A-Level Chemistry: Molecular Shapes and Geometric Structure Explained | A-Level 化学：分子形状与几何构型解析

📚 A-Level Chemistry: Molecular Shapes and Geometric Structure Explained | A-Level 化学：分子形状与几何构型解析

In A-Level Chemistry, molecular shape and geometry are not optional extras. The shape of a molecule controls its polarity, boiling point, chemical reactivity, and even the way it interacts with biological receptors. In the CIE syllabus, you are expected to predict shapes using the Valence Shell Electron Pair Repulsion model, explain bond angles, and link geometry to physical properties.

在 A-Level 化学中，分子形状与几何构型并不是可有可无的内容。分子的形状决定其极性、沸点、化学反应活性，甚至影响它与生物受体的作用方式。在 CIE 考纲中，你需要运用价层电子对互斥模型预测形状，解释键角，并将几何构型与物理性质联系起来。

1. Why Molecular Shape Matters | 为什么分子形状很重要

Molecular shape describes the three-dimensional arrangement of atoms in a molecule. It is determined only by the positions of the nuclei, not by the positions of lone pairs. Lone pairs still influence the shape because they occupy space around the central atom, but they are not counted when naming the shape.

分子形状描述的是分子中原子在三维空间中的排列方式。它只由原子核的位置决定，而不由孤对电子的位置决定。孤对电子仍然影响形状，因为它们占据了中心原子周围的空间，但在命名形状时不被计入。

A small change in shape can produce a large change in properties. Water is polar and can form hydrogen bonds because its molecule is bent. Carbon dioxide is non-polar because its molecule is linear, even though both C=O bonds are polar.

形状的微小变化可能带来性质的巨大改变。水是极性分子，能够形成氢键，因为水分子是弯曲形。二氧化碳是非极性分子，因为它的分子是直线形，尽管两个 C=O 键都带有极性。

2. The VSEPR Model | VSEPR 模型

The Valence Shell Electron Pair Repulsion model is the main tool for predicting molecular shape. It assumes that electron pairs in the valence shell repel one another and therefore arrange themselves as far apart as possible.

价层电子对互斥模型是预测分子形状的主要工具。它假设价层中的电子对互相排斥，因此会尽可能彼此远离。

There are three important types of electron-pair repulsion. Lone pair-lone pair repulsion is strongest, lone pair-bonding pair repulsion is intermediate, and bonding pair-bonding pair repulsion is weakest.

电子对之间存在三类重要的排斥作用：孤对电子-孤对电子排斥最强，孤对电子-成键电子对排斥次之，成键电子对-成键电子对排斥最弱。

Double and triple bonds are treated as a single electron domain because they are concentrated in one region of space. For example, CO₂ has two C=O double bonds and no lone pairs on carbon, so it has two electron domains and is linear.

双键和三键被看作一个电子域，因为它们集中占据同一空间区域。例如 CO₂ 含有两个 C=O 双键，碳原子上没有孤对电子，因此它有两个电子域，呈直线形。

3. Electron Domains and Their Arrangements | 电子域及其空间排布

An electron domain is any region of high electron density: a bonding pair, a lone pair, a double bond, or a triple bond. The total number of domains around the central atom determines the parent geometry, which is called the electron-pair arrangement.

电子域是任何电子密度较高的区域：成键电子对、孤对电子、双键或三键。中心原子周围的电子域总数决定母体几何构型，即电子对空间排布。

Two domains give a linear arrangement, three domains give a trigonal planar arrangement, four domains give a tetrahedral arrangement, five domains give a trigonal bipyramidal arrangement, and six domains give an octahedral arrangement.

两个电子域对应直线形排布，三个对应平面三角形排布，四个对应正四面体排布，五个对应三角双锥排布，六个对应八面体排布。

4. Lone Pairs and Distorted Geometries | 孤对电子与变形几何

Lone pairs are sometimes described as “invisible guests”. They occupy larger volumes than bonding pairs because they are attracted to only one nucleus, so their electron cloud spreads more freely.

孤对电子有时被形容为“隐形访客”。它们比成键电子对占据更大空间，因为它们只受一个原子核吸引，电子云扩展得更自由。

This means lone pairs compress nearby bond angles. Consider methane, ammonia, and water. All three have a tetrahedral electron-pair arrangement, but their molecular shapes are different because the number of lone pairs on the central atom increases from zero to two.

这意味着孤对电子会压缩邻近的键角。以甲烷、氨和水为例，它们都具有正四面体电子对排布，但由于中心原子上的孤对电子数从 0 增加到 2，分子形状各不相同。

The observed bond angles follow the repulsion order. Ammonia has one lone pair and a bond angle of approximately 107°. Water has two lone pairs and a bond angle of approximately 104.5°. Methane has no lone pairs and retains the full tetrahedral angle of 109.5°.

实际键角符合排斥力顺序。氨含有一个孤对电子，键角约为 107°。水含有两个孤对电子，键角约为 104.5°。甲烷没有孤对电子，因此保留完整的正四面体角 109.5°。

5. Common Molecular Geometries | 常见分子几何构型

The table below summarises the most common molecular shapes required for CIE A-Level Chemistry. The “shape” is named from the positions of the atoms only, not the lone pairs.

下表总结了 CIE A-Level 化学中最常见的分子形状。命名形状时只考虑原子的位置，不考虑孤对电子。

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