Group 7: The Halogens | 第7族：卤素

📚 Group 7: The Halogens | 第7族：卤素

Welcome to this revision guide on Group 7 of the Periodic Table — the halogens. These reactive non-metals include fluorine, chlorine, bromine and iodine, and they appear frequently across all three sciences in the Edexcel IGCSE syllabus. In this article, we will explore their electron structure, physical states, chemical behaviour, displacement reactions and everyday uses.

欢迎阅读本期的元素周期表第7族（卤素）复习指南。氟、氯、溴、碘都是活泼的非金属元素，它们在爱德思 IGCSE 科学三科（物理、化学、生物）的综合考纲中频繁出现。本文将详细讲解它们的电子结构、物理状态、化学性质、置换反应以及日常用途。

1. What Are the Halogens? | 什么是卤素？

The halogens are the elements found in Group 7 (also called Group VII) of the Periodic Table. The name comes from Greek, meaning ‘salt-former’, because they react directly with metals to produce salts. The first four members you must know are fluorine (F₂), chlorine (Cl₂), bromine (Br₂) and iodine (I₂). Astatine (At₂) is radioactive and rarely studied at IGCSE level.

卤素是元素周期表第7族（又称 VII 族）中的元素。”卤素”一词源自希腊语，意为”成盐元素”，因为它们能与金属直接反应生成盐。你首先需要掌握的四种卤素是氟（F₂）、氯（Cl₂）、溴（Br₂）和碘（I₂）。砹（At₂）具有放射性，在 IGCSE 阶段很少研究。

All halogens are non-metals and exist as diatomic molecules — meaning two atoms joined by a covalent bond. Their general formula is X₂, where X represents any halogen atom.

所有卤素都是非金属，并且以双原子分子形式存在——也就是说，两个原子通过共价键结合在一起。它们的一般通式是 X₂，其中 X 代表任意一个卤素原子。

2. Electron Configuration: The Key to Behaviour | 电子排布：行为的关键

Every halogen atom has seven electrons in its outer shell. This is the single most important fact about the group. Having seven outer electrons means each atom needs to gain just one more electron to achieve a stable noble-gas configuration, like that of the next inert gas in the Periodic Table.

每一个卤素原子在最外层都有七个电子。这是关于该族最重要的事实。最外层有七个电子意味着每个原子只需要再获得一个电子，就能达到与周期表中下一个稀有气体相同的稳定电子构型。

F: 2.7 Cl: 2.8.7 Br: 2.8.18.7 I: 2.8.18.18.7

Look at the electron structures above. The final number in each arrangement is always 7, confirming that all halogens belong to the same group. This shared feature explains why they have similar chemical properties.

请观察上方的电子排布。每一种排布的最后一个数字都是7，这证实了所有卤素属于同一族。这一共同特征解释了它们为什么具有相似的化学性质。

3. Physical Properties: States and Colours | 物理性质：状态与颜色

At room temperature (about 25 °C), the halogens exist in different physical states. This is a classic exam question: you must remember the state and colour of each halogen.

在室温（约 25 °C）下，卤素呈现出不同的物理状态。这是一个经典的考题：你必须牢记每种卤素的状态和颜色。

Halogen	State at 25 °C	Colour
Fluorine (F₂)	Gas	Pale yellow
Chlorine (Cl₂)	Gas	Greenish-yellow
Bromine (Br₂)	Liquid	Reddish-brown
Iodine (I₂)	Solid	Grey-black (violet vapour)

A useful memory trick is to think of a staircase: as you move down the group, the halogens become more massive and their boiling points increase, so fluorine and chlorine remain gases while bromine becomes a liquid and iodine a solid.

一个有用的记忆技巧是想象一个阶梯：随着你从该族上方走向下方，卤素的相对分子质量增大，沸点升高，因此氟和氯保持气态，溴变成液态，碘则变成固态。

4. Trends in Boiling Point | 沸点的变化趋势

The boiling point of the halogens increases steadily down the group. Fluorine boils at –188 °C, chlorine at –34 °C, bromine at 59 °C, and iodine at 184 °C. This trend appears because the molecules become larger and heavier as atomic number increases.

卤素的沸点沿该族向下稳步升高。氟的沸点为 –188 °C，氯为 –34 °C，溴为 59 °C，碘为 184 °C。出现这一趋势是因为随着原子序数增大，分子变得更大、更重。

The reason for the trend is intermolecular forces. All halogen molecules are simple covalent molecules held together by weak London dispersion forces. As the molecules get bigger, they contain more electrons, so these weak forces become stronger. More energy is needed to overcome them, so the boiling point rises.

这一趋势的原因是分子间作用力。所有卤素分子都是简单共价分子，通过微弱的伦敦色散力结合在一起。随着分子变大，所含电子增多，这些微弱作用力变得更强，因此需要更多能量来克服它们，沸点随之升高。

5. Reactivity Trend: Down the Group, Less Reactive | 反应活性趋势：沿族向下，活泼性减弱

This is one of the most important concepts in the whole course. The reactivity of the halogens decreases as you go down the group. Fluorine is the most reactive, and iodine is the least reactive of the four.

这是整个课程中最重要的概念之一。卤素的反应活性随族向下而减弱。氟是活泼性最强的，而在这四种卤素中碘的活泼性最弱。

Why? Reactivity depends on how easily the halogen atom gains an electron. Going down the group, atoms become larger and have more electron shells. The outer shell is further from the nucleus and is shielded by inner electrons. Therefore, the attraction between the positively charged nucleus and the incoming electron is weaker, making it harder for the atom to capture that extra electron.

这是为什么？反应活性取决于卤素原子获得电子的难易程度。沿族向下，原子变大，电子层数增多。最外层离原子核更远，且受到内层电子的屏蔽。因此，带正电的原子核对新来电子的吸引力更弱，使原子更难捕获那个额外的电子。

F₂ > Cl₂ > Br₂ > I₂ (reactivity order / 反应活性顺序)

6. Reaction with Metals: Making Salts | 与金属反应：生成盐

Halogens react with metals to form ionic compounds called halides. When a halogen meets a metal, the halogen gains an electron from the metal, becoming a negative ion (a halide ion, X⁻), while the metal loses electrons and becomes a positive ion.

卤素与金属反应生成称为卤化物的离子化合物。当卤素遇到金属时，卤素从金属那里获得一个电子，变成负离子（卤离子，X⁻），而金属则失去电子变成正离子。

A classic example is the reaction between iron and chlorine. If you gently heat iron wool and place it in a jar of chlorine gas, the iron glows brightly and produces a brown solid — iron(III) chloride.

一个经典例子是铁与氯气的反应。如果你微微加热铁绒，并放入装有氯气的集气瓶中，铁会发出明亮的光，并生成棕褐色固体——氯化铁（III）。

2Fe + 3Cl₂ → 2FeCl₃

Similarly, sodium reacts vigorously with chlorine to form sodium chloride, whose formula you certainly know — NaCl. This demonstrates the ‘salt-former’ origin of the word halogen.

类似地，钠与氯气剧烈反应生成氯化钠，其化学式你一定非常熟悉——NaCl。这正体现了”卤素”一词中”成盐元素”的本源。

7. Displacement Reactions: Who Is Stronger? | 置换反应：谁更强？

Because the halogens have different reactivities, a more reactive halogen can displace a less reactive halogen from its salt solution. This is exactly like the displacement reactions you study in the reactivity series of metals.

由于卤素的反应活性不同，一个活泼性更强的卤素可以将活泼性较弱的卤素从其盐溶液中置换出来。这与你所学的金属活动性顺序中的置换反应非常相似。

For example, if you add chlorine water to a colourless solution of potassium iodide, the solution turns reddish-brown. This is because chlorine displaces iodine from the solution:

例如，如果你将氯水加入无色的碘化钾溶液中，溶液会变为红棕色。这是因为氯将碘从溶液中置换出来：

Cl₂ + 2KI → 2KCl + I₂

The table below summarises which displacements occur:

下表总结了哪些置换反应会发生：

Halogen added	Potassium chloride (KCl)	Potassium bromide (KBr)	Potassium iodide (KI)
Chlorine (Cl₂)	No reaction	Brown solution (Br₂)	Brown solution (I₂)
Bromine (Br₂)	No reaction	No reaction	Brown solution (I₂)
Iodine (I₂)	No reaction	No reaction	No reaction

An easy way to remember this: a halogen that is higher in the group always displaces one that is lower. The more reactive halogen acts as the oxidising agent; it gains electrons and is reduced itself while oxidising the halide ion.

一个简单的记忆方法是：位于该族上方的卤素总能置换下方的卤素。活泼性更强的卤素作为氧化剂；它自身获得电子被还原，同时将卤离子氧化。

8. Reaction with Hydrogen | 与氢气的反应

The halogens also react with hydrogen to form covalent compounds called hydrogen halides. The ease of this reaction reflects the reactivity trend of the halogens.

卤素还能与氢气反应，生成称为卤化氢的共价化合物。该反应的难易程度同样反映了卤素的反应活性趋势。

Fluorine reacts explosively with hydrogen even in the dark and at low temperatures. Chlorine reacts in bright sunlight with a violent explosion, but only slowly in diffuse daylight. Bromine reacts slowly when heated, and iodine reacts even less readily, giving an equilibrium that only forms partially.

氟即使在黑暗和低温下也会与氢发生爆炸性反应。氯在强光照射下会猛烈爆炸，但在漫射光下反应缓慢。溴在加热时缓慢反应，而碘的反应更加困难，只能形成部分平衡。

The hydrogen halides dissolve in water to form acidic solutions, such as hydrochloric acid from hydrogen chloride:

卤化氢溶于水后形成酸性溶液，例如氯化氢在水中的溶液就是盐酸：

H₂ + Cl₂ → 2HCl

HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)

9. Uses of Halogens in Everyday Life | 卤素的日常用途

The halogens have many important applications, and you should know at least two or three for the exam.

卤素有许多重要的应用，你至少要记住两到三个供考试使用。

Chlorine is widely used to kill bacteria in drinking water and swimming pools. It is also used to manufacture disinfectants, plastics such as PVC (polyvinyl chloride), and bleach. However, chlorine is toxic and must be handled carefully.

氯气被广泛用于杀灭饮用水和游泳池中的细菌。它还用于制造消毒剂、塑料（如聚氯乙烯 PVC）以及漂白剂。然而，氯气有毒，必须小心处理。

Iodine is a violet solid and its solution in alcohol (tincture of iodine) is used as an antiseptic on minor wounds. Fluorine compounds are added to toothpaste to help prevent tooth decay, though fluorine gas itself is far too dangerous to use directly. Bromine is used in flame retardants and photographic film.

碘是紫色固体，其酒精溶液（碘酒）被用作轻微伤口的消毒剂。氟化物被添加到牙膏中帮助预防蛀牙，尽管氟气本身过于危险而不能直接使用。溴用于阻燃剂和照相胶片。

Remember: despite their usefulness, all halogens are toxic and corrosive in their elemental form. Chlorine and bromine are particularly hazardous, and iodine vapour can cause respiratory irritation.

请记住：尽管卤素非常有用，但它们的单质形态都具有毒性和腐蚀性。氯气和溴特别危险，碘蒸气会刺激呼吸道。

10. Examination Tips and Common Mistakes | 考试技巧与常见错误

Candidates often confuse the reactivity trend of halogens with that of alkali metals. Remember: in Group 1 (alkali metals), reactivity increases down the group; in Group 7 (halogens), reactivity decreases down the group. These are opposite trends, and mixing them up loses marks.

考生经常将卤素的活性趋势与碱金属的活性趋势混淆。请记住：在第1族（碱金属）中，反应活性沿族向下增强；在第7族（卤素）中，反应活性沿族向下减弱。这是相反的两种趋势，混淆会导致丢分。

Another common error is writing halogen atoms as X instead of X₂. Halogens exist as diatomic molecules, so in every equation you must write F₂, Cl₂, Br₂ or I₂. A third mistake is forgetting the state symbols: Cl₂(g), Br₂(l) and I₂(s) have different physical states at room temperature.

另一个常见错误是把卤素原子写成 X，而不是 X₂。卤素以双原子分子存在，因此在每个方程式中都必须写 F₂、Cl₂、Br₂ 或 I₂。第三个错误是忘记状态符号：室温下 Cl₂(g)、Br₂(l) 和 I₂(s) 具有不同的物理状态。

Finally, when answering displacement reaction questions, always state the colour change of the solution. For instance, chlorine displacing bromide produces an orange/brown colour; iodine in solution looks brown, and in an organic solvent it gives a purple layer. Stating the observation scores valuable marks.

最后，回答置换反应问题时，务必说明溶液的颜色变化。例如，氯置换溴离子时溶液变为橙棕色；碘在溶液中呈棕色，而在有机溶剂中则呈现紫色层。写出观察现象可以获得宝贵的分数。

11. Summary Table | 总结表

Here is a quick revision table to help you review the whole topic at a glance.

以下是一张快速复习表，帮助你一目了然地回顾整个主题。

Property	Trend down the group
Atomic radius / 原子半径	Increases / 增大
Melting and boiling point / 熔沸点	Increases / 升高
Reactivity / 反应活性	Decreases / 减弱
Attraction for outer electrons / 对外层电子的吸引力	Decreases / 减弱
Oxidising power / 氧化能力	Decreases / 减弱

Keep this table in mind as a final checkpoint before your exam. If you can explain why each trend occurs, you are well prepared for any halogen question the examiner throws at you.

请在考试前将这张表作为最后的检查点牢记于心。如果你能解释每一个趋势背后的原因，那么无论考官出什么样的卤素题目，你都能从容应对。

Halogens may be only one family of elements, but they connect many areas of chemistry — electron structure, bonding, redox reactions and industrial applications. Master the trends, practise the displacement equations, and always remember the colours. Good luck with your revision.

卤素虽然只是一个元素家族，但它串联了化学的许多领域——电子结构、化学键、氧化还原反应和工业应用。掌握变化趋势，勤练置换方程式，并始终记住颜色。祝你复习顺利。

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Group 7: The Halogens | 第7族：卤素