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Group 7(17) The Halogens - Properties, Trends and Reactions | 第7族(17族)卤素——性质、趋势与反应

2026-08-21 19:38 · 本文部分内容由AI辅助创作

📚 Group 7(17) The Halogens - Properties, Trends and Reactions | 第7族(17族)卤素——性质、趋势与反应

The Group 7 elements - fluorine, chlorine, bromine, iodine and astatine - are known as the halogens. Their name means 'salt former', and their chemistry is dominated by redox changes, especially the halogens acting as oxidising agents and halide ions acting as reducing agents.

第7族元素氟、氯、溴、碘和砹称为卤素。名称意为"成盐者"，其化学以氧化还原变化为主，尤其是卤素作氧化剂、卤离子作还原剂。

This guide covers the electronic structure of the halogens, the physical and chemical trends down the group, the key reactions you must be able to write and explain, and the standard laboratory tests that distinguish one halogen or halide ion from another. Every trend below can be traced back to one simple idea: atomic size and shielding increase down the group, so the attraction between the nucleus and the outer electrons decreases.

本指南涵盖卤素的电子结构、沿族向下的物理与化学趋势、必须会书写并解释的关键反应，以及区分不同卤素与卤离子的标准实验检验。下面每一个趋势都可以追溯到同一个简单原理：沿族向下原子半径与屏蔽效应增大，因此原子核对外层电子的吸引减弱。

1. Electronic Structure and Occurrence | 电子结构与存在形式

All halogens have five electrons in their outer shell, giving the valence configuration ns² np⁵. Each halogen is one electron short of a full outer shell, so halogen atoms tend to gain one electron to form halide ions X⁻. Because of this strong tendency to gain an electron, halogens are powerful oxidising agents and are never found uncombined in nature.

所有卤素最外层有五个电子，价层构型为 ns² np⁵。每个卤素离满层只差一个电子，因此卤素原子倾向于得到一个电子形成卤离子 X⁻。正因为这种强烈的得电子倾向，卤素是强氧化剂，在自然界中从不以单质形式存在。

In the elemental state halogens exist as diatomic molecules, X₂, held together by a single covalent bond. Fluorine and chlorine are diatomic gases at room temperature, bromine is a volatile red-brown liquid, and iodine is a grey-black solid. Astatine is radioactive and very rare, so its chemistry is less commonly studied.

在单质状态下，卤素以双原子分子 X₂ 存在，由一根共价单键相连。氟和氯在室温下是双原子气体，溴是易挥发的红棕色液体，碘是灰黑色固体。砹具有放射性且非常稀少，因此其化学性质研究较少。

In nature halogens appear as halide ions in seawater and in minerals: fluoride in fluorite (CaF₂) and cryolite (Na₃AlF₆), chloride in rock salt (NaCl), bromide in seawater and salt lakes, and iodide in seaweed and in Chile saltpetre. Astatine occurs only in trace amounts from radioactive decay, so its properties are largely estimated from the trend rather than measured.

自然界中卤素以卤离子形式存在于海水和矿物中：氟存在于萤石（CaF₂）和冰晶石（Na₃AlF₆）中，氯存在于岩盐（NaCl）中，溴存在于海水和盐湖中，碘存在于海藻和智利硝石中。砹仅由放射性衰变产生微量，因此其性质多由趋势推测而非实测。

2. Physical Properties: State, Colour and Volatility | 物理性质：状态、颜色与挥发性

The physical state of each halogen at room temperature follows directly from the strength of the London dispersion forces between its molecules. As the molecules get larger and possess more electrons, the temporary dipoles become stronger, so melting and boiling points rise and the element changes from gas to liquid to solid down the group.

各卤素在室温下的物理状态直接取决于其分子间伦敦色散力的强弱。分子越大、电子数越多，瞬时偶极越强，因此熔沸点升高，单质沿族向下由气体变为液体再变为固体。

Halogen 卤素State at 25 °C 室温状态Colour 颜色Melting / boiling point 熔点 / 沸点
Fluorine, F₂Gas 气体Pale yellow 淡黄色-220 °C / -188 °C
Chlorine, Cl₂Gas 气体Pale green 淡黄绿色-101 °C / -34 °C
Bromine, Br₂Liquid 液体Red-brown, volatile 红棕色，易挥发-7 °C / 59 °C
Iodine, I₂Solid 固体Grey-black; purple vapour 灰黑色；蒸气紫色114 °C / 184 °C
Astatine, At₂Solid 固体Radioactive, very rare 放射性，极稀少Estimated to be higher still 估计更高

Bromine is the only non-metal that is a liquid at room temperature, and iodine readily sublimes - it changes directly from a grey-black solid to a purple vapour on gentle warming. Both features are frequently examined as observations in practical questions.

溴是室温下唯一呈液态的非金属，碘则容易升华——缓慢加热时直接从灰黑色固体变为紫色蒸气。这两个特征常作为实验观察题出现在考试中。

Halogens are only slightly soluble in water but dissolve readily in non-polar organic solvents such as cyclohexane or hexane. This difference in solubility is exploited in the displacement tests, because the colours of the halogens are far more intense in organic solvents.

卤素在水中溶解度很小，但易溶于环己烷、己烷等非极性有机溶剂。置换反应检验正是利用这一溶解性差异，因为卤素在有机溶剂中的颜色要深得多。

3. Trends in Atomic Radius and Electronegativity | 原子半径与电负性趋势

Atomic radius increases down the group because extra electron shells are added and inner-shell shielding increases. The outer electrons are further from the nucleus and are less strongly attracted, so the effective nuclear charge felt by the outer shell falls.

原子半径沿族向下增大，因为增加了新的电子层，内层屏蔽增强。外层电子离核更远，受到的吸引更弱，因此外层感受到的有效核电荷下降。

Electronegativity decreases down the group for the same reason: more shells and greater shielding reduce the attraction for a bonding pair of electrons. Fluorine is the most electronegative element, which explains many of its extreme properties.

电负性沿族向下减小，原因相同：电子层增多、屏蔽增强，降低了对成键电子对的吸引力。氟是电负性最强的元素，这解释了它的许多极端性质。

These two trends explain a great deal of halogen chemistry. Because fluorine is so small and so electronegative, the H-F bond is very strong and very polar, which is why hydrogen fluoride is a weak acid and why fluoride salts have high lattice enthalpies. Because iodine atoms are large and only weakly electronegative, the H-I bond is weak and easily broken, which is why iodide is the strongest reducing agent of the halide ions.

这两个趋势解释了卤素的大量化学行为。由于氟原子极小且电负性极高，H-F 键非常强且极性很大，这正是氟化氢为弱酸、氟化物晶格焓较高的原因。由于碘原子较大且电负性较弱，H-I 键较弱、易断裂，这正是碘离子是卤离子中还原性最强者的原因。

Fluorine is also anomalous in that it shows only the -1 oxidation state in its compounds, whereas chlorine, bromine and iodine can display positive oxidation states such as +1, +3, +5 and +7 in oxoacids and oxoanions.

氟还有一个反常之处：其化合物中只显示 -1 氧化态，而氯、溴、碘在含氧酸和含氧酸根中可以显示 +1、+3、+5、+7 等正氧化态。

4. Melting and Boiling Points and Bond Enthalpy | 熔沸点与键焓趋势

Melting and boiling points increase down the group because larger halogen molecules have more electrons, so the London dispersion forces between molecules are stronger and require more energy to overcome. Iodine therefore melts at 114 °C, whereas fluorine boils at -188 °C.

熔点和沸点沿族向下升高，因为较大的卤素分子有更多电子，分子间的伦敦色散力更强，需要更多能量才能克服。因此碘的熔点为 114 °C，而氟的沸点低至 -188 °C。

Note carefully that the forces broken on melting and boiling are intermolecular forces, not covalent bonds. The covalent X-X bond itself is not broken when a halogen boils, so the trend in boiling point has nothing to do with bond enthalpy.

需要特别注意的是，熔化或沸腾时被克服的是分子间作用力，而不是共价键。卤素沸腾时共价 X-X 键本身并未断裂，因此沸点趋势与键焓无关。

The X-X bond enthalpy, by contrast, generally decreases down the group. This is not the same as the boiling point trend, and the two are frequently confused in exams.

相反，X-X 键焓沿族向下总体减小。这一趋势与沸点趋势不同，考试中两者常被混淆。

Bond 键Mean bond enthalpy 平均键焓 / kJ mol⁻¹
F-F158
Cl-Cl242
Br-Br193
I-I151

The F-F bond is anomalously weak. Fluorine atoms are so small that the lone pairs on the two bonded atoms repel each other strongly, and there is relatively little opportunity for effective orbital overlap. The low F-F bond enthalpy, combined with the very strong H-F bond formed, is the reason why fluorine reacts so violently with hydrogen and with water.

F-F 键异常地弱。氟原子太小，两个成键原子的孤对电子之间排斥很强，而且有效轨道重叠的机会相对较少。F-F 键焓很低，再加上生成的 H-F 键非常强，这正是氟与氢气、与水反应如此剧烈的原因。

5. Halogens as Oxidising Agents | 卤素作氧化剂

As oxidising agents, the halogens accept electrons and are themselves reduced. Their oxidising power decreases down the group, which is shown clearly by the standard electrode potentials for the half-equation below.

作为氧化剂时，卤素接受电子，自身被还原。其氧化能力沿族向下减弱，下面的半反应标准电极电势清楚地说明了这一点。

X₂ + 2e⁻ → 2X⁻

Half-equation 半反应E° / V
F₂ + 2e⁻ → 2F⁻+2.87
Cl₂ + 2e⁻ → 2Cl⁻+1.36
Br₂ + 2e⁻ → 2Br⁻+1.07
I₂ + 2e⁻ → 2I⁻+0.54

The explanation is again about size and shielding. Down the group the halogen atom becomes larger, the incoming electron is added further from the nucleus, and the attraction it experiences is weaker. Less energy is released when the halide ion is formed, so the halogen is less willing to be reduced.

解释仍然与半径和屏蔽有关。沿族向下卤素原子变大，进入的电子离核更远，受到的吸引更弱。形成卤离子时释放的能量更少，因此卤素被还原的倾向下降。

Fluorine is so strongly oxidising that it oxidises water even in the cold, displacing oxygen:

氟的氧化性极强，即使在冷水中也能氧化水并置换出氧气：

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2F₂ + 2H₂O → 4HF + O₂

Chlorine also reacts with water, but by disproportionation rather than by displacing oxygen:

氯也和水反应，但发生的是歧化反应，而不是置换出氧气：

Cl₂ + H₂O ⇌ HCl + HClO

The same disproportionation behaviour appears with cold dilute sodium hydroxide, giving bleach:

在冷稀氢氧化钠中表现出同样的歧化行为，生成漂白液：

Cl₂ + 2NaOH → NaCl + NaClO + H₂O

6. Displacement Reactions between Halogens and Halides | 卤素与卤离子间的置换反应

A more reactive halogen will displace a less reactive halide ion from solution. This is a direct consequence of the electrode potential trend: the halogen higher in the group is the stronger oxidising agent, so it takes the electrons from the halide ion below it.

较活泼的卤素可以把较不活泼的卤离子从溶液中置换出来。这直接来自电极电势趋势：位于较上方的卤素是更强的氧化剂，会从下方的卤离子中夺取电子。

Cl₂ + 2KBr → 2KCl + Br₂

Cl₂ + 2KI → 2KCl + I₂

Br₂ + 2KI → 2KBr + I₂

Iodine cannot displace bromide or chloride, and bromine cannot displace chloride, because in each case the halogen present is the weaker oxidising agent. If you write one of these reactions the wrong way round, you have contradicted the whole trend.

碘不能置换溴离子或氯离子，溴也不能置换氯离子，因为在每种情况下现存的卤素都是较弱的氧化剂。如果把这类反应写反了，就等于违背了整个趋势。

The observations are colour changes. In the aqueous layer the colours are faint, but on adding cyclohexane and shaking, the halogen is extracted into the organic layer where the colour is much more intense: chlorine pale yellow-green, bromine orange-brown, iodine purple.

实验现象是颜色变化。在水层中颜色很浅，但加入环己烷并振荡后，卤素被萃取到有机层，颜色深得多：氯为淡黄绿色，溴为橙棕色，碘为紫色。

In the exam, always state the colour change in words that a blind examiner could follow, for example 'colourless solution turns orange-brown' rather than simply naming the product.

考试中务必用文字描述颜色变化，做到即使看不到图也能理解，例如写"无色溶液变为橙棕色"，而不要仅仅写出产物名称。

7. Halide Ions as Reducing Agents and Reaction with Sulfuric Acid | 卤离子作还原剂与浓硫酸反应

Halide ions can act as reducing agents by donating electrons, and their reducing power increases down the group: I⁻ > Br⁻ > Cl⁻ > F⁻. Iodide is the strongest reducing agent because the I-I bond formed is weak and the large iodide ion loses its outer electron most easily.

卤离子可以给出电子而表现还原性，其还原能力沿族向下增强：I⁻ > Br⁻ > Cl⁻ > F⁻。碘离子还原性最强，因为生成的 I-I 键较弱，而且体积大的碘离子最容易失去外层电子。

The standard demonstration is the reaction of solid sodium halides with concentrated sulfuric acid. In every case the first step is not a redox reaction but an acid-base reaction that produces the hydrogen halide:

经典演示实验是固体卤化钠与浓硫酸的反应。每一种情况下，第一步都不是氧化还原反应，而是生成卤化氢的酸碱反应：

NaX + H₂SO₄ → NaHSO₄ + HX

With sodium fluoride and sodium chloride, that is as far as it goes: HF and HCl are not reducing enough to be oxidised by concentrated sulfuric acid, although the fumes produced are acidic and corrosive.

对氟化钠和氯化钠而言，反应到此为止：HF 和 HCl 的还原性不足以被浓硫酸氧化，尽管产生的气体具有酸性和腐蚀性。

With sodium bromide, the hydrogen bromide formed is then oxidised, and bromine vapour is seen as a red-brown gas together with the choking smell of sulfur dioxide:

对溴化钠而言，生成的溴化氢会进一步被氧化，可观察到红棕色溴蒸气以及刺鼻的二氧化硫气味：

NaBr + H₂SO₄ → NaHSO₄ + HBr

2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

With sodium iodide, reduction goes much further. Iodide reduces sulfuric acid all the way to sulfur dioxide, to sulfur, and even to hydrogen sulfide, which gives the unmistakable smell of rotten eggs:

对碘化钠而言，还原程度要深得多。碘离子把硫酸一直还原到二氧化硫、单质硫，甚至硫化氢，后者有明显的臭鸡蛋气味：

NaI + H₂SO₄ → NaHSO₄ + HI

8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O

In the exam, this sequence is used as evidence for the order of reducing power. The deeper the reduction of sulfuric acid, the stronger the reducing agent. Remember that sulfur in H₂SO₄ is at +6, in SO₂ at +4, in S at 0 and in H₂S at -2.

考试中，这一序列被用作还原性强弱顺序的证据。硫酸被还原的程度越深，该卤离子的还原性越强。记住硫在 H₂SO₄ 中为 +6，在 SO₂ 中为 +4，在 S 中为 0，在 H₂S 中为 -2。

8. Hydrogen Halides: Formation, Acidity and Thermal Stability | 卤化氢：生成、酸性与热稳定性

All the halogens react with hydrogen to give hydrogen halides, HX, but the conditions needed become progressively more vigorous down the group:

所有卤素都能与氢反应生成卤化氢 HX，但沿族向下所需的反应条件越来越剧烈：

H₂ + X₂ → 2HX