📚 Shapes of Molecules and Ions | 分子与离子的形状
The three-dimensional arrangement of atoms in a molecule or ion is not random – it is determined by the repulsion between electron pairs around the central atom. Mastering this topic is essential for A-Level Chemistry, as it explains physical properties, bond angles and reactivity patterns.
分子或离子中原子的三维排列并非随机——它由中心原子周围电子对之间的排斥力决定。掌握这一主题对 A-Level 化学至关重要，因为它能解释物理性质、键角以及反应活性规律。
1. Introduction to VSEPR Theory | VSEPR 理论简介
VSEPR stands for Valence Shell Electron Pair Repulsion. The theory states that the shape of a molecule or polyatomic ion is determined by the total number of electron pairs (both bonding and non-bonding) around the central atom. These electron pairs arrange themselves as far apart as possible to minimise electrostatic repulsion, thereby defining the molecular geometry.
VSEPR 代表价层电子对互斥。该理论指出，分子或多原子离子的形状由中心原子周围的电子对总数（包括成键电子对和非键电子对）决定。这些电子对会尽可能远离彼此，以最小化静电排斥，从而确定分子的几何形状。
2. Electron Pair Repulsion Rules | 电子对排斥规则
Electron pair repulsion follows a strict order: lone pair–lone pair repulsion is strongest, lone pair–bonding pair repulsion is intermediate, and bonding pair–bonding pair repulsion is weakest. Lone pairs are more diffuse and occupy a larger volume than bonding pairs, causing them to squeeze bond angles together.
电子对排斥遵循严格的顺序：孤对电子–孤对电子的排斥最强，孤对电子–成键电子对的排斥次之，成键电子对–成键电子对的排斥最弱。孤对电子比成键电子对更弥散且占据更大空间，因此会压缩键角。
- Lone pair–lone pair > Lone pair–bonding pair > Bonding pair–bonding pair
- 孤对–孤对 > 孤对–键对 > 键对–键对
Multiple bonds (double or triple) are treated as one region of electron density in VSEPR. However, they exert slightly greater repulsion than single bonds, which can lead to slight distortions of bond angles.
双键和三键在 VSEPR 中被视为一个电子密度区域。然而，它们比单键产生稍大的排斥，这可能导致键角发生轻微变化。
3. Steric Number and Basic Geometries | 空间位阻数与基本构型
The steric number of a central atom is the sum of bonded atoms plus lone pairs. Each steric number from 2 to 6 gives a characteristic electron-pair geometry. The molecular shape is derived from that geometry by ignoring the positions of lone pairs.
中心原子的空间位阻数是与其结合的原子数加上孤对电子数。从 2 到 6 的每个空间位阻数都对应一种特征性的电子对排布。分子形状就是从该排布中忽略孤对电子位置后得到的。
| Steric Number | Electron-pair geometry | Example shapes |
|---|---|---|
| 2 | Linear | Linear |
| 3 | Trigonal planar | Trigonal planar, Bent |
| 4 | Tetrahedral | Tetrahedral, Trigonal pyramidal, Bent |
| 5 | Trigonal bipyramidal | Trigonal bipyramidal, Seesaw, T-shaped, Linear |
| 6 | Octahedral | Octahedral, Square pyramidal, Square planar |
4. Linear Molecules | 直线形分子
A linear shape occurs when there are exactly two electron pairs around the central atom and no lone pairs, or when the steric number is 2. The bond angle is exactly 180°. Classic examples are beryllium chloride (BeCl₂) and carbon dioxide (CO₂).
当中心原子周围恰好有两对电子对且没有孤对电子，或者空间位阻数为 2 时，就形成直线形。键角精确为 180°。典型例子是氯化铍 (BeCl₂) 和二氧化碳 (CO₂)。
Bond angle = 180°
In BeCl₂, beryllium has two bonding pairs and no lone pairs, so the two Be–Cl bonds point in opposite directions. In CO₂, the central carbon forms two double bonds to oxygen; each double bond counts as one region of electron density, again giving a linear arrangement.
在 BeCl₂ 中，铍有两个成键电子对且没有孤对电子，因此两个 Be–Cl 键指向相反方向。在 CO₂ 中，中心碳与氧形成两个双键；每个双键计为一个电子密度区域，同样形成直线形排列。
5. Trigonal Planar Molecules | 平面三角形分子
When the central atom has three electron pairs, all of which are bonding pairs, the shape is trigonal planar with bond angles of 120°. Key examples are boron trifluoride (BF₃) and the sulfate(IV) oxide molecule SO₃.
当中心原子有三对电子对且全部为成键电子对时，形状为平面三角形，键角为 120°。主要例子有三氟化硼 (BF₃) 和三氧化硫 (SO₃)。
Bond angle = 120°
Boron in BF₃ has three single bonds and an incomplete octet (6 electrons), but the three bonding pairs spread out evenly in a plane. In the carbonate ion CO₃²⁻, resonance gives three equivalent C–O bonds, and the ion is also trigonal planar.
BF₃ 中的硼有三个单键且八隅体不完整（6 个电子），但三对成键电子在同一平面内均匀分布。在碳酸根离子 CO₃²⁻ 中，共振产生了三个等效的 C–O 键，该离子也呈平面三角形。
6. Tetrahedral Molecules and Ions | 四面体分子与离子
With four electron pairs on the central atom and no lone pairs, the basic shape is tetrahedral. All bond angles are approximately 109.5°. This is the most important geometry in organic chemistry and is seen in methane (CH₄) and the ammonium ion (NH₄⁺).
当中心原子有四对电子对且没有孤对电子时，基本形状为四面体形。所有键角约为 109.5°。这是有机化学中最重要的几何构型，见于甲烷 (CH₄) 和铵根离子 (NH₄⁺)。
Bond angle ≈ 109.5°
Methane has four identical C–H bonds arranged in a symmetrical tetrahedron. The ammonium ion, NH₄⁺, is isoelectronic with methane and also adopts a perfect tetrahedral shape despite carrying a positive charge.
甲烷有四个相同的 C–H 键，对称排列成四面体。铵根离子 NH₄⁺ 与甲烷是等电子体，尽管带正电荷，也呈现完美的四面体形状。
7. Trigonal Pyramidal: Ammonia (NH₃) | 三角锥形：氨分子 (NH₃)
When the central atom has four electron pairs but one of them is a lone pair, the molecule takes the shape of a trigonal pyramid. The bonding pairs are pushed closer together, reducing the bond angle from the ideal tetrahedral 109.5° to about 107°.
当中心原子有四对电子对，但其中一对是孤对电子时，分子呈三角锥形。成键电子对被推得更近，使键角从理想的四面体角 109.5° 减小到约 107°。
Bond angle ≈ 107°
In ammonia, the nitrogen uses three bonding pairs to link with hydrogen atoms and retains one lone pair. The lone pair–bonding pair repulsion forces the N–H bonds closer together. This is a common exam example for the effect of a lone pair on bond angles.
在氨分子中，氮用三个成键电子对与氢原子结合，并保留一对孤对电子。孤对–键对排斥迫使 N–H 键互相靠拢。这是考试中说明孤对电子对键角影响的常见例子。
8. Bent Shapes: Water and Sulfur Dioxide | 弯曲形：水和二氧化硫
A molecule with four electron pairs and two lone pairs is bent or V-shaped. Water (H₂O) is the classic case, with an experimental bond angle of 104.5°. The two lone pairs on oxygen exert strong repulsion, narrowing the H–O–H angle even further compared with ammonia.
有四对电子对和两对孤对电子的分子呈弯曲形或 V 形。水 (H₂O) 是经典例子，实验键角为 104.5°。氧原子上的两对孤对电子产生强烈排斥，使 H–O–H 键角比氨分子缩小得更多。
Bond angle ≈ 104.5°
Sulfur dioxide (SO₂) is another important bent molecule. Here the central sulfur has three electron domains: two double bonds to oxygen and one lone pair. The shape is bent with a bond angle of about 119°. The deviation from the 120° of a perfect trigonal planar arrangement is caused by the greater repulsion of the lone pair.
二氧化硫 (SO₂) 是另一个重要的弯曲形分子。中心硫有三个电子区域：两个与氧的双键和一个孤对电子。形状为弯曲形，键角约为 119°。偏离完美平面三角形 120° 的原因是孤对电子具有更大的排斥力。
9. Expanded Octet: Trigonal Bipyramidal and Octahedral | 扩展八隅体：三角双锥与八面体
Elements in Period 3 or below can expand their octet and accommodate five or six electron pairs. With five electron pairs (steric number 5), the electron-pair geometry is trigonal bipyramidal. A molecule with all five positions occupied by bonding pairs, such as phosphorus pentachloride (PCl₅), shows two distinct bond angles: 90° between axial and equatorial positions, and 120° between equatorial positions.
第三周期及以下的元素可以扩展其八隅体，容纳五对或六对电子对。空间位阻数为 5 时，电子对排布为三角双锥形。所有五个位置都被成键电子对占据的分子，例如五氯化磷 (PCl₅)，会表现出两种不同的键角：轴向与平伏位置之间为 90°，平伏位置之间为 120°。
Axial–equatorial: 90°; Equatorial–equatorial: 120°
With six electron pairs (steric number 6), the geometry is octahedral. Sulfur hexafluoride (SF₆) is a perfect example: all six S–F bonds are equivalent, and all bond angles are exactly 90°. If one or two lone pairs replace bonding pairs, the shapes become square pyramidal or square planar.
空间位阻数为 6 时，几何构型为八面体形。六氟化硫 (SF₆) 是一个完美例子：所有六个 S–F 键等同，所有键角精确为 90°。如果一个或两个成键电子对被孤对电子取代，形状将变为四方锥形或平面四边形。
10. Summary Table of Common Shapes | 常见形状总结表
| Total electron pairs | Bonding pairs | Lone pairs | Shape name | Bond angle(s) | Example |
|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | 180° | BeCl₂, CO₂ |
| 3 | 3 | 0 | Trigonal planar | 120° | BF₃, SO₃ |
| 3 | 2 | 1 | Bent (V-shaped) | ~119° | SO₂ |
| 4 | 4 | 0 | Tetrahedral | 109.5° | CH₄, NH₄⁺ |
| 4 | 3 | 1 | Trigonal pyramidal | ~107° | NH₃ |
| 4 | 2 | 2 | Bent (V-shaped) | 104.5° | H₂O |
| 5 | 5 | 0 | Trigonal bipyramidal | 90°, 120° | PCl₅ |
| 6 | 6 | 0 | Octahedral | 90° | SF₆ |
11. Common Exam Pitfalls and Tips | 常见考试陷阱与技巧
Many marks are lost by simply stating 'tetrahedral' for a molecule like NH₃. You must adjust the shape name and bond angle to account for lone pairs. Always start by counting the total number of electron pairs around the central atom, then decide which electron-pair geometry applies, and finally name the molecular shape by the positions of the atoms only.
许多失分是因为对 NH₃ 这样的分子简单地写 '四面体'。你必须根据孤对电子调整形状名称和键角。始终先从计算中心原子周围的电子对总数开始，然后确定属于哪种电子对排布，最后只根据原子的位置来命名分子形状。
- Always clearly state the number of lone pairs and bonding pairs.
- Use a stepwise method: count outer electrons, add/subtract for charge, determine bonds and lone pairs, identify shape.
- Remember that double and triple bonds count as ONE region of electron density.
- Draw a 3D wedge‑dash diagram where required.
- 对于要求画图的题目，务必使用楔形线和虚线表示三维结构。
- 务必清晰说明孤对电子和成键电子对的数目。
- 使用分步法：数外层电子、根据电荷加减、确定键和孤对电子、判断形状。
- 记住双键和三键只计为一个电子密度区域。
12. Practice and Application | 练习与应用
The best way to master molecular shapes is to practise with a wide variety of examples, including charged ions like NH₄⁺, CO₃²⁻, and NO₃⁻, as well as molecules with multiple bonds. Try drawing the Lewis structure first, then applying VSEPR rules systematically.
掌握分子形状的最佳方法是大量练习各种例子，包括 NH₄⁺、CO₃²⁻ 和 NO₃⁻ 等带电离子，以及含有双键和叁键的分子。先画出路易斯结构，再系统地应用 VSEPR 规则。
Common exam questions will ask you to compare bond angles, explain why water is bent while carbon dioxide is linear, or predict the shape of an unfamiliar species such as XeF₄ or I₃⁻. Always relate your answer back to electron pair repulsion arguments.
常见的考题会要求你比较键角、解释为什么水是弯曲形而二氧化碳是直线形，或者预测不熟悉物种如 XeF₄ 或 I₃⁻ 的形状。务必让你的答案回归到电子对排斥的论述上。
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