📚 The Group 7 Elements: Halogens | 第7族元素：卤素
Group 7 elements, also known as the halogens, are a family of highly reactive non-metals found in the periodic table. They have similar chemical properties because they all have seven electrons in their outer shell.
第7族元素，也称为卤素，是元素周期表中一族非常活泼的非金属。它们具有相似的化学性质，因为它们的最外层都有七个电子。
1. Where Are the Group 7 Elements? | 第7族元素在哪里？
The Group 7 elements are found in the vertical column second from the right of the periodic table. In modern IUPAC numbering this is group 17, but the name 'Group 7' is still common in many school courses.
第7族元素位于元素周期表右侧第二列。在现代 IUPAC 编号中这是第17族，但许多学校课程中仍常称为第7族。
They are placed after the noble gases and before the transition metals in the periodic table. The elements become more metallic in character as you move down the group, but they all remain non-metals.
它们位于稀有气体之后、过渡金属之前。随着往下，这些元素的金属性略有增强，但它们仍然都是非金属。
2. Meet the Halogens: Names and Symbols | 认识卤素：名称与符号
The first four Group 7 elements are fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). Astatine (At) is also in Group 7 but it is rare and radioactive, so it is usually not studied in detail at KS3.
前四个第7族元素是氟 (F)、氯 (Cl)、溴 (Br) 和碘 (I)。砹 (At) 也属于第7族，但它稀有且具有放射性，因此在 KS3 阶段通常不详细学习。
At room temperature, fluorine is a pale yellow gas, chlorine is a yellow-green gas, bromine is a red-brown liquid and iodine is a grey-black solid.
在室温下，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。
| Element | Symbol | State at room temperature | Colour |
|---|---|---|---|
| Fluorine | F | Gas | Pale yellow |
| Chlorine | Cl | Gas | Yellow-green |
| Bromine | Br | Liquid | Red-brown |
| Iodine | I | Solid | Grey-black |
3. Diatomic Molecules | 双原子分子
All halogens exist as diatomic molecules, meaning two atoms are joined together by a covalent bond. Their formulas are written as F₂, Cl₂, Br₂ and I₂.
所有卤素都以双原子分子形式存在，即两个原子通过共价键连接在一起。它们的化学式写为 F₂、Cl₂、Br₂ 和 I₂。
When chlorine gas is written in an equation, the formula is Cl₂, never Cl. The '2' shows that two chlorine atoms make up one molecule.
在方程式中书写氯气时，化学式是 Cl₂，而不是 Cl。这里的 '2' 表示一个氯分子由两个氯原子组成。
You should always show the correct diatomic formula when writing word or symbol equations involving halogens.
在书写涉及卤素的文字方程式或符号方程式时，应始终写出正确的双原子化学式。
4. Physical Appearance and State | 物理外观与状态
There is a clear trend in appearance and state as you go down Group 7. The elements get darker in colour, and their melting and boiling points increase.
第7族元素从上到下外观和状态有明显趋势。元素颜色逐渐变深，熔点和沸点逐渐升高。
Fluorine is a pale yellow gas, chlorine is a yellow-green gas, bromine is a dark red liquid, and iodine is a shiny grey-black solid. Iodine sublimes to form a purple vapour when heated gently.
氟是淡黄色气体，氯是黄绿色气体，溴是深红棕色液体，碘是有光泽的灰黑色固体。碘受热时会升华，产生紫色蒸气。
This trend can help you predict properties: any halogen above bromine is likely a gas, while iodine and elements below it are solids at room temperature.
这一趋势可以帮助你预测性质：溴上方的卤素很可能是气体，而碘及其下方的元素在室温下是固体。
5. Reactivity Trend Down the Group | 第7族从上到下的反应性趋势
In Group 7, reactivity decreases as you go down the group. Fluorine is the most reactive halogen, followed by chlorine, then bromine, and iodine is the least reactive of these four.
在第7族中，从上到下反应性逐渐减弱。氟是最活泼的卤素，其次是氯，然后是溴，碘在这四种元素中最不活泼。
This is the opposite trend to Group 1, where reactivity increases down the group. It is important not to confuse these two trends.
这与第1族的趋势相反，第1族从上到下反应性增强。注意不要混淆这两个趋势。
A useful way to remember is: 'the higher up and further right, the more reactive the non-metal is.' For halogens, higher in the group means more reactive.
一个有用的记忆方法是：'位置越靠上、越靠右，非金属越活泼。' 对卤素来说，位置越靠上，就越活泼。
6. Explaining the Trend in Reactivity | 解释反应性趋势
Halogen atoms have seven electrons in their outer shell. They need to gain one more electron to achieve a stable full outer shell, forming a halide ion such as F⁻, Cl⁻, Br⁻ or I⁻.
卤素原子最外层有七个电子。它们需要再获得一个电子才能达到稳定的满层结构，形成卤离子，如 F⁻、Cl⁻、Br⁻ 或 I⁻。
The easier it is for a halogen atom to attract and gain an electron, the more reactive it is. Fluorine has the smallest atom, so its outer shell is close to the nucleus and the incoming electron is strongly attracted.
卤素原子越容易吸引并获得电子，它的反应性就越强。氟的原子最小，因此它的最外层离原子核很近，对进来的电子吸引力很强。
Going down the group, the atoms get larger and the outer shell is further from the nucleus. The attraction for an extra electron becomes weaker, so reactivity decreases.
从上到下，原子半径增大，最外层离原子核更远。原子核对额外电子的吸引力变弱，因此反应性降低。
7. Displacement Reactions | 置换反应
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. This is called a displacement reaction.
较活泼的卤素可以将较不活泼的卤素从其盐溶液中置换出来。这种反应称为置换反应。
For example, chlorine displaces bromine from potassium bromide solution, turning the solution orange-brown because bromine is produced.
例如，氯可以从溴化钾溶液中置换出溴，使溶液变成橙棕色，因为生成了溴。
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Chlorine also displaces iodine from potassium iodide solution, producing a dark brown solution.
氯也可以从碘化钾溶液中置换出碘，生成深棕色溶液。
Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)
Bromine can displace iodine from potassium iodide solution, but it cannot displace chlorine from potassium chloride solution.
溴可以从碘化钾溶液中置换出碘
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