📚 More Molecular Shapes | 更多分子形状
In A-Level Chemistry, understanding molecular shape is essential for predicting polarity, reactivity and physical properties. The Valence Shell Electron Pair Repulsion (VSEPR) theory provides a simple yet powerful model to determine the three-dimensional arrangement of atoms in a molecule. This article extends beyond the basic shapes (linear, bent, trigonal planar, tetrahedral) to explore more complex geometries such as trigonal bipyramidal and octahedral, and explains how lone pairs modify bond angles and shapes.
在A-Level化学中，理解分子形状对于预测极性、反应性和物理性质至关重要。价层电子对互斥（VSEPR）理论提供了一个简单而强大的模型，用于确定分子中原子的三维排列。本文在基本形状（直线形、角形、平面三角形、四面体形）的基础上，进一步探讨更复杂的几何构型，如三角双锥形和八面体形，并解释孤对电子如何改变键角和形状。
1. VSEPR Theory Recap | VSEPR理论回顾
VSEPR theory states that electron pairs around a central atom arrange themselves to minimise repulsion. Both bonding pairs (shared between atoms) and lone pairs (non-bonding) occupy space around the central atom. The shape is determined by the total number of electron pairs, but the molecular shape name refers only to the positions of atoms, not lone pairs.
VSEPR理论指出，中心原子周围的电子对会自行排列以使排斥力最小化。成键电子对（原子间共享）和孤对电子（未成键）都占据中心原子周围的空间。分子形状由电子对总数决定，但分子形状的名称仅指原子的位置，不包括孤对电子。
The electron-pair geometry describes the arrangement of all electron pairs, whereas the molecular geometry describes the arrangement of atoms only. For example, ammonia (NH₃) has four electron pairs (tetrahedral electron-pair geometry) but the atoms form a trigonal pyramidal shape because one position is occupied by a lone pair.
电子对几何构型描述所有电子对的排列，而分子几何构型仅描述原子的排列。例如，氨（NH₃）有四对电子对（四面体电子对几何构型），但原子形成三角锥形，因为其中一个位置被孤对电子占据。
2. Electron Pair Repulsion Hierarchy | 电子对排斥层级
Repulsion strengths follow this order: lone-pair/lone-pair > lone-pair/bonding-pair > bonding-pair/bonding-pair. This hierarchy explains why the presence of lone pairs compresses bond angles below the ideal values for a given electron-pair geometry.
排斥力的强弱顺序为：孤对-孤对 > 孤对-成键 > 成键-成键。这一层级解释了为什么孤对电子的存在会使键角压缩到低于给定电子对几何构型的理想值。
Lone pairs are held closer to the nucleus and occupy a larger spatial domain, causing stronger repulsion. Multiple bonds (double or triple) repel more strongly than single bonds because they contain higher electron density, which can subtly alter bond angles.
孤对电子更靠近原子核，占据更大的空间区域，因此产生更强的排斥力。多重键（双键或三键）由于电子密度更高，比单键排斥更强，这可以微妙地改变键角。
3. Two Electron Pairs: Linear | 两对电子对：直线形
When a central atom is surrounded by two electron pairs and both are bonding pairs, the optimal arrangement is linear with a bond angle of 180°. Carbon dioxide (CO₂) is the classic example: O=C=O. Beryllium chloride (BeCl₂) in the gas phase also adopts a linear shape.
当中心原子被两对电子对包围且均为成键电子对时，最佳排列是直线形，键角为180°。二氧化碳（CO₂）是经典例子：O=C=O。气态氯化铍（BeCl₂）也呈直线形。
There is also a linear shape that arises from five electron pairs with three lone pairs (discussed later). However, the simplest linear molecules have only two bonding pairs and no lone pairs on the central atom.
此外，还有一种直线形来源于五对电子对搭配三对孤对电子的情况（稍后讨论）。然而，最简单的直线形分子中心原子只有两对成键电子对且没有孤对电子。
4. Three Electron Pairs: Trigonal Planar and Bent | 三对电子对：平面三角形和角形
Three electron pairs adopt a trigonal planar arrangement with 120° angles. If all three pairs are bonding, the molecular shape is trigonal planar, e.g. BF₃. If one pair is a lone pair, the shape is bent (or V-shaped), with a bond angle slightly less than 120° due to lone-pair repulsion, as in SO₂ (approx. 119°).
三对电子对采用平面三角形排列，键角120°。如果三对均为成键电子对，分子形状为平面三角形，例如BF₃。如果有一对是孤对电子，形状为角形（或V形），由于孤对电子的排斥，键角略小于120°，如SO₂（约119°）。
The electron-pair geometry remains trigonal planar, but the molecular geometry only considers the positions of the two oxygen atoms and the sulfur atom. Tin(II) chloride (SnCl₂) is another bent molecule with a lone pair on tin.
电子对几何构型仍为平面三角形，但分子几何构型仅考虑两个氧原子和硫原子的位置。二氯化锡（SnCl₂）是另一种角形分子，锡上有一对孤对电子。
5. Four Electron Pairs: Tetrahedral, Trigonal Pyramidal, Bent | 四对电子对：四面体形、三角锥形、角形
Four electron pairs arrange tetrahedrally with ideal bond angles of 109.5°. With four bonding pairs, the molecule is tetrahedral (e.g. CH₄, SiCl₄). With three bonding pairs and one lone pair, the shape becomes trigonal pyramidal – ammonia (NH₃) has a bond angle of 107°. With two bonding pairs and two lone pairs, the shape is bent – water (H₂O) has a bond angle of 104.5°.
四对电子对呈四面体排列，理想键角为109.5°。四对均为成键时分子为四面体形（如CH₄、SiCl₄）。三对成键加一对孤对时，形状变为三角锥形——氨（NH₃）的键角为107°。两对成键加两对孤对时，形状为角形——水（H₂O）的键角为104.5°。
The progressive decrease in bond angle from 109.5° to 107° to 104.5° is due to the increasing number of lone pairs, which exert stronger repulsion and compress the bonding pairs closer together.
键角从109.5°逐步减小到107°再到104.5°，是因为孤对电子数量增加，它们施加更强的排斥力，将成键电子对挤压得更近。
6. Five Electron Pairs: Trigonal Bipyramidal and Derivatives | 五对电子对：三角双锥形及其衍生物
Five electron pairs adopt a trigonal bipyramidal geometry, with two distinct positions: axial (perpendicular to the equatorial plane, 90° to equatorial bonds) and equatorial (in the trigonal plane, 120° to each other). In PCl₅, all five pairs are bonding, giving a trigonal bipyramidal shape with axial Cl–P–Cl angles of 90° and equatorial angles of 120°.
五对电子对采用三角双锥几何构型，有两种不同的位置：轴向（垂直于赤道平面，与赤道键成90°）和赤道（在三角平面内，彼此成120°）。在PCl₅中，五对均为成键电子对，形成三角双锥形，轴向Cl–P–Cl角为90°，赤道角为120°。
Lone pairs occupy equatorial positions to minimise repulsion. If there is one lone pair, the shape is seesaw (or sawhorse) with a bond angle around 90° and 120°, e.g. SF₄. Two lone pairs give a T-shaped molecule, e.g. ClF₃ (bond angles approx. 87.5°). Three lone pairs yield a linear shape, e.g. XeF₂ (bond angle 180°), as the three lone pairs occupy the equatorial plane, leaving two axial bonding pairs.
孤对电子占据赤道位置以最小化排斥力。若有一对孤对，形状为跷跷板形（或锯马形），键角约为90°和120°，例如SF₄。两对孤对产生T形分子，例如ClF₃（键角约87.5°）。三对孤对则产生直线形，例如XeF₂（键角180°），因为三对孤对占据赤道平面，留下两个轴向成键电子对。
7. Six Electron Pairs: Octahedral and Derivatives | 六对电子对：八面体形及其衍生物
Six electron pairs form an octahedral arrangement with bond angles of 90°. Sulfur hexafluoride (SF₆) is a perfect octahedron. If one bonding pair is replaced by a lone pair, the shape becomes square pyramidal – as in BrF₅, where the lone pair occupies one vertex of the octahedron, pushing the four equatorial fluorine atoms slightly upward. If two lone pairs are present, they occupy opposite vertices to minimise repulsion, giving a square planar shape, as in XeF₄ (bond angles 90°).
六对电子对形成八面体排列，键角为90°。六氟化硫（SF₆）是完美的八面体。如果一个成键电子对被孤对替换，形状变为四方锥形——如BrF₅，孤对占据八面体的一个顶点，将四个赤道氟原子略微上推。若存在两对孤对，它们占据相对顶点以最小化排斥力，形成平面正方形，例如XeF₄（键角90°）。
The square pyramidal shape can be visualised as an octahedron missing one corner; the square planar shape is like an octahedron missing two opposite corners. Both shapes are common in transition metal complexes.
四方锥形可以想象为八面体缺少一个顶点；平面正方形相当于八面体缺少两个相对的顶点。这两种形状在过渡金属配合物中很常见。
8. Effect of Lone Pairs on Bond Angles | 孤对电子对键角的影响
Each lone pair reduces the bond angle by roughly 2° to 2.5° compared to the ideal angle, depending on the central atom and bonding atoms. For example, CH₄ (no lone pairs) has a bond angle of 109.5°; NH₃ (one lone pair) has 107°; H₂O (two lone pairs) has 104.5°. In group 15 hydrides, the angle decreases as the central atom becomes larger and less electronegative (NH₃ 107°, PH₃ 93.5°, AsH₃ 91.8°), which is also explained by reduced bond-pair repulsion due to diffuse bonding orbitals.
每对孤对电子会使键角相比理想角大约减少2°至2.5°，具体取决于中心原子和键合原子。例如，CH₄（无孤对）键角为109.5°；NH₃（一对孤对）为107°；H₂O（两对孤对）为104.5°。在第15族氢化物中，随着中心原子变大、电负性降低，键角减小（NH₃ 107°，PH₃ 93.5°，AsH₃ 91.8°），这也可以用扩散的键合轨道导致成键电子对斥力减小来解释。
The position of lone pairs in trigonal bipyramidal and octahedral geometries is crucial: they always go where they experience the least repulsion – equatorial for 5-pair systems, opposite for 6-pair systems.
孤对电子在三角双锥和八面体几何构型中的位置至关重要：它们总是位于排斥力最小的地方——五对体系占据赤道位置，六对体系占据相对位置。
9. Multiple Bonds and Molecular Shape | 多重键与分子形状
In VSEPR theory, a double or triple bond is treated as one electron-pair region, but it occupies more space than a single bond because of the higher electron density. This can slightly distort bond angles. For instance, in carbonyl dichloride (COCl₂), the C=O double bond repels the two C–Cl bonds, causing the Cl–C–Cl angle to contract to around 111° instead of the ideal 120°.
在VSEPR理论中，双键或三键被视为一个电子对区域，但由于电子密度更高，它比单键占据更多空间。这可能会轻微扭曲键角。例如，在碳酰氯（COCl₂）中，C=O双键排斥两个C–Cl键，导致Cl–C–Cl角收缩至约111°，而非理想的120°。
Similarly, in molecules like SO₂, the S=O double bonds are considered as two regions along with one lone pair, giving a bent shape. The bond angle is reduced from 120° to 119° due to lone-pair repulsion and the additional space requirement of the double bonds.
同样，在SO₂等分子中，S=O双键被视为两个区域加上一对孤对电子，产生角形。键角从120°减小到119°，这是由于孤对排斥和双键额外空间需求共同作用。
10. Step-by-Step Prediction Guide | 逐步预测指南
To predict a molecular shape: Step 1 – Count the central atom’s valence electrons, add electrons from surrounding atoms (each atom contributes one electron per bond), adjust for charges, and divide by two to get the number of electron pairs. Step 2 – Determine the electron-pair geometry (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral). Step 3 – Deduct lone pairs to find the molecular shape. Step 4 – Estimate bond angles, taking lone-pair compression into account.
预测分子形状的步骤：第一步 – 计算中心原子的价电子数，加上周边原子提供的电子（每个原子每个键提供一个电子），考虑电荷调整，除以二得到电子对数。第二步 – 确定电子对几何构型（直线形、平面三角形、四面体、三角双锥、八面体）。第三步 – 扣除孤对电子，得出分子形状。第四步 – 估算键角，考虑孤对电子压缩效应。
For example, for XeF₂: Xe has 8 valence electrons, each F provides 1 electron, total = 10 electrons, giving 5 electron pairs. With 2 bonding pairs and 3 lone pairs, the electron-pair geometry is trigonal bipyramidal; the molecular shape is linear (axial bonds, equatorial lone pairs). Bond angle = 180°.
例如XeF₂：Xe有8个价电子，每个F提供1个电子，总数为10个电子，即5对电子对。2对成键加3对孤对，电子对几何构型为三角双锥；分子形状为直线形（轴向键，赤道孤对）。键角=180°。
11. Common Mistakes to Avoid | 常见错误避免
Many students confuse electron-pair geometry with molecular shape. Remember: molecular shape only describes atom positions. Another error is forgetting to count lone pairs properly, leading to an incorrect shape. Also, assume each double or triple bond is ONE electron-pair region, not multiple. Finally, do not apply octahedral geometry to central atoms with fewer than six electron pairs; expanded octets are only possible from period 3 onwards.
许多学生混淆电子对几何构型与分子形状。请记住：分子形状仅描述原子位置。另一个错误是未正确计算孤对电子，导致形状判断错误。此外，双键或三键应被视为一个电子对区域，而不是多个。最后，不要将八面体几何构型应用于少于六对电子对的中心原子；扩展八隅体仅从第3周期起才有可能。
Another frequent pitfall is placing lone pairs in axial positions of a trigonal bipyramid – they must go equatorial. Similarly, in octahedral species, two lone pairs go opposite each other, not adjacent.
另一个常见陷阱是将孤对电子放在三角双锥的轴向位置——它们必须放在赤道。同样，在八面体物种中，两对孤对电子应占据相对位置，而非相邻。
12. Summary Table of Shapes | 形状总结表
| Electron Pairs | Bonding Pairs / Lone Pairs | Molecular Shape | Bond Angle (°) | Examples |
|---|---|---|---|---|
| 2 | 2 / 0 | Linear | 180 | CO₂, BeCl₂ |
| 3 | 3 / 0 | Trigonal planar | 120 | BF₃ |
| 3 | 2 / 1 | Bent (V-shaped) | <120 (e.g. 119) | SO₂, SnCl₂ |
| 4 | 4 / 0 | Tetrahedral | 109.5 | CH₄, SiCl₄ |
| 4 | 3 / 1 | Trigonal pyramidal | ~107 | NH₃, PH₃ |
| 4 | 2 / 2 | Bent (V-shaped) | 104.5 | H₂O, SCl₂ |
| 5 | 5 / 0 | Trigonal bipyramidal | 90, 120 | PCl₅ |
| 5 | 4 / 1 | Seesaw | <90, <120 | SF₄ |
| 5 | 3 / 2 | T-shaped | ~87.5 | ClF₃ |
| 5 | 2 / 3 | Linear | 180 | XeF₂ |
| 6 | 6 / 0 | Octahedral | 90 | SF₆ |
| 6 | 5 / 1 | Square pyramidal | Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信） CommentsMore posts |
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