📚 Shapes of Molecules | 分子的形状
The three-dimensional arrangement of atoms in a molecule is not random; it follows predictable patterns governed by electron-pair repulsions. In Cambridge A-Level Chemistry, mastering molecular shapes is essential for understanding polarity, bonding and reactivity. This article covers VSEPR theory, the influence of lone pairs, multiple bonds and electronegativity, and provides a systematic method for predicting shape.
分子中原子的三维排布并非杂乱无章，而是由电子对排斥力决定的可预测模式。在剑桥A-Level化学中，掌握分子形状是理解极性、键合和反应活性的基础。本文涵盖VSEPR理论、孤对电子、多重键和电负性的影响，并提供预测形状的系统方法。
1. The VSEPR Theory | 价层电子对互斥理论
Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs surrounding a central atom will arrange themselves as far apart as possible to minimise repulsion. The geometry adopted depends on the total number of electron pairs (bonding plus lone pairs), often called the steric number.
价层电子对互斥理论 (VSEPR) 指出，中心原子周围的电子对会尽可能远离彼此以便使排斥最小化。所采取的几何构型取决于电子对总数（成键对加孤对），常被称为位阻数。
Each electron pair is a region of negative charge, and since like charges repel, the molecular shape is a direct result of repulsion minimisation. The theory treats double and triple bonds as a single region of electron density, but with increased repulsion strength compared to single bonds.
每个电子对是一个负电荷区域，由于同种电荷相斥，分子形状直接源于排斥最小化。该理论将双键和三键视为单个电子密度区域，但与单键相比具有更强的排斥力。
2. Types of Electron Pairs and Repulsion Order | 电子对种类与排斥顺序
Electron pairs on a central atom are classified as bonding pairs (shared between atoms) or lone pairs (non-bonding). Lone pairs are held closer to the nucleus and occupy a larger angular space, so they repel other electron pairs more strongly than bonding pairs do.
中心原子上的电子对分为成键电子对（原子间共享）和孤对电子（非键合）。孤对电子受核吸引更近并占据更大的角空间，因此它们对其他电子对的排斥力强于成键电子对。
The repulsion strength order is: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. This order explains progressive reductions in bond angles as the number of lone pairs increases in a given geometry.
排斥力的大小顺序为：孤对–孤对 > 孤对–键对 > 键对–键对。这一顺序解释了在给定几何构型中，随着孤对电子数量增加而键角逐渐减小的现象。
3. Two Electron Pairs: Linear Geometry | 两对电子：直线形
When a central atom has two electron pairs, both bonding, the most stable arrangement is linear with a bond angle of 180°. Beryllium chloride (BeCl2) in the vapour phase and carbon dioxide (CO2) are classic examples.
当中心原子有两对电子且均为成键对时，最稳定的排布是直线形，键角180°。气态氯化铍 (BeCl2) 和二氧化碳 (CO2) 是典型例子。
Even if both regions are double bonds (as in CO2), the shape remains linear because the two regions of electron density position themselves on opposite sides of the central atom.
即使两个区域都是双键（如CO2），形状仍为直线形，因为两个电子密度区域位于中心原子的相反两侧。
4. Three Electron Pairs: Trigonal Planar and Bent | 三对电子：平面三角形与弯曲形
With three bonding pairs and no lone pairs, electron pairs spread evenly in a plane, giving a trigonal planar shape with bond angles of 120°. Boron trifluoride (BF3) illustrates this perfectly.
拥有三个成键电子对且无孤对电子时，电子对在平面上均匀展开，形成平面三角形，键角120°。三氟化硼 (BF3) 恰好展现了这一点。
If one bonding pair is replaced by a lone pair, the molecular shape becomes bent (or V-shaped). The repulsive power of the lone pair compresses the remaining bond angle to slightly below 120°; in sulfur dioxide (SO2), the O–S–O angle is approximately 119°.
如果一个成键对被一个孤对取代，分子形状变为弯曲形（或V形）。孤对的排斥力将剩余的键角压缩至略低于120°；二氧化硫 (SO2) 中 O–S–O 角约为119°。
The electron-pair geometry is still trigonal planar, but the molecular shape (atom positions) is bent. This distinction is important when naming the shape.
电子对几何构型仍然是平面三角形，但分子形状（原子位置）为弯曲形。在命名形状时这一区别很重要。
5. Four Electron Pairs: The Tetrahedral Family | 四对电子：四面体家族
A central atom with four bonding pairs adopts a tetrahedral arrangement to minimise repulsion, with ideal bond angles of 109.5°. Methane (CH4) and ammonium ion (NH4+) are perfect tetrahedral examples.
中心原子有四对成键电子对时采取四面体排布以最小化排斥，理想键角为109.5°。甲烷 (CH4) 和铵根离子 (NH4+) 是完美四面体的例子。
When one lone pair is present, the shape becomes trigonal pyramidal. The lone pair squeezes the bond angles; ammonia (NH3) has an H–N–H angle of 107°.
存在一个孤对电子时，形状变为三角锥形。孤对压缩了键角；氨 (NH3) 的 H–N–H 角为107°。
With two lone pairs, the shape is bent (or angular). Water (H2O) has a bond angle of 104.5°, illustrating the cumulative effect of two lone-pair repulsions.
有两个孤对时，形状为弯曲形（或角形）。水 (H2O) 的键角为104.5°，体现了两个孤对排斥的累积效应。
The trend 109.5° → 107° → 104.5° demonstrates that each additional lone pair decreases the bond angle by roughly 2–2.5°.
109.5° → 107° → 104.5° 的趋势表明每增加一个孤对，键角约减小2–2.5°。
6. Five Electron Pairs: Trigonal Bipyramidal | 五对电子：三角双锥
Five bonding pairs give a trigonal bipyramidal shape. There are two distinct positions: three equatorial positions (120° apart) and two axial positions (90° to the equatorial plane). Phosphorus pentachloride (PCl5) is a standard example.
五个成键电子对形成三角双锥形状。存在两类不同的位置：三个赤道位（彼此120°）和两个轴位（与赤道平面成90°）。五氯化磷 (PCl5) 是标准例子。
When lone pairs are introduced, they preferentially occupy equatorial positions. This is because equatorial sites have only two 90° interactions with axial pairs, whereas an axial lone pair would destabilise the molecule with three 90° repulsions.
引入孤对电子时，它们优先占据赤道位置。因为赤道位只有两个与轴对呈90°的相互作用，而轴位孤对会产生三个90°排斥，使分子不稳定。
With one lone pair, the shape is seesaw (SF4); with two lone pairs, it is T-shaped (ClF3); with three lone pairs, the remaining two bonding pairs occupy axial positions, giving a linear shape (XeF2).
有一个孤对时，形状为翘翘板形 (SF4)；有两个孤对时是T形 (ClF3)；有三个孤对时，剩下的两个成键对占据轴位，形成直线形 (XeF2)。
7. Six Electron Pairs: Octahedral and Beyond | 六对电子：八面体及其变体
Six bonding pairs around a central atom produce an octahedral shape with all bond angles equal to 90°. Sulfur hexafluoride (SF6) is a textbook octahedral molecule.
中心原子周围的六个成键电子对产生八面体形状，所有键角均为90°。六氟化硫 (SF6) 是典型的八面体分子。
One lone pair results in a square pyramidal shape, as in bromine pentafluoride (BrF5). The lone pair occupies one vertex, pushing the opposite four atoms into a square base.
一个孤对电子导致四角锥形，如五氟化溴 (BrF5)。孤对占据一个顶点，将相对的四个原子推入正方形底面。
Two lone pairs adopt opposite positions to minimise repulsion, yielding a square planar shape. Xenon tetrafluoride (XeF4) is a perfect example, with four fluorine atoms at the corners of a square and lone pairs above and below the plane.
两个孤对占据相对位置以减少排斥，产生平面正方形。四氟化氙 (XeF4) 是典型例子，四个氟原子位于正方形的角上，孤对在平面的上方和下方。
8. Electronegativity Effects on Bond Angles | 电负性对键角的影响
The identity of the atoms bonded to the central atom can significantly alter bond angles. When highly electronegative atoms such as fluorine replace hydrogen, the bonding electron cloud is pulled away from the central atom, weakening repulsion among bonding pairs.
与中心原子键合的原子的种类会显著改变键角。当高电负性的原子（如氟）取代氢时，成键电子云被拉离中心原子，减弱了成键对之间的排斥。
For instance, the bond angle in NF3 (102°) is smaller than in NH3 (107°), despite both having a trigonal pyramidal shape. The fluorine atoms reduce bonding-pair repulsion around nitrogen, allowing the lone pair to compress the angle further.
例如，NF3 的键角 (102°) 小于 NH3 的 (107°)，尽管两者都是三角锥形。氟原子减少了氮周围成键对排斥，使得孤对能够进一步压缩键角。
Conversely, if a less electronegative atom such as phosphorus serves as the central atom, the bonding pairs stay closer and the angle opens slightly compared to nitrogen analogues, but this is often overshadowed by other factors such as orbital hybridisation.
相反地，如果中心原子是电负性较小的磷，成键电子对保持更近，键角相对于氮类似物会有所增大，但这通常被轨道杂化等其他因素掩盖。
9. Influence of Multiple Bonds on Geometry | 多重键对几何形状的影响
Double and triple bonds contain greater electron density than single bonds and thus repel neighbouring bonding pairs more strongly. This distorts ideal bond angles.
双键和三键比单键含有更高的电子密度，因此对相邻成键对的排斥更强，导致理想键角变形。
In formaldehyde (H2CO), the carbon atom has three regions of electron density (two single bonds to H and one double bond to O). The C=O double bond repels the C–H bonds more than they repel each other, so the H–C–H angle is compressed to about 118°, while the O=C–H angle expands to roughly 121°.
在甲醛 (H2CO) 中，碳原子有三个电子密度区域（两个 C–H 单键和一个 C=O 双键）。C=O 双键对 C–H 键的排斥强于 C–H 键之间的排斥，因此 H–C–H 角被压缩至约118°，而 O=C–H 角扩大至约121°。
This effect is also seen in alkenes, where the >C=C< bond leads to a planar arrangement around the carbon atoms, with bond angles close to 120°, but slight deviations can occur due to substituent repulsions.
这种效应也见于烯烃，其中 >C=C< 键导致碳原子周围平面排布，键角接近120°，但由于取代基排斥可能发生微小偏差。
10. Systematic Method to Determine Shape | 确定分子形状的系统方法
To confidently predict any molecule’s shape, follow these steps: (1) Identify the central atom (usually the least electronegative, or the one with the highest bonding capacity). (2) Count its valence electrons. (3) Add one electron for each single bond formed with surrounding atoms. (4) Adjust for charge: add one electron for each negative charge; remove one for each positive charge. (5) Divide the total by 2 to get the number of electron pairs (steric number). (6) Subtract the number of bonded atoms from the steric number to find lone pairs. (7) Name the electron-pair geometry and then the molecular shape based on the positions of atoms only.
为了准确预测任何分子的形状，可遵循以下步骤：(1) 确定中心原子（通常是电负性最低的或键合能力最强的原子）；(2) 计算其价电子数；(3) 为每个与周围原子形成的单键添加一个电子；(4) 根据电荷调整：每个负电荷加一个电子，每个正电荷减一个电子；(5) 总数除以2得到电子对数（位阻数）；(6) 从位阻数中减去键合原子数以获得孤对电子数；(7) 先命名电子对几何构型，然后仅根据原子位置命名分子形状。
For CO2: C has 4 valence e–, no charge, forms two double bonds (treated as 2 bonding regions) → steric number = 2 → 0 lone pairs → linear. For SO2: S has 6, no charge, two bonding regions → total pairs = 3, with one lone pair → bent.
以 CO2 为例：C 有4个价电子，无电荷，形成两个双键（视为2个成键区域）→ 位阻数 = 2 → 0个孤对 → 直线形。以 SO2 为例：S 有6个价电子，无电荷，两个成键区域 → 总电子对 = 3，包含一个孤对 → 弯曲形。
11. Why Molecular Shape Matters | 分子形状为何重要
Molecular shape determines whether a molecule is polar or non-polar. A symmetrical shape can cancel individual bond dipoles, making the molecule non-polar (e.g., CO2, CCl4), while asymmetry leaves a net dipole moment (H2O, NH3).
分子形状决定了分子是极性还是非极性。对称的形状可抵消单个键的偶极矩，使分子非极性（如 CO2、CCl4），而不对称结构则留下净偶极矩（H2O、NH3）。
Shape also influences intermolecular forces, boiling points, solubility, and how molecules interact with biological receptors. The bent shape of water enables strong hydrogen bonding, giving it unusual properties vital for life. In organic chemistry, the tetrahedral shape of carbon leads to chirality, which affects drug design and enzyme specificity.
形状还影响分子间作用力、沸点、溶解度以及分子与生物受体的相互作用。水的弯曲形状使其能形成强氢键，赋予其对生命至关重要的不寻常性质。在有机化学中，碳的四面体形状导致手性，影响药物设计和酶特异性。
12. Common Pitfalls and Exam Tips | 常见错误与考试技巧
Students often confuse electron-pair geometry with molecular shape. Remember that the shape name refers only to the positions of atoms; lone pairs are ‘invisible’ in the final shape. For example, NH3 has tetrahedral electron-pair geometry but trigonal pyramidal molecular shape.
学生们常混淆电子对几何构型与分子形状。请记住，形状名称仅指原子的位置；孤对电子在最终形状中是“不可见的”。例如 NH3 具有四面体电子对几何构型，但分子形状为三角锥形。
Miscounting valence electrons is another frequent mistake. Always draw a Lewis structure first and double-check the number of electron pairs. Practice with charged species like NH4+, SO42- and unusual cases like XeF2 to build confidence.
价电子计数错误是另一个常见错误。务必先画出路易斯结构，并复核电子对数。练习带电荷物种如 NH4+、SO42- 以及特殊例子如 XeF2，以建立自信。
Finally, when describing the effect of lone pairs on bond angles, use the correct repulsion order and relate it to observed angles (e.g., NH3 107°, H2O 104.5°). Examiners expect precise values and a qualitative explanation.
最后，在描述孤对对键角的影响时，使用正确的排斥顺序并将其与观测的键角联系起来（如 NH3 107°、H2O 104.5°）。考官期望精确的数值和定性解释。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导