的，但数值最负的通常是氯而非氟。氟的电子亲和能意外地放热较少，是因为其极小的 2p 轨道中存在强烈的电子–电子排斥作用。
5. Boiling Points and Intermolecular Forces | 沸点与分子间作用力
Halogen molecules are non-polar diatomic molecules, so the only intermolecular forces between them are London dispersion forces. These temporary dipole–induced dipole attractions arise from instantaneous fluctuations in electron distribution.
卤素分子是非极性双原子分子，因此它们之间唯一的分子间作用力是伦敦色散力。这种瞬时偶极–诱导偶极吸引力来源于电子分布的瞬时波动。
Down the group, the number of electrons per molecule increases from F₂ to I₂. More electrons mean stronger and more frequent instantaneous dipoles, so the London forces become stronger. This explains why boiling points increase down the group: F₂ −188°C, Cl₂ −34°C, Br₂ 59°C, I₂ 184°C.
沿族向下，每个分子的电子数从 F₂ 到 I₂ 逐渐增多。电子数越多，瞬时偶极越强、越频繁，因此伦敦色散力越强。这解释了沸点沿族向下升高的原因：F₂ −188°C、Cl₂ −34°C、Br₂ 59°C、I₂ 184°C。
6. Oxidising Ability of Halogens | 卤素的氧化性
A halogen acts as an oxidising agent because it can accept an electron from another species and be reduced to a halide ion. The general half-equation is:
卤素可以作为氧化剂，因为它能从其他物种接受一个电子，本身被还原为卤离子。通用半反应方程式为：
X₂ + 2e⁻ → 2X⁻
The oxidising strength decreases down the group: F₂ > Cl₂ > Br₂ > I₂. This is because a smaller halogen atom has a stronger attraction between the nucleus and the incoming electron, so it is reduced more readily.
氧化性沿族向下依次减弱：F₂ > Cl₂ > Br₂ > I₂。这是因为较小的卤素原子核对进入电子的吸引力更强，因此更容易被还原。
Fluorine is the strongest oxidising agent and can oxidise water, while iodine is much weaker and is easily displaced by chlorine or bromine from iodide solutions. These differences are used in displacement reactions.
氟是最强的氧化剂，甚至能氧化水；碘的氧化性则弱得多，很容易被氯或溴从碘化物溶液中置换出来。这些差异可用于置换反应。
7. Displacement Reactions of Halide Ions | 卤离子的置换反应
A more reactive halogen will displace a less reactive halogen from an aqueous solution of its halide salt. For example, chlorine gas bubbled into sodium bromide solution displaces bromine:
较活泼的卤素能从卤化物盐的水溶液中置换出较不活泼的卤素。例如，将氯气通入溴化钠溶液中会置换出溴：
Cl₂(aq) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq)
The ionic equation shows the redox process more clearly:
离子方程式更清楚地显示了氧化还原过程：
Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
Observations depend on the halogen displaced. Chlorine displaces bromide to give an orange solution, and iodide to give a brown solution. Bromine displaces iodide to give a brown solution, but cannot displace chloride because bromine is a weaker oxidising agent than chlorine.
实验现象取决于被置换出的卤素。氯置换溴离子得到橙色溶液，置换碘离子得到棕色溶液。溴可以置换碘离子得到棕色溶液，但不能置换氯离子，因为溴的氧化性弱于氯。
8. Hydrogen Halides: Preparation and Acidity | 卤化氢：制备与酸性
Hydrogen halides HX are colourless gases at room temperature that fume in moist air because they dissolve readily in water to form acidic solutions. They can be prepared by reacting a halide salt with concentrated sulfuric acid or by direct combination of hydrogen and the halogen.
卤化氢 HX 在室温下是无色气体，在潮湿空气中会发烟，因为它们极易溶于水形成酸性溶液。它们可以通过卤化物盐与浓硫酸反应制备，或由氢与卤素直接化合制得。
When sodium chloride is warmed with concentrated sulfuric acid, hydrogen chloride gas is produced:
氯化钠与浓硫酸共热时，会产生氯化氢气体：
NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g)
The acid strength of hydrogen halides in aqueous solution increases down the group. HF is a weak acid because the H–F bond is very strong and the molecule ionises only partially. HCl, HBr and HI are strong acids, with HI being the strongest due to the weak H–I bond.
卤化氢在水溶液中的酸性沿族向下增强。HF 是弱酸，因为 H–F 键非常强，分子仅部分电离。HCl、HBr 和 HI 是强酸，其中 HI 最强，因为 H–I 键较弱。
9. Reducing Power of Halide Ions | 卤离子的还原性
Halide ions can act as reducing agents by losing an electron to form the halogen molecule:
卤离子可以通过失去一个电子形成卤素分子，从而充当还原剂：
2X⁻ → X₂ + 2e⁻
Reducing power increases down the group: F⁻ < Cl⁻ < Br⁻ < I⁻. Iodide is the strongest reducing agent because its ionic radius is largest, so the outer electron is held least tightly and is lost most easily.
还原性沿族向下增强：F⁻ < Cl⁻ < Br⁻ < I⁻。碘离子是最强的还原剂，因为其离子半径最大，外层电子被束缚得最不牢固，最容易失去。
This trend can be seen in reactions with concentrated sulfuric acid. Chloride ions are not oxidised, and hydrogen chloride gas is evolved. Bromide ions are oxidised to bromine, while iodide ions are oxidised to iodine and can reduce sulfuric acid further to hydrogen sulfide:
这一趋势可从与浓硫酸的反应中观察到。氯离子不被氧化，只生成氯化氢气体；溴离子被氧化为溴；碘离子被氧化为碘，并能将硫酸进一步还原为硫化氢：
H₂SO₄ + 2HBr → SO₂ + Br₂ + 2H₂O
H₂SO₄ + 8HI → H₂S + 4I₂ + 4H₂O
10. Testing for Halide Ions with Silver Nitrate | 用硝酸银检验卤离子
Halide ions in aqueous solution can be identified by adding dilute nitric acid followed by aqueous silver nitrate. The nitric acid removes carbonate and sulfite ions that would otherwise give confusing precipitates. Silver halide precipitates form with characteristic colours:
水溶液中的卤离子可通过加入稀硝酸后再加硝酸银溶液来鉴别。稀硝酸可除去碳酸根和亚硫酸根离子，以免产生干扰性沉淀。卤化银沉淀具有特征颜色：
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) white
Ag⁺(aq) + Br⁻(aq) → AgBr(s) cream
Ag⁺(aq) + I⁻(aq) → AgI(s) pale yellow
The precipitates can be distinguished further by their solubility in ammonia solution. Silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide is insoluble even in concentrated ammonia. This test is a reliable way to identify chloride, bromide and iodide ions.
这些沉淀还可通过它们在氨水中的溶解性进一步区分。氯化银溶于稀氨水，溴化银仅溶于浓氨水，碘化银即使在浓氨水中也不溶。该检验是鉴别氯离子、溴离子和碘离子的可靠方法。
11. Disproportionation Reactions of Chlorine | 氯的歧化反应
Chlorine can undergo disproportionation, a reaction in which the same element is both oxidised and reduced. When chlorine dissolves in water, it forms a mixture of hydrochloric acid and hypochlorous acid:
氯可以发生歧化反应，即同一种元素在反应中既被氧化又被还原。氯溶于水时，会生成盐酸和次氯酸的混合物：
Cl₂(g) + H₂O(l) ⇌ HCl(aq) + HClO(aq)
In this equilibrium, the oxidation state of chlorine changes from 0 in Cl₂ to −1 in HCl and +1 in HClO. Hypochlorous acid is a weak acid and an oxidising agent, which makes chlorine water effective as a disinfectant.
在此平衡中，氯的氧化态从 Cl₂ 中的 0 变为 HCl 中的 −1 和 HClO 中的 +1。次氯酸是一种弱酸和氧化剂，这使氯水成为有效的消毒剂。
With cold dilute sodium hydroxide, chlorine forms sodium chloride and sodium hypochlorite, the active ingredient in household bleach:
氯与冷的稀氢氧化钠溶液反应，生成氯化钠和次氯酸钠，后者是家用漂白剂的有效成分：
Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l)
With hot concentrated sodium hydroxide, further disproportionation occurs and sodium chlorate(V) is formed:
与热的浓氢氧化钠溶液反应时，会发生进一步的歧化反应，生成氯酸钠(V)：
3Cl₂(g) + 6NaOH(aq) → 5NaCl(aq) + NaClO₃(aq) + 3H₂O(l)
12. Industrial Uses and Environmental Notes | 工业用途与环境注意事项
Chlorine is widely used in water purification because it kills bacteria and viruses. It is also essential in the manufacture of polyvinyl chloride (PVC), solvents and bleach. Fluoride compounds are added to toothpaste to prevent tooth decay, while iodine is used in antiseptics and as a dietary supplement for thyroid function.
氯广泛用于水的净化，因为它能杀灭细菌和病毒。氯也是制造聚氯乙烯（PVC）、溶剂和漂白剂的重要原料。氟化物被添加到牙膏中预防蛀牙，碘则用于消毒剂以及作为甲状腺功能所需的膳食补充剂。
However, halogenated organic compounds such as chlorofluorocarbons (CFCs) have caused serious environmental damage, including depletion of the ozone layer. The persistence of halogenated plastics and pesticides also creates long-term waste and pollution problems, so the use of halogens must be carefully managed.
然而，氯氟烃（CFCs）等卤代有机化合物已造成严重的环境破坏，包括臭氧层耗竭。卤代塑料和农药的持久性也带来了长期的废弃物和污染问题，因此必须谨慎管理卤素的使用。
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