📚 Mastering Shapes of Molecules and Ions for Edexcel A-Level Chemistry | 掌握 Edexcel A-Level 化学：分子与离子的空间构型
Shape is one of the most important ideas in A-Level Chemistry. It controls how molecules pack in solids, how they interact with water, how drugs bind to enzymes, and how reactants meet in a mechanism. For Edexcel A-Level Chemistry, you must be able to predict and explain the shapes of simple molecules and ions using electron-pair repulsion theory.
空间构型是 A-Level 化学中最重要的概念之一。它决定了分子在固体中如何堆积、如何与水作用、药物分子如何与酶结合，以及反应物如何在反应机理中相互接近。在 Edexcel A-Level 化学中，你必须能够使用电子对互斥理论预测并解释简单分子和离子的空间构型。
1. Why Shape Matters | 为什么空间构型重要
Molecular shape determines polarity, boiling point, solubility, and reactivity. For example, CO₂ is non-polar because its two polar C=O bonds cancel in a linear arrangement, while H₂O is polar because its bent shape prevents cancellation. A-Level exam questions often link shape to physical properties, so never treat shape as an isolated fact.
分子构型决定极性、沸点、溶解度和反应活性。例如 CO₂ 是非极性分子，因为两个极性 C=O 键在直线排列中相互抵消；而 H₂O 是极性分子，因为 V 形结构使键的极性不能抵消。考试中常把构型与物理性质联系起来，因此不能把构型当作孤立知识点。
2. VSEPR Theory: The Core Principle | 电子对互斥理论的核心原理
VSEPR stands for Valence Shell Electron Pair Repulsion. The central idea is that electron pairs around a central atom repel one another and therefore arrange themselves as far apart as possible. Both bonding pairs and lone pairs occupy positions around the central atom, but lone pairs repel more strongly than bonding pairs.
VSEPR 是价层电子对互斥理论的英文缩写。其核心思想是：中心原子周围的电子对相互排斥，因此会尽可能远离。成键电子对和孤电子对都占据中心原子周围的位置，但孤电子对的排斥力比成键电子对更强。
The repulsion order is:
排斥力从大到小的顺序是：
lone pair–lone pair > lone pair–bond pair > bond pair–bond pair
This explains why molecules with lone pairs have bond angles smaller than the ideal angles for their base shape.
这就解释了为什么含有孤电子对的分子，其键角小于理想键角。
3. Counting Electron Domains | 计算电子域数
An electron domain is any region of electron density around the central atom. A single bond, a double bond, or a triple bond counts as one domain. A lone pair also counts as one domain. Use the Lewis structure to count the domains around the central atom before deciding the shape.
电子域是中心原子周围的电子密度区域。单键、双键或三键都只算作一个电子域；一个孤电子对也算作一个电子域。判断形状前，应先画出路易斯结构，并数出中心原子周围的电子域数。
The base electron-domain arrangements are summarised below:
基本电子域排列总结如下：
| Electron domains | Base shape | Ideal bond angles | Example |
|---|---|---|---|
| 2 | Linear | 180° | BeCl₂, CO₂ |
| 3 | Trigonal planar | 120° | BF₃, NO₃⁻ |
| 4 | Tetrahedral | 109.5° | CH₄, NH₄⁺ |
| 5 | Trigonal bipyramidal | 90°, 120°, 180° | PCl₅ |
| 6 | Octahedral | 90° | SF₆ |
4. Steric Number 2: Linear Shape | 空间位阻数为 2：直线形
When the central atom has two electron domains, the shape is linear with a bond angle of 180°. This occurs in molecules such as BeCl₂, where beryllium forms two single bonds, and CO₂, where carbon forms two double bonds. The two double bonds in CO₂ count as only two domains.
当中心原子有两个电子域时，分子为直线形，键角 180°。例如 BeCl₂ 中铍形成两个单键，CO₂ 中碳形成两个双键。CO₂ 中的两个双键只算作两个电子域。
In CO₂, the central carbon has no lone pairs, so the two C=O regions are placed on opposite sides. The polar bonds cancel exactly, giving a non-polar molecule.
在 CO₂ 中，中心碳原子没有孤电子对，因此两个 C=O 区域位于相反方向。极性键完全抵消，分子为非极性。
5. Steric Number 3: Trigonal Planar and Bent | 空间位阻数为 3：三角平面与 V 形
Three electron domains around a central atom give a trigonal planar base shape with 120° bond angles. Examples include BF₃, NO₃⁻ and CO₃²⁻. These species have no lone pairs on the central atom, so all three bonding domains remain in one plane.
中心原子周围有三个电子域时，基本形状为三角平面，键角 120°。例子包括 BF₃、NO₃⁻ 和 CO₃²⁻。这些粒子在中心原子上没有孤电子对，因此三个成键域保持在同一平面内。
If one of the three domains is a lone pair, the actual shape is bent or angular. The bond angle is compressed from 120° to about 118°. Examples include SO₂ and ozone, O₃.
如果三个电子域中有一个是孤电子对，实际形状为 V 形或角形。键角从 120° 压缩到约 118°。例子包括 SO₂ 和臭氧 O₃。
6. Steric Number 4: Tetrahedral, Pyramidal and Bent | 空间位阻数为 4：四面体、三角锥与 V 形
Four electron domains give a tetrahedral base shape with 109.5° bond angles. CH₄ and NH₄⁺ are classic examples. All four hydrogen atoms are bonded to the central atom, and there are no lone pairs, so the shape is perfectly tetrahedral.
四个电子域形成四面体基本形状，键角 109.5°。CH₄ 和 NH₄⁺ 是经典例子。四个氢原子都与中心原子成键，且没有孤电子对，因此形状为规则四面体。
With one lone pair and three bonding pairs, the shape becomes trigonal pyramidal. The bond angle decreases to about 107° because the lone pair repels the bonding pairs more strongly. NH₃ and H₃O⁺ are typical examples.
如果有一个孤电子对和三个成键对，形状变为三角锥。由于孤电子对排斥成键对的能力更强，键角减小到约 107°。NH₃ 和 H₃O⁺ 是典型的三角锥分子。
With two lone pairs and two bonding pairs, the shape is bent or V-shaped. The bond angle is further reduced to about 104.5°. Water, H₂O, is the most important example.
如果有两个孤电子对和两个成键对，形状为 V 形。键角进一步减小到约 104.5°。水 H₂O 是最重要的例子。
7. Steric Number 5: Trigonal Bipyramidal and Related Shapes | 空间位阻数为 5：三角双锥及相关形状
Five electron domains arrange themselves as a trigonal bipyramid. There are three equatorial positions separated by 120° and two axial positions at 90° to the equatorial plane. PCl₅ is a standard example with no lone pairs.
五个电子域排列为三角双锥。三个赤道位置彼此相隔 120°，两个轴向位置与赤道平面成 90°。PCl₅ 是没有孤电子对的典型例子。
When lone pairs are present in a five-domain system, they always occupy equatorial positions. This is because equatorial–equatorial repulsion is less than axial–equatorial repulsion. One lone pair gives a seesaw shape, two lone pairs give a T-shape, and three lone pairs give a linear shape, as in I₃⁻.
当五个电子域体系中含有孤电子对时，孤电子对总是占据赤道位置。这是因为赤道-赤道之间的排斥力小于轴向-赤道之间的排斥力。一个孤电子对形成跷跷板形，两个孤电子对形成 T 形，三个孤电子对形成直线形，如 I₃⁻。
8. Steric Number 6: Octahedral and Square Planar | 空间位阻数为 6：八面体与平面正方形
Six electron domains around a central atom produce an octahedral shape. All bond angles are 90°. SF₆ is the standard example because sulfur has six single bonds to fluorine and no lone pairs.
中心原子周围有六个电子域时形成八面体。所有键角均为 90°。SF₆ 是标准例子，因为硫与氟形成六个单键，且没有孤电子对。
When two lone pairs are present, they occupy opposite positions to minimise repulsion. This leaves four bonding pairs in a square plane, giving a square planar shape with 90° angles. XeF₄ is a common example. One lone pair in a six-domain system gives a square pyramidal shape.
当存在两个孤电子对时，它们占据相反位置以尽量减少排斥。剩余的四个成键对位于同一平面内，形成平面正方形，
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