📚 Shapes of Molecules | 分子形状
Molecular shape controls how a molecule interacts with other particles, how it packs into a solid, and how it behaves in a reaction. At Cambridge A-level, the main tool for predicting shape is VSEPR theory, which treats electron pairs around a central atom as negative clouds that repel one another and therefore move as far apart as possible.
分子形状决定分子如何与其他粒子相互作用、如何堆积成固体以及在反应中如何表现。在剑桥 A-level 阶段，预测分子形状的主要工具是 VSEPR 理论，该理论把中心原子周围的电子对视为互相排斥的负电荷云，因此它们会尽量彼此远离。
1. VSEPR Theory: Electron Pair Repulsion | VSEPR 理论：电子对互斥
VSEPR stands for Valence Shell Electron Pair Repulsion. The central idea is that both bonding pairs and lone pairs occupy space around the central atom. Because all electron pairs are negatively charged, they repel each other, so they arrange themselves to minimise the total repulsion. The final molecular shape is the arrangement of the atoms, not the arrangement of the electron pairs.
VSEPR 表示价层电子对互斥。核心思想是成键电子对和孤对电子都占据中心原子周围的空间。由于所有电子对都带负电荷，它们互相排斥，因此会排列成使总排斥最小的方式。最终的分子形状是原子的排列方式，而不是电子对的排列方式。
A very important addition is that lone pairs repel more strongly than bonding pairs. A lone pair is held by only one nucleus and its electron cloud spreads out more widely, so it occupies a larger angular space and pushes bonding pairs closer together.
一个非常重要的补充是孤对电子的排斥作用比成键电子对更强。孤对电子只受一个原子核吸引，电子云分布更弥散，因此占据更大的角度空间，把成键电子对压得更近。
2. Counting Electron Domains | 计算电子域
To predict a shape, first draw a Lewis structure. Then count the electron domains around the central atom. A single bond counts as one electron domain. A double bond or triple bond also counts as one electron domain because all the electrons in that multiple bond occupy the same region between the two nuclei. Each lone pair counts as one electron domain.
要预测形状，先画出路易斯结构。然后计算中心原子周围的电子域。单键计作一个电子域。双键或三键也计作一个电子域，因为多重键中的电子都占据两个原子核之间的同一区域。每个孤对电子计作一个电子域。
For example, in CO₂ the central carbon has two double bonds and no lone pairs, so it has two electron domains. In H₂O the central oxygen has two single bonds and two lone pairs, so it has four electron domains. Multiple bonds contain more electron density than single bonds, but for shape prediction they still count as only one domain.
例如，在 CO₂ 中，中心碳原子有两个双键且没有孤对电子，因此有两个电子域。在 H₂O 中，中心氧原子有两个单键和两个孤对电子，因此有四个电子域。多重键比单键含有更高的电子密度，但在形状预测中仍只计作一个域。
3. Two Electron Domains: Linear | 两个电子域：直线形
When a central atom has two electron domains, they move to opposite sides of the atom. This gives a bond angle of 180° and a linear molecular shape. The two bonding regions lie along one straight line.
当中心原子有两个电子域时，它们移动到原子的两侧。这给出 180° 的键角和直线形分子。两个成键区域沿一条直线排列。
Common examples are BeCl₂, in which beryllium has two single bonds, and CO₂, in which carbon has two double bonds. In both cases the shape is linear and the angle is 180°.
常见例子有 BeCl₂，其中铍有两个单键；以及 CO₂，其中碳有两个双键。两种情况下分子都是直线形，键角为 180°。
2 bonding domains + 0 lone pairs → linear, angle 180°
4. Three Electron Domains: Trigonal Planar | 三个电子域：平面三角形
Three electron domains spread out in the same plane at 120° angles, forming the corners of an equilateral triangle. If all three domains are bonding pairs, the molecular shape is trigonal planar and the bond angle is 120°.
三个电子域在同一平面上以 120° 角展开，形成等边三角形的顶点。如果三个域都是成键电子对，则分子形状为平面三角形，键角为 120°。
Boron trifluoride, BF₃, is a standard example: boron has three single bonds and no lone pairs. The carbonate ion, CO₃²⁻, is also trigonal planar because carbon has three bonding domains overall.
三氟化硼 BF₃ 是一个典型例子：硼有三个单键且没有孤对电子。碳酸根离子 CO₃²⁻ 也是平面三角形，因为碳原子总共有三个成键域。
3 bonding domains + 0 lone pairs → trigonal planar, angle 120°
5. Four Electron Domains: Tetrahedral Family | 四个电子域：四面体家族
Four electron domains adopt a tetrahedral arrangement because this places them as far apart as possible in three dimensions. The ideal bond angle is 109.5°. If all four domains are bonding pairs, the molecular shape is tetrahedral. Examples include CH₄ and NH₄⁺.
四个电子域采取四面体排列，因为这样能在三维空间中使它们彼此最远。理想键角为 109.5°。如果四个域都是成键电子对，则分子形状为四面体。例子包括 CH₄ 和 NH₄⁺。
If one lone pair replaces one bonding pair, the electron-domain arrangement is still based on a tetrahedron, but the molecular shape is trigonal pyramidal. Ammonia, NH₃, has three bonding pairs and one lone pair; its bond angle is reduced to about 107°. If two lone pairs replace two bonding pairs, the shape is bent or V-shaped. Water, H₂O, has two bonding pairs and two lone pairs; its bond angle is about 104.5°.
如果一个孤对电子替代一个成键电子对，电子域排列仍以四面体为基础，但分子形状变为三角锥形。氨 NH₃ 有三个成键对和一个孤对，键角减小到约 107°。如果两个孤对替代两个成键对，形状为弯曲形或 V 形。水 H₂O 有两个成键对和两个孤对，键角约为 104.5°。
- 4 bonding + 0 lone → tetrahedral, 109.5°
- 3 bonding + 1 lone → trigonal pyramidal, about 107°
- 2 bonding + 2 lone → bent / V-shaped, about 104.5°
6. Five Electron Domains: Trigonal Bipyramidal | 五个电子域：三角双锥
Five electron domains arrange themselves in a trigonal bipyramidal shape. Three positions, called equatorial, lie in a plane at 120° to each other. Two positions, called axial, lie above and below the plane at 90° to the equatorial positions. When all five domains are bonding pairs, the molecular shape is trigonal bipyramidal, as in PCl₅.
五个电子域排列成三角双锥形。三个位置称为赤道位，在同一平面上彼此成 120°。两个位置称为轴向位，位于平面的上方和下方，与赤道位成 90°。当五个域都是成键电子对时，分子形状为三角双锥，如 PCl₅。
If lone pairs are present, they always occupy equatorial positions first. This is because an equatorial lone pair makes only two 90° interactions, whereas an axial lone pair would make three 90° interactions, producing greater repulsion.
如果有孤对电子，它们总是优先占据赤道位。这是因为赤道位孤对只产生两个 90° 相互作用，而轴向位孤对会产生三个 90° 相互作用，排斥更大。
- 5 bonding + 0 lone → trigonal bipyramidal, 90° and 120°
- 4 bonding + 1 lone → seesaw, angles slightly less than 90° and 120°
- 3 bonding + 2 lone → T-shaped, angles slightly less than 90°
- 2 bonding + 3 lone → linear, angle 180°
Examples include SF₄ for the seesaw shape, ClF₃ for the T-shape, and XeF₂ for the linear shape with three lone pairs.
例子包括 SF₄ 为跷跷板形，ClF₃ 为 T 形，XeF₂ 为含三个孤对的直线形。
7. Six Electron Domains: Octahedral | 六个电子域：八面体
Six electron domains point towards the corners of an octahedron. All angles between adjacent domains are 90°. When all six domains are bonding pairs, the molecular shape is octahedral, as in SF₆.
六个电子域指向八面体的顶点。相邻域之间的角度均为 90°。当六个域都是成键电子对时，分子形状为八面体，如 SF₆。
With one lone pair and five bonding pairs, the shape is square pyramidal, as in BrF₅. With two lone pairs and four bonding pairs, the lone pairs occupy opposite positions to minimise repulsion, and the molecular shape is square planar, as in XeF₄.
含一个孤对和五个成键对时，形状为四方锥，如 BrF₅。含两个孤对和四个成键对时，孤对占据相对位置以使排斥最小，分子形状为平面正方形，如 XeF₄。
- 6 bonding + 0 lone → octahedral, 90°
- 5 bonding + 1 lone → square pyramidal, angles slightly less than 90°
- 4 bonding + 2 lone → square planar, 90°
8. Lone Pairs and Bond Angle Compression | 孤对电子与键角压缩
Lone pairs are held closer to the central nucleus than bonding pairs and spread out more in space. As a result, the repulsion order is: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair. This explains the trend in bond angles across CH₄, NH₃ and H₂O.
孤对电子比成键电子对更靠近中心原子核，在空间中分布更弥散。因此排斥顺序为：孤对-孤对 > 孤对-成键 > 成键-成键。这解释了 CH₄、NH₃ 和 H₂O 的键角变化趋势。
CH₄ has four bonding pairs, so the angle is the ideal 109.5°. NH₃ has one lone pair, which repels the three bonding pairs more strongly, reducing the angle to about 107°. H₂O has two lone pairs, giving even greater repulsion and reducing the angle to about 104.5°.
CH₄ 有四个成键对，因此键角为理想的 109.5°。NH₃ 有一个孤对，对三个成键对产生更强排斥，键角减小到约 107°。H₂O 有两个孤对，排斥更强，键角减小到约 104.5°。
Multiple bonds also repel more strongly than single bonds. In SO₂, the sulfur atom has two double bonds and one lone pair, giving three electron domains. The O=S=O angle is compressed to about 118° rather than the ideal 120°.
多重键也比单键排斥更强。在 SO₂ 中，硫原子有两个双键和一个孤对，形成三个电子域。O=S=O 键角被压缩到约 118°，而不是理想的 120°。
9. Common Exam Examples and Naming | 常见考试实例与命名
The table below summarises the main shapes, bond angles and examples required at Cambridge A-level. In an exam, you should be able to name the shape, state the bond angle and explain the shape using VSEPR.
下表总结了剑桥 A-level 要求的主要形状、键角和实例。在考试中，你需要能够命名形状、给出键角并用 VSEPR 解释形状。
| Electron domains | Bonding pairs | Lone pairs | Shape | Bond angle | Example |
|---|---|---|---|---|---|
| 2 | 2 | 0 | Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
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