📚 Physical Properties of Group 7 Elements (Halogens) | 第七主族元素（卤素）的物理性质
The Group 7 elements, also known as the halogens, include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). In this article, we will examine their physical properties, focusing on the trends down the group and the underlying reasons based on molecular structure and intermolecular forces.
第七主族元素又称卤素，包括氟（F）、氯（Cl）、溴（Br）、碘（I）和砹（At）。本文将讨论它们的物理性质，重点分析沿周期表向下变化的趋势，以及这些趋势背后的分子结构和分子间作用力原因。
1. Introduction to the Halogens | 卤素概述
Halogens are non-metals found in Group 7 of the periodic table. They exist as diatomic molecules (X₂) in their elemental form because each atom has seven valence electrons and needs one more electron to achieve a stable noble gas configuration.
卤素是元素周期表第七主族的非金属元素。它们以双原子分子（X₂）形式存在，因为每个原子有七个价电子，还需要获得一个电子才能达到稳定的稀有气体电子构型。
The physical properties of halogens change gradually down the group. These changes can be explained by increasing atomic size, increasing electron number, and stronger van der Waals forces between molecules.
卤素的物理性质沿族向下逐渐变化。这些变化可由原子半径增大、电子数增多以及分子间范德华力增强来解释。
2. Electronic Configuration and Trends | 电子构型与趋势
All halogens have the general outer-shell electronic configuration ns²np⁵. For example, fluorine is 1s²2s²2p⁵, chlorine is [Ne]3s²3p⁵, bromine is [Ar]3d¹⁰4s²4p⁵, and iodine is [Kr]4d¹⁰5s²5p⁵.
所有卤素的最外层电子构型均为 ns²np⁵。例如，氟为 1s²2s²2p⁵，氯为 [Ne]3s²3p⁵，溴为 [Ar]3d¹⁰4s²4p⁵，碘为 [Kr]4d¹⁰5s²5p⁵。
As we move down the group, the number of occupied electron shells increases, leading to a larger atomic radius and a greater shielding effect. These factors influence nearly every physical property of the halogens.
沿族向下，占据的电子层数增加，导致原子半径增大、屏蔽效应增强。这些因素几乎影响卤素的所有物理性质。
3. Atomic Radius | 原子半径
The atomic radius of halogens increases down the group. Fluorine has the smallest atomic radius, while iodine (and astatine) has the largest.
卤素的原子半径沿族向下增大。氟的原子半径最小，碘（和砹）的原子半径最大。
This increase occurs because each successive element has an additional electron shell. Although the nuclear charge also increases, the shielding effect of inner electrons outweighs the increased attraction, so the outer electrons are held less tightly and the radius expands.
这一增长是因为每个后续元素都增加了一个电子层。尽管核电荷也增加，但内层电子的屏蔽效应超过了吸引力的增加，因此外层电子被束缚得更松，半径增大。
- F: ~0.064 nm
- Cl: ~0.099 nm
- Br: ~0.114 nm
- I: ~0.133 nm
- 氟：约 0.064 nm
- 氯：约 0.099 nm
- 溴：约 0.114 nm
- 碘：约 0.133 nm
4. Electronegativity | 电负性
Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. In Group 7, electronegativity decreases down the group.
电负性是原子在共价键中吸引成键电子的能力。在第七主族中，电负性沿族向下减小。
Fluorine is the most electronegative element in the periodic table, with a Pauling value of 3.98. Chlorine is 3.16, bromine is 2.96, and iodine is 2.66.
氟是元素周期表中电负性最大的元素，鲍林值为 3.98。氯为 3.16，溴为 2.96，碘为 2.66。
Even though nuclear charge increases down the group, the atomic radius increases significantly, so the attraction between the nucleus and the bonding electron pair becomes weaker. Therefore, electronegativity decreases.
尽管核电荷沿族向下增加，但原子半径增加更显著，因此原子核对成键电子对的吸引力变弱。所以电负性降低。
5. Melting and Boiling Points | 熔点和沸点
The melting and boiling points of the halogens increase steadily down the group. Fluorine melts at −220 °C and boils at −188 °C; chlorine melts at −102 °C and boils at −34 °C; bromine melts at −7.2 °C and boils at 58.8 °C; iodine melts at 113.7 °C and boils at 184.3 °C.
卤素的熔点和沸点沿族向下逐渐升高。氟的熔点为 −220 °C，沸点为 −188 °C；氯的熔点为 −102 °C，沸点为 −34 °C；溴的熔点为 −7.2 °C，沸点为 58.8 °C；碘的熔点为 113.7 °C，沸点为 184.3 °C。
The reason for this trend is that the halogen molecules are non-polar. The only intermolecular forces are induced dipole–dipole interactions, also called London dispersion forces. As the molecular size increases down the group, the number of electrons increases, making the molecules more polarisable and increasing the strength of dispersion forces. More energy is required to overcome these forces, so melting and boiling points rise.
这一趋势的原因是卤素分子是非极性的。分子间只存在诱导偶极–偶极相互作用，即伦敦色散力。随着分子大小沿族向下增大，电子数增加，分子更易极化，色散力增强。需要更多能量克服这些力，因此熔点和沸点升高。
6. States and Colour at Room Temperature | 常温下的状态和颜色
At room temperature, halogens exist in different physical states due to the trend in melting and boiling points:
在室温下，由于熔沸点趋势，卤素呈现不同的物理状态：
| Halogen | State at RT | Colour |
| F₂ | Gas | Pale yellow |
| Cl₂ | Gas | Yellow-green |
| Br₂ | Liquid | Red-brown |
| I₂ | Solid | Dark grey / violet vapour |
| 卤素 | 室温状态 | 颜色 |
| F₂ | 气体 | 浅黄色 |
| Cl₂ | 气体 | 黄绿色 |
| Br₂ | 液体 | 红棕色 |
| I₂ | 固体 | 暗灰色 / 紫色蒸气 |
These colours arise from the absorption of visible light that causes electronic transitions between molecular orbitals. As the molecules get larger, the energy gap between the highest occupied and lowest unoccupied molecular orbitals decreases, so the absorbed light shifts toward longer wavelengths and the observed colour deepens.
这些颜色来源于分子轨道之间电子跃迁对可见光的吸收。随着分子变大，最高占据分子轨道与最低未占据分子轨道之间的能隙减小，因此吸收的光向更长波长移动，观察到的颜色加深。
7. Volatility and Intermolecular Forces | 挥发性和分子间作用力
Volatility is the tendency of a substance to evaporate. Since the intermolecular forces in halogens are weak London dispersion forces, all halogens are volatile to some extent, but volatility decreases down the group.
挥发性是物质蒸发的倾向。由于卤素分子间作用力是较弱的伦敦色散力，所有卤素都有一定挥发性，但挥发性沿族向下减弱。
Fluorine and chlorine are gases at room temperature, bromine is a volatile liquid that gives off brown fumes, and iodine is a solid that sublimes easily, producing a purple vapour when heated gently.
氟和氯在室温下是气体，溴是易挥发的液体，会放出棕色蒸气；碘是固体，容易升华，稍加热即产生紫色蒸气。
This decrease in volatility is directly linked to the increasing strength of dispersion forces with larger electron clouds, so more energy is required for molecules to escape into the vapour phase.
挥发性的降低直接与电子云越大色散力越强相关，因此分子逸入气相需要更多能量。
8. Bond Energies of Halogen Molecules | 卤素单质的键能
The X–X bond energy in halogen molecules changes as follows: F–F has a bond energy of about 158 kJ mol⁻¹, Cl–Cl is 242 kJ mol⁻¹, Br–Br is 193 kJ mol⁻¹, and I–I is 151 kJ mol⁻¹.
卤素分子中 X–X 键能如下：F–F 约为 158 kJ mol⁻¹，Cl–Cl 为 242 kJ mol⁻¹，Br–Br 为 193 kJ mol⁻¹，I–I 为 151 kJ mol⁻¹。
The bond energy does not decrease smoothly down the group. Fluorine has an unexpectedly low bond energy because of the small size of the fluorine atoms and the high electron–electron repulsion between non-bonding pairs. Chlorine has the highest bond energy because its atoms are large enough to reduce repulsion but still form a strong bond.
键能沿族向下并非平滑减小。氟的键能异常低，因为氟原子很小，非键电子对之间排斥力大。氯的键能最高，因为其原子足够大，减少了排斥作用，但仍能形成较强键。
This irregularity is an important exception to general periodic trends and is often tested in exams.
这种不规则性是周期趋势的重要例外，也常在考试中出现。
9. Solubility | 溶解性
Halogens are only slightly soluble in water. Chlorine, bromine, and iodine dissolve to some extent, but their aqueous solutions are not true solutions because the halogens also react slowly with water.
卤素在水中的溶解度很小。氯、溴、碘能一定程度溶解，但它们的溶液并非真正意义上的简单溶液，因为卤素还会与水缓慢反应。
Fluorine reacts violently with water, so it cannot be dissolved in the normal sense. The solubility of halogens in organic solvents such as hexane or cyclohexane is much higher, and the colour of the halogen in an organic layer is different from that in water, which is used in extraction and displacement experiments.
氟与水剧烈反应，因此不能以通常意义溶解。卤素在己烷或环己烷等有机溶剂中的溶解度要高得多，且卤素在有机层中的颜色不同于水层，这常用于萃取和置换反应实验。
10. Density | 密度
The density of halogens increases down the group. Fluorine gas is the least dense, while iodine is a dense solid.
卤素的密度沿族向下增大。氟气密度最小，碘则是密度较大的固体。
This trend can be explained by the increase in relative atomic mass. Although atomic radius also increases, the mass increase is more significant, so the atoms are packed more closely in terms of mass per unit volume.
这一趋势可由相对原子质量的增加来解释。虽然原子半径也增大，但质量增加更显著，因此单位体积的质量更大。
- F₂: 1.7 g dm⁻³ (gas)
- Cl₂: 3.2 g dm⁻³ (gas)
- Br₂: 3.12 g cm⁻³ (liquid)
- I₂: 4.93 g cm⁻³ (solid)
- F₂：1.7 g dm⁻³（气体）
- Cl₂：3.2 g dm⁻³（气体）
- Br₂：3.12 g cm⁻³（液体）
- I₂：4.93 g cm⁻³（固体）
11. Summary of Physical Property Trends | 物理性质趋势总结
The physical properties of the halogens change in a fairly predictable way down Group 7:
卤素的物理性质在第七主族中沿族向下以相当可预测的方式变化：
| Property | Trend down Group 7 |
| Atomic radius | Increases |
| Electronegativity | Decreases |
| Melting and boiling points | Increase |
| Volatility | Decreases |
| Density | Increases |
| Colour intensity | Deepens |
| 性质 | 沿第七主族趋势 |
| 原子半径 | 增大 |
| 电负性 | 减小 |
| 熔点和沸点 | 升高 |
| 挥发性 | 降低 |
| 密度 | 增大 |
| 颜色强度 | 加深 |
Most of these trends can be explained by three key ideas: increasing atomic size, increasing number of electrons, and strengthening London dispersion forces. Remember that fluorine is anomalous in its bond energy due to electron repulsion, which is a common exam trap.
上述大多数趋势都可用三个关键概念解释：原子尺寸增大、电子数增多、伦敦色散力增强。请记住，氟的键能因电子排斥而异常，这是常见的考试陷阱。
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