Group 17: The Halogens | 第17族：卤素

📚 Group 17: The Halogens | 第17族：卤素

The Group 17 elements – fluorine, chlorine, bromine, iodine and astatine – are known as the halogens. Their name comes from the Greek for ‘salt former’, reflecting their ability to react with metals to produce salts. The chemistry of the halogens is dominated by their high electronegativity, strong oxidising power and the clear trends in physical and chemical properties down the group. Understanding these trends and the characteristic reactions of the halogens and their compounds is essential for success in Cambridge A Level Chemistry.

第17族元素——氟、氯、溴、碘和砹——被称为卤素。它们的名称源于希腊语“成盐者”，体现了它们与金属反应生成盐的能力。卤素的化学性质主要由其高电负性、强氧化性以及沿族向下的物理和化学性质递变所主导。理解这些递变规律以及卤素及其化合物的特征反应对于在剑桥A Level化学考试中取得成功至关重要。

1. Introduction to Group 17 | 第17族简介

Group 17 elements each have seven electrons in their outermost shell, giving an electronic configuration of ns²np⁵. They exist as diatomic molecules (F₂, Cl₂, Br₂, I₂) under standard conditions and require just one more electron to achieve a stable noble gas configuration. This makes them highly reactive non‑metals, with reactivity decreasing as the group is descended.

第17族元素的最外层都有7个电子，电子排布为ns²np⁵。它们在标准条件下以双原子分子（F₂、Cl₂、Br₂、I₂）形式存在，只需再获得一个电子便可达到稳定的稀有气体构型。这使它们成为高度活泼的非金属，活泼性沿族向下递减。

2. Physical Properties of the Halogens | 卤素的物理性质

The physical appearance of the halogens changes dramatically down the group. Fluorine is a pale yellow gas, chlorine is a greenish‑yellow gas, bromine is a red‑brown liquid and iodine is a grey‑black solid that sublimes to a purple vapour. Astatine is a black solid, but it is radioactive and rarely studied in the laboratory.

卤素的物理外观沿族向下发生显著变化。氟是淡黄色气体，氯是黄绿色气体，溴是红棕色液体，碘是灰黑色固体，升华为紫色蒸气。砹是黑色固体，但具有放射性，在实验室中很少研究。

Boiling points and melting points increase down the group. This is because the size of the diatomic molecules increases, leading to more electrons and stronger instantaneous dipole–induced dipole (London) forces between molecules. The trend is clearly seen in the table below.

沸点和熔点沿族向下递增。这是因为双原子分子体积增大，电子数增多，分子间的瞬时偶极–诱导偶极（伦敦力）作用增强。下表中的数据清晰展示了这一趋势。

Halogen	Formula	Colour	State (r.t.)	Melting point / °C	Boiling point / °C
Fluorine	F₂	pale yellow	gas	−220	−188
Chlorine	Cl₂	greenish‑yellow	gas	−101	−34
Bromine	Br₂	red‑brown	liquid	−7	+59
Iodine	I₂	grey‑black	solid	+114	+184

3. Trends in Electronegativity and Electron Affinity | 电负性与电子亲和势的变化趋势

Electronegativity decreases down Group 17. Fluorine is the most electronegative element (Pauling scale 4.0), while iodine is much lower (2.5). This decrease occurs because atomic radius increases and shielding by inner electron shells reduces the attraction of the nucleus for a bonding pair of electrons.

电负性沿第17族向下递减。氟是电负性最强的元素（鲍林标度4.0），而碘则低得多（2.5）。这种下降是因为原子半径增大，内层电子屏蔽作用减弱了原子核对键合电子对的吸引力。

The magnitude of electron affinity (the energy released when an electron is added to a gaseous atom) is generally high for halogens, but the trend is not perfectly smooth. Chlorine actually has a slightly more exothermic electron affinity than fluorine because the very small fluorine atom experiences significant electron‑electron repulsion when an extra electron is added. Down the group, electron attachment becomes less exothermic as the incoming electron is added to a larger orbital further from the nucleus.

卤素的电子亲和势（气态原子获得一个电子时释放的能量）通常很大，但递变并不完全规则。氯的电子亲和势实际比氟的略放热，因为氟原子非常小，加入额外电子时电子间排斥显著。沿族向下，电子附着放热减少，因为外来电子填入距核更远、更大的轨道。

4. Oxidising Power of the Halogens | 卤素的氧化能力

The halogens are strong oxidising agents because they readily gain one electron to form a halide ion (X⁻). The oxidising strength decreases down the group: F₂ > Cl₂ > Br₂ > I₂. This is explained by the decreasing ability of the atom to attract an extra electron as atomic size increases and nuclear attraction is weakened by shielding.

卤素是强氧化剂，因为它们容易获得一个电子形成卤离子（X⁻）。氧化能力沿族向下递减：F₂ > Cl₂ > Br₂ > I₂。这是因为随着原子体积增大、屏蔽效应削弱核吸引力，原子吸引额外电子的能力下降。

Standard electrode potentials confirm this trend:

F₂ + 2e⁻ → 2F⁻ E° = +2.87 V
Cl₂ + 2e⁻ → 2Cl⁻ E° = +1.36 V
Br₂ + 2e⁻ → 2Br⁻ E° = +1.07 V
I₂ + 2e⁻ → 2I⁻ E° = +0.54 V

标准电极电势证实了这一趋势：

F₂ + 2e⁻ → 2F⁻ E° = +2.87 V
Cl₂ + 2e⁻ → 2Cl⁻ E° = +1.36 V
Br₂ + 2e⁻ → 2Br⁻ E° = +1.07 V
I₂ + 2e⁻ → 2I⁻ E° = +0.54 V

The more positive the E° value, the stronger the oxidising agent. Fluorine is therefore the most powerful oxidiser, capable of oxidising even water to oxygen.

E°值越正，氧化能力越强。因此氟是最强的氧化剂，甚至能将水氧化成氧气。

5. Displacement Reactions of Halogens | 卤素的置换反应

A more reactive halogen can displace a less reactive halogen from an aqueous solution of its halide salt. For example, chlorine water (pale green) added to potassium bromide solution (colourless) gives a red‑brown colour due to the formation of bromine:

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

更活泼的卤素可以将较不活泼的卤素从其卤化物水溶液中置换出来。例如，将氯水（淡绿色）加入溴化钾溶液（无色）中，因生成溴而呈红棕色：

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

Similarly, chlorine displaces iodine (giving a brown solution that turns blue‑black with starch) and bromine displaces iodine. No displacement occurs if a weaker oxidising agent is added to a halide of a stronger oxidiser – for example, iodine cannot displace chlorine or bromine. Observations can be summarised as follows:

类似地，氯置换碘（得到棕色溶液，遇淀粉变蓝黑色），溴也能置换碘。若将较弱氧化剂加到较强氧化剂的卤化物中，则不会发生置换——例如，碘不能置换氯或溴。观察结果总结如下：

Halogen added	KCl	KBr	KI
Cl₂	no reaction	red‑brown Br₂	brown I₂
Br₂	no reaction	—	brown I₂
I₂	no reaction	no reaction	—

6. Reactions of Halogens with Other Elements | 卤素与其他元素的反应

Halogens react directly with most metals to form ionic halides. For instance, warm iron wool burns vigorously in chlorine gas to produce iron(III) chloride (FeCl₃). With hydrogen, halogens form hydrogen halides. The reaction becomes less vigorous down the group: fluorine reacts explosively in the dark, chlorine reacts slowly in the dark but explosively in sunlight, bromine requires heating, and iodine reacts only partially in a reversible process.

卤素能与大多数金属直接反应生成离子型卤化物。如温热的铁丝在氯气中剧烈燃烧生成氯化铁（FeCl₃）。卤素与氢气反应生成卤化氢，反应剧烈程度沿族向下减弱：氟在黑暗中爆炸性化合，氯在暗处缓慢但在光照下爆炸，溴需加热，碘只发生部分可逆反应。

H₂ + X₂ → 2HX (X = F, Cl, Br, I)

With water, fluorine oxidises water vigorously, whereas chlorine undergoes disproportionation to form hydrochloric acid and chloric(I) acid (HOCl). Bromine behaves similarly but less readily, and iodine dissolves only slightly.

对于水，氟剧烈氧化水，而氯发生歧化反应生成盐酸和次氯酸（HOCl）。溴反应类似但较不活泼，碘仅微溶。

2F₂ + 2H₂O → 4HF + O₂

Cl₂ + H₂O ⇌ HCl + HOCl

7. Hydrogen Halides: Preparation, Trends in Bond Strength and Acidity | 卤化氢：制备、键强与酸性趋势

Hydrogen halides (HX) can be prepared by reacting a metal halide with concentrated phosphoric acid or, for HCl, with concentrated sulfuric acid. Direct synthesis from the elements is also possible but becomes increasingly difficult for HBr and HI due to their thermal decomposition.

卤化氢（HX）可通过金属卤化物与浓磷酸反应制得，对于HCl也可用浓硫酸。直接由单质合成亦可，但HBr和HI因热分解而愈加困难。

The H–X bond strength decreases down the group, as shown by bond enthalpies: H–F 568, H–Cl 432, H–Br 366, H–I 298 kJ mol⁻¹. This trend explains the acidic strength of the hydrogen halides in water. HF is a weak acid because its bond is very strong and the proton is not fully donated; the acid dissociation constant K_a increases from HF to HI, making HI a strong acid.

H–X键能沿族向下递减，键焓数据为：H–F 568，H–Cl 432，H–Br 366，H–I 298 kJ mol⁻¹。这一趋势解释了卤化氢在水中的酸性强度。HF是弱酸，因为其键非常牢固，质子不能完全给出；从HF到HI，酸解离常数K_a增大，HI为强酸。

The boiling points of hydrogen halides generally increase from HCl to HI due to stronger London forces. HF shows an anomalously high boiling point because of extensive hydrogen bonding between molecules.

卤化氢的沸点从HCl到HI总体上因伦敦力增强而升高。HF由于分子间强烈的氢键作用表现出反常高的沸点。

8. Reactions of Halide Ions with Concentrated Sulfuric Acid | 卤离子与浓硫酸的反应

Concentrated sulfuric acid reacts differently with solid sodium chloride, sodium bromide and sodium iodide because the halide ions differ in their reducing power, which increases down the group (F⁻ is not a reducing agent, I⁻ is a strong reducing agent).

浓硫酸与固体氯化钠、溴化钠和碘化钠的反应各不相同，因为卤离子的还原能力沿族向下递增（F⁻非还原剂，I⁻为强还原剂）。

With NaCl, an acid–base reaction occurs, producing hydrogen chloride gas (visible as misty fumes):

与NaCl发生酸碱反应，生成氯化氢气体（可见雾状白烟）：

NaCl + H₂SO₄ → NaHSO₄ + HCl

With NaBr, HBr is first formed, but part of it is oxidised by sulfuric acid to bromine (red‑brown fumes) and sulfur dioxide:

与NaBr先形成HBr，但部分HBr被硫酸氧化为溴（红棕色烟雾）和二氧化硫：

NaBr + H₂SO₄ → NaHSO₄ + HBr

2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

With NaI, the HI produced is such a powerful reducing agent that it reduces the sulfur in sulfuric acid all the way from +6 to −2, giving a complex mixture of products: purple iodine vapour, hydrogen sulfide (rotten egg smell), sulfur dioxide, and even yellow solid sulfur.

与NaI生成的HI是强还原剂，能将硫酸中的硫从+6价一直还原到−2价，产生复杂混合物：紫色碘蒸气、硫化氢（臭鸡蛋味）、二氧化硫，甚至黄色固体硫。

NaI + H₂SO₄ → NaHSO₄ + HI

8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O

9. Precipitation Reactions with Silver Nitrate | 与硝酸银的沉淀反应（检验卤离子）

The presence of chloride, bromide or iodide ions in aqueous solution can be confirmed by adding dilute nitric acid followed by silver nitrate solution. The acid eliminates carbonate or sulfite interferences. Silver halide precipitates form with characteristic colours:

水溶液中氯离子、溴离子或碘离子的存在可通过加入稀硝酸后滴加硝酸银溶液来确认。酸可消除碳酸根或亚硫酸根离子的干扰。生成具有特征颜色的卤化银沉淀：

AgCl: white precipitate, soluble in dilute ammonia
AgBr: cream precipitate, soluble in concentrated ammonia only
AgI: yellow precipitate, insoluble in ammonia

AgCl：白色沉淀，溶于稀氨水
AgBr：奶油色沉淀，仅溶于浓氨水
AgI：黄色沉淀，不溶于氨水

The difference in solubility is due to the decreasing stability of the complex ion [Ag(NH₃)₂]⁺ as the halide ion becomes larger and the precipitate more covalent in character. This test is often combined with the halogen displacement test to identify the halide completely.

溶解度的差异是因为随着卤离子变大、沉淀共价性增强，配离子[Ag(NH₃)₂]⁺的稳定性下降。该检验常与卤素置换实验结合，以全面鉴别卤离子。

10. Uses of Halogens and Their Compounds | 卤素及其化合物的用途

Chlorine is used worldwide for water purification and as a bleaching agent. It is also essential in the manufacture of PVC (polyvinyl chloride) via the monomer chloroethene. Sodium chlorate(I) (NaClO) is the active ingredient in household bleach. Fluorine compounds, such as sodium fluoride, are added to toothpaste to prevent dental decay, and polytetrafluoroethylene (PTFE, Teflon) is widely used as a non‑stick coating. Bromine compounds serve as flame retardants, while silver bromide is used in photographic film. Iodine is an antiseptic in solution, and potassium iodide is added to table salt to prevent iodine deficiency disorders. These applications rely directly on the high reactivity and distinctive properties of the halogens.

氯在全球用于水净化和漂白。它也是制造PVC（聚氯乙烯）的关键原料，通过单体氯乙烯聚合获得。次氯酸钠（NaClO）是家用漂白水的有效成分。氟的化合物如氟化钠添加在牙膏中防龋齿，聚四氟乙烯（PTFE，特氟龙）广泛用作不粘涂层。溴化合物用作阻燃剂，而溴化银用于照相胶卷。碘溶液是消毒剂，碘酸钾或碘化钾添加到食盐中以预防碘缺乏症。这些用途直接依赖于卤素的高反应活性和独特性质。

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Group 17: The Halogens | 第17族：卤素