📚 VSEPR Theory: Predicting Molecular and Ionic Shapes | VSEPR模型预测分子与离子形状
The Valence Shell Electron Pair Repulsion (VSEPR) theory is a powerful model that predicts the three-dimensional arrangement of atoms in molecules and polyatomic ions. It is based on the simple idea that electron pairs around a central atom repel one another and therefore adopt a geometry that minimises this repulsion.
价层电子对互斥理论（VSEPR）是一种强有力的模型，用于预测分子和多原子离子的三维空间排列。其基本思想是：中心原子周围的电子对相互排斥，因此会采取能最大程度减小这种排斥的几何构型。
1. Core Principle of VSEPR | VSEPR的核心原理
Electron pairs in the valence shell of a central atom are localised in bonding regions and lone pairs. Because negatively charged electron clouds repel each other, the most stable arrangement is the one that maximises the distance between all electron pairs.
中心原子价层中的电子对分布于成键区域和孤对电子区域。由于带负电的电子云相互排斥，最稳定的排列方式就是使所有电子对之间距离最大的几何构型。
The geometry of a molecule is determined by the total number of electron pairs around the central atom, not just by the number of atoms bonded to it. Lone pairs occupy space just like bonding pairs, and they influence the overall shape.
分子的几何构型由中心原子周围电子对的总数决定，而不仅仅是由与其成键的原子数决定。孤对电子与成键电子对一样占据空间，并影响整体形状。
2. Counting Valence Electron Pairs | 计算价层电子对数
To apply VSEPR, first determine the total number of valence electrons on the central atom, then add electrons from bonded atoms or adjust for ionic charge.
应用VSEPR理论时，首先确定中心原子的价电子总数，然后加上成键原子提供的电子，并根据离子电荷进行调整。
For a neutral molecule ABₙ, the steric number (SN) equals the number of atoms bonded to the central atom plus the number of lone pairs on the central atom. A common method is: SN = (number of valence electrons on central atom + number of monovalent atoms bonded + charge) ÷ 2.
对于中性分子ABₙ，空间位数（SN）等于中心原子成键原子数加上中心原子上的孤对电子数。常用计算方法是：SN =（中心原子价电子数 + 成键的单价原子数 + 电荷数）÷ 2。
For example, in CO₂, carbon has 4 valence electrons, each oxygen contributes no monovalent count, so SN = 4 ÷ 2 = 2, giving a linear shape.
例如，在CO₂中，碳有4个价电子，每个氧不贡献单价原子数，因此SN = 4 ÷ 2 = 2，得到直线形。
3. Electron-Pair Geometries | 电子对几何构型
The total number of electron pairs (bonding and lone pairs) around the central atom determines the electron-pair geometry. For SN = 2, 3, 4, 5, and 6, the ideal geometries are linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, respectively.
中心原子周围电子对总数（成键对和孤对）决定了电子对几何构型。当SN = 2、3、4、5、6时，理想几何构型分别为直线形、平面三角形、正四面体形、三角双锥形和正八面体形。
These ideal geometries place electron pairs as far apart as possible. For example, four electron pairs point to the corners of a tetrahedron, with bond angles of approximately 109.5°.
这些理想几何构型使电子对尽可能远离。例如，四对电子指向正四面体的顶点，键角约为109.5°。
| Steric Number | Electron-Pair Geometry | Ideal Bond Angle |
| 2 | Linear | 180° |
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal bipyramidal | 90°, 120° |
| 6 | Octahedral | 90° |
4. Molecular Shapes with No Lone Pairs | 无孤对电子的分子形状
When all electron pairs are bonding pairs, the molecular shape is identical to the electron-pair geometry. For example, CH₄ (SN = 4, no lone pairs) is tetrahedral with 109.5° bond angles, and BF₃ (SN = 3, no lone pairs) is trigonal planar.
当所有电子对都是成键对时，分子形状与电子对几何构型相同。例如，CH₄（SN = 4，无孤对）呈正四面体形，键角为109.5°；BF₃（SN = 3，无孤对）呈平面三角形。
Common examples include PCl₅ (trigonal bipyramidal) and SF₆ (octahedral). These symmetric molecules have ideal bond angles because all electron pairs experience equivalent repulsion.
常见例子包括PCl₅（三角双锥形）和SF₆（正八面体形）。这些对称分子具有理想的键角，因为所有电子对都受到等价的排斥作用。
5. Effect of Lone Pairs on Bond Angles | 孤对电子对键角的影响
Lone pairs occupy more space than bonding pairs because they are held by only one nucleus. Consequently, lone-pair–bonding-pair repulsions are stronger than bonding-pair–bonding-pair repulsions, compressing bond angles.
孤对电子比成键对占据更多空间，因为孤对电子仅受一个原子核吸引。因此，孤对电子与成键对之间的排斥力大于成键对之间的排斥力，从而压缩键角。
For instance, in NH₃ the H–N–H bond angle is approximately 107°, less than the ideal tetrahedral angle of 109.5°, because the lone pair pushes the bonding pairs closer together.
例如，在NH₃中，H–N–H键角约为107°，小于理想四面体角109.5°，因为孤对电子将成键对推得更近。
In H₂O, with two lone pairs, the H–O–H bond angle is about 104.5°, showing an even greater compression.
在H₂O中，由于有两个孤对电子，H–O–H键角约为104.5°，显示出更大的压缩效应。
6. Molecular Shape Categories | 分子形状分类
Starting from a tetrahedral electron-pair geometry, replacing bonding pairs with lone pairs gives different molecular shapes: NH₃ is trigonal pyramidal (SN = 4, one lone pair), and H₂O is bent or V-shaped (SN = 4, two lone pairs).
从正四面体电子对几何构型出发，用孤对电子替换成键对可以得到不同的分子形状：NH₃呈三角锥形（SN = 4，一个孤对），H₂O呈弯曲形或V形（SN = 4，两个孤对）。
For SN = 3 with one lone pair, the molecular shape is bent, as seen in SO₂ or O₃. For SN = 2 with no lone pairs, the shape is linear, such as BeCl₂ or CO₂.
当SN = 3且有1个孤对时，分子形状为弯曲形，如SO₂或O₃。当SN = 2且无孤对时，形状为直线形，如BeCl₂或CO₂。
| Total Electron Pairs | Lone Pairs | Molecular Shape | Example |
| 2 | 0 | Linear | BeCl₂ |
| 3 | 0 | Trigonal planar | BF₃ |
| 3 | 1 | Bent | SO₂ |
| 4 | 0 | Tetrahedral | CH₄ |
| 4 | 1 | Trigonal pyramidal | NH₃ |
| 4 | 2 | Bent | H₂O |
7. Treatment of Multiple Bonds | 多重键的处理
In VSEPR theory, a double or triple bond is treated as a single electron-pair region for geometry prediction. The electron density in a multiple bond is greater, but it still occupies only one position around the central atom.
在VSEPR理论中，双键或三键被视为单个电子对区域来进行几何预测。多重键中的电子密度更大，但仍然只占据中心原子周围的一个位置。
For example, CO₂ has two double bonds (O=C=O) and SN = 2, so it is linear. The carbonate ion CO₃²⁻ has one double bond and two single bonds, giving SN = 3 and a trigonal planar shape.
例如，CO₂有两个双键（O=C=O），SN = 2，因此呈直线形。碳酸根离子CO₃²⁻有一个双键和两个单键，SN = 3，呈平面三角形。
Multiple bonds exert slightly stronger repulsion than single bonds, but for shape prediction they are counted as one region. Thus, the presence of resonance structures does not alter the predicted geometry.
多重键产生的排斥力略大于单键，但在形状预测中仍算作一个区域。因此，共振结构的存在不会改变预测的几何构型。
8. Predicting the Shapes of Ions | 预测离子形状
VSEPR applies equally to polyatomic ions. The ionic charge must be included when counting valence electrons. For a cation, subtract electrons; for an anion, add electrons.
VSEPR理论同样适用于多原子离子。计算价电子时必须考虑离子电荷。阳离子要减去电子，阴离子要加上电子。
For NH₄⁺, nitrogen has 5 valence electrons, each hydrogen contributes 1 (4 H atoms), and the +1 charge means we subtract one electron: (5 + 4 − 1) ÷ 2 = 4. So NH₄⁺ is tetrahedral.
对于NH₄⁺，氮有5个价电子，每个氢贡献1个（4个H原子），+1电荷意味着减去1个电子：(5 + 4 − 1) ÷ 2 = 4。因此NH₄⁺呈正四面体形。
For NO₃⁻, nitrogen has 5 valence electrons, oxygen contributes 0 for monovalent counting (since oxygen is divalent), but the −1 charge adds one electron: (5 + 0 + 1) ÷ 2 = 3. The nitrate ion is trigonal planar.
对于NO₃⁻，氮有5个价电子，氧的二价特性使其在单价计数中贡献0，但−1电荷需要加1个电子：(5 + 0 + 1) ÷ 2 = 3。硝酸根离子呈平面三角形。
9. Expanded Octets in Period 3 and Beyond | 第三周期及以后的超价分子
Elements from the third period, such as phosphorus and sulfur, can have more than eight valence electrons because they access vacant d orbitals. This allows SN values of 5 and 6.
第三周期及以后的元素，如磷和硫，可以拥有超过8个价电子，因为它们能利用空的d轨道。这允许SN值为5和6。
In PCl₅, phosphorus has 5 bonding pairs and no lone pairs, giving a trigonal bipyramidal shape. In SF₆, sulfur has 6 bonding pairs, producing an octahedral shape.
在PCl₅中，磷有5个成键对且无孤对，形成三角双锥形。在SF₆中，硫有6个成键对，形成正八面体形。
When lone pairs exist in trigonal bipyramidal geometries, they prefer the equatorial positions (120° apart) because those positions have more space and reduce repulsion. For example, SF₄ has a seesaw shape, and BrF₃ has a T-shape.
在三角双锥几何构型中，孤对电子优先占据赤道位（相距120°），因为这些位置空间更大，能减少排斥。例如，SF₄呈跷跷板形，BrF₃呈T形。
10. Limitations and Exceptions of VSEPR | VSEPR的局限与例外
VSEPR works well for many compounds, but it fails to explain the shapes of molecules with transition metals, which often involve d-orbital hybridisation and crystal field effects. It also struggles with molecules where lone pairs are delocalised.
VSEPR对许多化合物适用，但无法解释过渡金属分子的形状，这些往往涉及d轨道的杂化和晶体场效应。对于孤对电子离域化的分子也有困难。
Some species, such as XeF₂ (SN = 5, three lone pairs), have a linear shape rather than a bent one because lone pairs occupy all three equatorial positions, leaving the two bonding pairs in axial positions.
某些物种，如XeF₂（SN = 5，三个孤对），呈直线形而不是弯曲形，因为孤对占据全部三个赤道位，剩下两个成键对位于轴位。
Moreover, VSEPR does not explain why some electron-pair geometries are distorted beyond simple lone-pair repulsion. For such cases, molecular orbital theory or valence bond theory may be needed.
此外，VSEPR不能解释为什么某些电子对几何构型的畸变超出了简单的孤对排斥。对于这些情况，可能需要用分子轨道理论或价键理论来解释。
11. Worked Examples for Exam Success | 考试典型例题精讲
Example 1: Predict the shape of SO₃. Sulfur has 6 valence electrons, each oxygen contributes 0 for monovalent count, and no charge: SN = 6 ÷ 2 = 3. All three positions are bonding pairs, so SO₃ is trigonal planar with 120° bond angles.
例1：预测SO₃的形状。硫有6个价电子，每个氧在单价计数中贡献0，且无电荷：SN = 6 ÷ 2 = 3。三个位置均为成键对，因此SO₃呈平面三角形，键角为120°。
Example 2: Predict the shape of ClO₂⁻. Chlorine has 7 valence electrons, two oxygen atoms (0 monovalent contribution), and a −1 charge: SN = (7 + 2 + 1) ÷ 2 = 5. The central Cl has 2 bonding pairs and 3 lone pairs, but with two bonding pairs, the shape is bent (based on the trigonal bipyramidal electron arrangement where lone pairs occupy equatorial positions).
例2：预测ClO₂⁻的形状。氯有7个价电子，两个氧原子（单价贡献为0），且带−1电荷：SN = (7 + 2 + 1) ÷ 2 = 5。中心Cl有2个成键对和3个孤对，但只有2个成键对，因此形状为弯曲形（基于三角双锥电子排布，孤对占据赤道位）。
Example 3: Predict the shape of I₃⁻. Iodine has 7 valence electrons, two iodine atoms (monovalent), and a −1 charge: SN = (7 + 2 + 1) ÷ 2 = 5. With three lone pairs in the equatorial plane, the molecule is linear.
例3：预测I₃⁻的形状。碘有7个价电子，两个碘原子（单价），且带−1电荷：SN = (7 + 2 + 1) ÷ 2 = 5。孤对占据赤道平面后，分子呈直线形。
12. VSEPR and Polarity | VSEPR与分子极性
Molecular shape directly affects polarity. A molecule with polar bonds may be nonpolar if the bond dipoles cancel due to symmetrical geometry, such as in CO₂ and CCl₄.
分子形状直接决定极性。具有极性键的分子如果因为几何对称使键偶极矩抵消，则可能为非极性分子，如CO₂和CCl₄。
In contrast, bent molecules like H₂O and trigonal pyramidal molecules like NH₃ are polar because the bond dipoles do not cancel. The lone pairs also enhance polarity.
相反，弯曲形的H₂O和三角锥形的NH₃是极性分子，因为键偶极矩不能相互抵消，孤对电子还增强了极性。
Thus, VSEPR not only predicts shape but also helps determine whether a molecule has a permanent dipole moment, an essential concept in intermolecular forces and physical properties.
因此，VSEPR不仅能预测形状，还能帮助判断分子是否具有永久偶极矩，这是理解分子间作用力和物理性质的重要概念。
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