📚 Shapes of Molecules and Ions: Electron-Pair Repulsion Theory | 分子与离子的空间形状：电子对互斥理论
This Edexcel A-Level Chemistry topic focuses on using electron-pair repulsion theory to predict the three-dimensional shapes of simple molecules and polyatomic ions. Understanding bond angles and the influence of lone pairs is essential for explaining molecular geometry, polarity and reactivity.
本 Edexcel A-Level 化学主题重点介绍如何利用电子对互斥理论预测简单分子和多原子离子的三维形状。理解键角以及孤电子对的影响，对于解释分子几何结构、极性和反应性至关重要。
1. The Core Principle: Repulsion Between Electron Pairs | 核心原理：电子对之间的排斥
Electron-pair repulsion theory, often called VSEPR theory, is based on one simple idea. Electron pairs around a central atom are regions of negative charge, so they repel each other and arrange themselves as far apart as possible to minimise repulsion.
电子对互斥理论（通常称为 VSEPR 理论）基于一个简单的思想。中心原子周围的电子对是负电荷区域，因此它们彼此排斥，并尽可能远离，以将排斥力降到最低。
The shape of a molecule is therefore determined by the number of electron pairs around the central atom, not by the number of atoms alone. Both bonding pairs and lone pairs contribute to the total electron-pair count.
因此，分子的形状取决于中心原子周围电子对的数量，而不仅仅取决于原子的数量。成键电子对和孤电子对都计入电子对总数。
Repulsion strength follows the order: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair. This is because lone pairs are less tightly held by the nucleus and spread out more widely in space.
排斥力大小顺序为：孤电子对-孤电子对 > 孤电子对-成键电子对 > 成键电子对-成键电子对。这是因为孤电子对受原子核约束较弱，在空间中扩展更宽。
2. Counting Electron Domains | 计算电子域
To use the theory correctly, first count the total number of electron domains around the central atom. A single bond, double bond or triple bond each counts as one bonding pair. Each non-bonding pair of electrons on the central atom counts as one lone pair.
要正确使用该理论，首先要计算中心原子周围的电子域总数。单键、双键或三键各计为一个成键电子对。中心原子上的每个非键电子对各计为一个孤电子对。
Add the number of bonding pairs and lone pairs to find the total number of electron domains. For example, CO₂ has two double bonds around carbon, so it has two electron domains. H₂O has two single bonds and two lone pairs, giving four electron domains.
将成键电子对数和孤电子对数相加，得到总电子域数。例如，CO₂ 中碳原子周围有两个双键，因此有两个电子域；H₂O 有两个单键和两个孤电子对，总共四个电子域。
Always draw the Lewis structure first and make sure the central atom follows the octet rule where possible, but be aware that elements in Period 3 or below can have expanded octets and therefore more than four electron domains.
始终先画出 Lewis 结构，并确保中心原子尽可能遵循八电子规则，但要注意第三周期及以下元素可以具有扩展八电子，因此可拥有四个以上的电子域。
3. Two Electron Domains: Linear Geometry | 两个电子域：直线形
When a central atom has only two electron domains, both are usually bonding pairs. The two regions of negative charge move to opposite sides of the central atom, producing a linear arrangement with a bond angle of 180°.
当中心原子只有两个电子域时，两者通常都是成键电子对。两个负电荷区域位于中心原子的两侧，形成直线形排列，键角为 180°。
Typical examples are BeCl₂ and CO₂. In CO₂, carbon forms two double bonds to oxygen atoms, and the molecule is linear. The same geometry applies to the ion N₂O, where the central nitrogen is joined by two double bonds.
典型例子是 BeCl₂ 和 CO₂。在 CO₂ 中，碳与氧原子形成两个双键，分子为直线形。同样的几何结构也适用于 N₂O 离子，其中中心氮原子通过两个双键连接。
4. Three Electron Domains: Trigonal Planar and Bent | 三个电子域：平面三角形与角形
Three electron domains arrange themselves in one plane at angles of 120°, giving a trigonal planar arrangement. When all three domains are bonding pairs, the molecular shape is trigonal planar, as in BF₃ and SO₃.
三个电子域在同一平面内以 120° 排列，形成平面三角形。当三个域均为成键电子对时，分子形状为平面三角形，如 BF₃ 和 SO₃。
If one of the three domains is a lone pair, the shape of the molecule is bent or V-shaped. Examples include SO₂ and O₃. The bond angle is slightly less than 120° because the lone pair repels bonding pairs more strongly than bonding pairs repel each other.
若三个域中有一个为孤电子对，则分子形状为角形或 V 形。例子包括 SO₂ 和 O₃。由于孤电子对对成键电子对的排斥力大于成键电子对之间的排斥力，因此键角略小于 120°。
5. Four Electron Domains: Tetrahedral, Trigonal Pyramidal and Bent | 四个电子域：四面体、三角锥与角形
Four electron domains adopt a tetrahedral arrangement with ideal bond angles of 109.5°. When all four domains are bonding pairs, the molecule is tetrahedral. Common examples are CH₄, CCl₄ and NH₄⁺.
四个电子域采取四面体排列，理想键角为 109.5°。当四个域均为成键电子对时，分子为四面体形。常见例子包括 CH₄、CCl₄ 和 NH₄⁺。
With one lone pair and three bonding pairs, the shape is trigonal pyramidal. NH₃ and H₃O⁺ are typical examples. The bond angle is about 107°, reduced from 109.5° because the lone pair occupies more space and presses the three bonding pairs closer together.
若含有一个孤电子对和三个成键电子对，则为三角锥形。NH₃ 和 H₃O⁺ 是典型例子。键角约为 107°，比 109.5° 有所减小，因为孤电子对占据更大空间并将三个成键电子对压得更近。
With two lone pairs and two bonding pairs, the shape is bent or V-shaped. H₂O and H₂S show this geometry. The bond angle in H₂O is about 104.5°, compressed by the two lone pairs.
若含有两个孤电子对和两个成键电子对，则为角形或 V 形。H₂O 和 H₂S 呈现这种几何结构。H₂O 中的键角约为 104.5°，被两个孤电子对压缩。
6. Five Electron Domains: Trigonal Bipyramidal and Related Shapes | 五个电子域：三角双锥及相关形状
Five electron domains form a trigonal bipyramidal arrangement. Two axial positions lie above and below the central atom, and three equatorial positions lie in a plane. Axial-equatorial angles are 90°, while equatorial-equatorial angles are 120°.
五个电子域形成三角双锥排列。两个轴向位置位于中心原子的上方和下方，三个赤道位置位于同一平面内。轴向与赤道之间的夹角为 90°，赤道与赤道之间的夹角为 120°。
When all five domains are bonding pairs, the molecule is trigonal bipyramidal. PF₅ and PCl₅ are well-known examples. No lone pair is present, so the ideal angles are 90° and 120°.
当五个域均为成键电子对时，分子为三角双锥形。PF₅ 和 PCl₅ 是常见例子。没有孤电子对，因此理想角度为 90° 和 120°。
Replacing bonding pairs with lone pairs gives further shapes. One lone pair produces a see-saw shape, as in SF₄. Two lone pairs produce a T-shaped molecule, as in ClF₃. Three lone pairs leave two bonding pairs opposite each other, producing a linear molecule, as in XeF₂. Lone pairs always occupy equatorial positions first, because this minimises repulsion with other electron pairs.
用孤电子对替代成键电子对会产生更多形状。一个孤电子对产生跷跷板形，如 SF₄；两个孤电子对产生 T 形分子，如 ClF₃；三个孤电子对使两个成键电子对彼此相对，产生直线形分子，如 XeF₂。孤电子对总是优先占据赤道位置，因为这样可使与其他电子对的排斥力最小。
7. Six Electron Domains: Octahedral and Square Planar | 六个电子域：八面体与平面正方形
Six electron domains arrange octahedrally, with all bond angles equal to 90°. SF₆ and PCl₆⁻ are classic examples of octahedral molecules and ions, with six bonding pairs and no lone pairs.
六个电子域以八面体排列，所有键角均为 90°。SF₆ 和 PCl₆⁻ 是八面体分子和离子的典型例子，它们有六个成键电子对且没有孤电子对。
With one lone pair, the shape becomes square pyramidal. Examples are BrF₅ and IF₅. The bond angles are slightly less than 90° because the lone pair pushes the five bonding pairs away.
若含有一个孤电子对，则形状变为四方锥形。例子包括 BrF₅ 和 IF₅。由于孤电子对推开五个成键电子对，键角略小于 90°。
With two lone pairs, the shape is square planar. XeF₄ is the most common example. The two lone pairs occupy opposite positions above and below the plane, and the four fluorine atoms lie at the corners of a square with 90° bond angles.
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