📚 Understanding Group 7 (17): The Halogens | 深度解析第七主族（17族）：卤素
The halogens are a group of non-metallic elements in Group 7 (IUPAC Group 17) of the periodic table. They share a characteristic outer electron configuration of ns² np⁵ and are highly reactive, especially as oxidising agents. This article explores their trends in physical properties, their ability to displace one another from halide salts, and their important reactions with water and alkalis, which underpin practical uses such as water treatment.
卤素是元素周期表第7族（IUPAC 17族）中的一组非金属元素。它们具有共同的外层电子构型 ns² np⁵，反应活性很高，尤其作为氧化剂。本文将探讨其物理性质的递变规律、从卤化物盐中相互置换的能力，以及与水和碱的重要反应，这些原理支撑着诸如水处理等实际应用。
1. Introduction to the Halogens | 卤素简介
The halogens include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive astatine (At). At standard conditions they exist as diatomic molecules (X₂), because each atom needs to share one electron to attain a stable noble-gas configuration.
卤素包括氟（F）、氯（Cl）、溴（Br）、碘（I）以及放射性的砹（At）。在标准状况下，它们以双原子分子（X₂）的形式存在，因为每个原子只需共享一个电子即可达到稳定的稀有气体电子构型。
Their reactivity as a family is dominated by the ease with which they gain one electron to form X⁻ ions. This electron-accepting behaviour makes them strong oxidising agents.
作为一族，它们的反应活性主要取决于获得一个电子形成 X⁻ 离子的难易程度。这种接受电子的行为使它们成为强氧化剂。
2. Electron Configuration and Atomic Trends | 电子构型与原子趋势
The general outer electronic configuration of the halogens is ns² np⁵. For example, chlorine is [Ne] 3s² 3p⁵. There are seven valence electrons, so a halogen atom only needs one more electron to complete an octet.
卤素的一般外层电子构型为 ns² np⁵。例如，氯为 [Ne] 3s² 3p⁵。其共有7个价电子，因此卤素原子只需再获得一个电子就能填满八隅体。
Moving down the group, the atomic radius increases because each successive element has an additional electron shell. The outer electrons are increasingly shielded from the nucleus, which alters properties such as electronegativity and oxidising power.
随着原子序数增加，原子半径增大，因为每个后续元素都增加了一个电子壳层。外层电子受到核的屏蔽作用增强，从而影响电负性和氧化能力等性质。
3. Physical Properties: Appearance and Boiling Points | 物理性质：外观与沸点
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 with a metallic sheen. Their boiling points increase down the group, as summarised below.
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，带有金属光泽。它们的沸点沿族向下升高，如下所示。
| Halogen | Appearance at r.t. | Boiling point (°C) |
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