📚 VSEPR: Shapes of Molecules and Ions | VSEPR理论：分子与离子的形状
The Valence Shell Electron Pair Repulsion (VSEPR) theory is a cornerstone model in chemistry used to predict the three‑dimensional geometries of molecules and polyatomic ions. It is built on a simple electrostatic principle: electron pairs in the valence shell of a central atom repel one another and will therefore arrange themselves as far apart as possible. By minimising repulsion, the molecule adopts a specific shape that determines many of its physical and chemical properties. Mastering VSEPR is essential for IB Chemistry, as it directly connects Lewis structures to polarity, intermolecular forces and reactivity.
价层电子对互斥理论（VSEPR）是化学中用于预测分子和多原子离子三维几何形状的核心模型。它建立在简单的静电原理之上：中心原子价层中的电子对相互排斥，因此会尽可能远离彼此。通过将排斥降至最低，分子获得特定的形状，这决定了它的许多物理和化学性质。掌握VSEPR对IB化学至关重要，因为它将路易斯结构直接与极性、分子间作用力和反应性联系起来。
1. The Foundation of VSEPR | VSEPR的基础
VSEPR theory starts from a valid Lewis structure. Once all valence electrons are placed, you identify the regions of electron density around the central atom. Each of these regions – whether it is a single bond, a double bond, a triple bond or a non‑bonding lone pair – counts as one electron domain. The number of electron domains determines the basic electron‑pair geometry, while the number of bonding vs. non‑bonding domains determines the observed molecular shape.
VSEPR理论从正确的路易斯结构出发。当所有价电子都排布好之后，你找出中心原子周围的电子密度区域。每一个这样的区域——无论是单键、双键、三键还是非键合的孤对电子——都计作一个电子域。电子域的数目决定了基本的电子对几何构型，而成键域与非键域的数量则决定了实际观察到的分子形状。
For example, the carbon atom in carbon dioxide, CO₂, is surrounded by two double bonds. Even though double bonds contain four electrons, they act as a single electron domain. Thus CO₂ has two electron domains, leading to a linear shape. In the ammonium ion NH₄⁺, nitrogen is surrounded by four single bonds and no lone pair – four electron domains give a tetrahedral shape.
例如，二氧化碳CO₂中的碳原子被两个双键包围。尽管双键含有四个电子，但它们作为一个电子域起作用。因此CO₂有两个电子域，呈现直线形。在铵离子NH₄⁺中，氮被四个单键包围且没有孤对电子——四个电子域产生四面体形。
2. How to Count Electron Domains | 如何计数电子域
A systematic approach for any molecule or ion is: (1) draw the Lewis structure using the total valence electrons, adjusting for any charge; (2) count the number of bonding pairs (single, double or triple all count as one domain each) and lone pairs around the central atom; (3) add them to obtain the total number of electron domains. Remember that a double or triple bond is not two or three domains – it is still one region of electron density.
对任意分子或离子的系统方法是：（1）利用总价电子数画出路易斯结构，并根据电荷进行调整；（2）计算中心原子周围的成键电子对数（单键、双键或三键每一个都计作一个域）和孤对电子数；（3）将它们相加得到总的电子域数。记住：双键或三键并不是两个或三个域——它们依然只是一个电子密度区域。
Common pitfall: students often forget to add lone pairs when the central atom possesses them, or they mistakenly count a double bond as two electron pairs. Always treat a multiple bond as a single domain. For ions like NO₃⁻, the resonance hybrid has three N–O bonding regions and no lone pair on nitrogen, giving three electron domains and a trigonal planar shape.
常见误区：学生经常忘记在中心原子有孤对电子时加上它们，或者错误地将一个双键计作两个电子对。始终将多重键视为单个域。对于像NO₃⁻这样的离子，共振杂化体中有三个N–O成键区域，且氮上没有孤对电子，因此有三个电子域，呈平面三角形。
3. Two Electron Domains – Linear Geometry | 两个电子域 – 直线形
When a central atom has two electron domains, they position themselves on opposite sides of the atom to minimise repulsion, resulting in a linear shape with a bond angle of 180°. This geometry occurs for molecules like BeCl₂ (two single bonds) and CO₂ (two double bonds), and also for ions such as the cyanide ion CN⁻ (one triple bond). The subtlety here is that regardless of bond order, two domains always give linear.
当中心原子有两个电子域时，它们会位于原子的相对两侧以将排斥降至最低，产生直线形，键角为180°。这种几何构型出现在像BeCl₂（两个单键）和CO₂（两个双键）这样的分子中，也出现在像氰根离子CN⁻（一个三键）这样的离子中。这里的微妙之处在于：无论键级如何，两个域总是产生直线形。
For example, both H–Be– CN, the carbon is bonded to hydrogen with a single bond and to nitrogen with a triple bond – is linear. The 180° angle maximises separation and eliminates lateral repulsion entirely.
例如，H–Be–H和O=C=O都是直线形。在HCN中，碳以一个单键与氢相连，以一个三键与氮相连——仍然是两个域，所以分子是直线形。180°键角使分离程度最大化，完全消除了侧向排斥。
4. Three Electron Domains – Trigonal Planar and Bent | 三个电子域 – 平面三角形与弯曲形
With three electron domains around the central atom, the ideal arrangement is trigonal planar, with bond angles of 120°. The electron domains point toward the corners of an equilateral triangle. If all three domains are bonding pairs (AX₃), the molecular shape is trigonal planar. Examples include BF₃, the nitrate ion NO₃⁻, and the carbonate ion CO₃²⁻.
当中心原子周围有三个电子域时，理想的排列是平面三角形，键角为120°。电子域指向等边三角形的顶点。如果三个域都是成键电子对（AX₃），分子形状即为平面三角形。例子包括BF₃、硝酸根离子NO₃⁻和碳酸根离子CO₃²⁻。
When one of the three domains is a lone pair (AX₂E), the observed shape is bent or V‑shaped. The lone pair occupies more space than a bonding pair, compressing the bond angle to slightly less than 120°. Sulfur dioxide, SO₂, and ozone, O₃, are classic examples with bond angles around 119° and 117° respectively. The nitrite ion, NO₂⁻, also falls into this category.
当三个域中有一个是孤对电子（AX₂E）时，观察到的形状是弯曲形或V形。孤对电子比成键电子对占据更多空间，将键角压缩至略小于120°。二氧化硫SO₂和臭氧O₃是经典的例子，键角分别约为119°和117°。亚硝酸根离子NO₂⁻也属于这一类。
5. Four Electron Domains – Tetrahedral, Trigonal Pyramidal and Bent | 四个电子域 – 四面体形、三角锥形与弯曲形
A central atom with four electron domains adopts a tetrahedral electron‑pair geometry to maximise separation, with ideal bond angles of 109.5°. If all four domains are bonding (AX₄), the molecular shape is tetrahedral. Methane, CH₄, ammonium ion NH₄⁺, and sulfate ion SO₄²⁻ are perfect examples. The 109.5° angle is a hallmark of sp³ hybridisation.
中心原子有四个电子域时，会采取四面体电子对几何构型以最大化距离，理想键角为109.5°。如果所有四个域都是成键的（AX₄），则分子形状为四面体形。甲烷CH₄、铵离子NH₄⁺和硫酸根离子SO₄²⁻是完美的例子。109.5°的键角是sp³杂化的标志。
Replacing one bonding domain with a lone pair yields AX₃E, a trigonal pyramidal shape. The lone pair‑bonding pair repulsion reduces the bond angles to about 107°, as seen in ammonia NH₃ and the hydronium ion H₃O⁺. When two lone pairs are present (AX₂E₂), the shape becomes bent with an even smaller angle of approximately 104.5°, as in water H₂O and hydrogen sulfide H₂S.
用一个孤对电子替换一个成键域得到AX₃E，即三角锥形。孤对电子与成键电子对的排斥使键角减小到约107°，如氨NH₃和水合氢离子H₃O⁺所示。当存在两个孤对电子（AX₂E₂）时，形状变为弯曲形，角度更小，约为104.5°，如水H₂O和硫化氢H₂S。
₂O (104.5°) illustrates how each additional lone pair progressively squeezes the bonding pairs closer together. The lone pairs are held closer to the nucleus and exert a stronger repulsive force on the bonding electrons.
CH₄（109.5°）→ NH₃（107°）→ H₂O（104.5°）的趋势说明了每增加一个孤对电子，如何逐步将成键电子对挤压得更紧密。孤对电子更靠近原子核，对成键电子施加更强的排斥力。
6. Five Electron Domains – Trigonal Bipyramidal Family | 五个电子域 – 三角双锥族
When the central atom is surrounded by five electron domains, the electron‑pair geometry is trigonal bipyramidal. The five positions are not all equivalent: three equatorial positions are separated by 120° in a plane, and two axial positions lie perpendicular to that plane, making 90° angles with the equatorials. An AX₅ molecule like PF₅ or PCl₅ exhibits this geometry.
当中心原子被五个电子域包围时，电子对几何构型为三角双锥。五个位置并不全部等同：三个赤道位置在平面内以120°分隔，两个轴向位置垂直于该平面，与赤道位置呈90°角。像PF₅或PCl₅这样的AX₅分子就呈现这种几何形状。
Lone pairs always occupy equatorial positions in a trigonal bipyramid because this minimises repulsions (equatorial positions have only two 90° neighbours, whereas axial positions have three). Removing one equatorial bonding pair yields AX₄E, a seesaw shape. SF₄ is a prime example, with bond angles of approximately 101° and 173° due to lone pair compression.
在三角双锥中，孤对电子总是占据赤道位置，因为这样能将排斥减到最小（赤道位置只有两个90°相邻位置，而轴向位置有三个）。去掉一个赤道成键电子对得到AX₄E，即跷跷板形。SF₄是一个典型例子，由于孤对电子的压缩，键角约为101°和173°。
With two lone pairs (AX₃E₂), the three bonding pairs arrange in a T‑shape. ClF₃ adopts a T‑shaped structure with bond angles near 87.5° (axial‑equatorial) and 175° (equatorial angle widened). When three lone pairs are present (AX₂E₃), the remaining two bonds are forced opposite each other, producing a linear shape. XeF₂ is a classic linear molecule with three equatorial lone pairs.
存在两个孤对电子（AX₃E₂）时，三个成键电子对排列成T形。ClF₃采取T形结构，键角接近87.5°（轴向-赤道间）和175°（赤道角被扩大）。当存在三个孤对电子（AX₂E₃）时，其余两个键被迫彼此相对，产生直线形。XeF₂是一个经典的直线形分子，具有三个赤道孤对电子。
7. Six Electron Domains – Octahedral Family | 六个电子域 – 八面体族
Six electron domains arrange at the corners of an octahedron, giving all bond angles 90°. An AX₆ molecule like sulfur hexafluoride, SF₆, is perfectly octahedral. This geometry is common for central atoms with an expanded valence shell, such as p‑block elements in period 3 and beyond.
六个电子域排列在八面体的顶点，所有键角为90°。像六氟化硫SF₆这样的AX₆分子是完美的八面体。这种几何构型在具有扩展价层的中心原子上很常见，例如第3周期及以后的p区元素。
Removing one bonding pair and placing a lone pair in one of the octahedral positions yields AX₅E, a square pyramidal shape. Bromine pentafluoride, BrF₅, adopts this geometry, with the lone pair opposite the square base. The bond angles between adjacent equatorial fluorine atoms are slightly less than 90° due to lone pair repulsion.
去掉一个成键电子对并在八面体顶点放置一个孤对电子得到AX₅E，即四方锥形。五氟化溴BrF₅采取这种几何构型，孤对电子位于四方形底面的对面。由于孤对电子的排斥，相邻赤道氟原子之间的键角略小于90°。
With two lone pairs (AX₄E₂), the most stable arrangement places them opposite each other, giving a square planar shape with 90° bond angles. Xenon tetrafluoride, XeF₄, is a well‑known example. The two lone pairs occupy axial positions, and the four fluorine atoms lie in a plane at 90° angles.
存在两个孤对电子（AX₄E₂）时，最稳定的排列是将它们置于彼此相对的位置，形成平面四方形，键角为90°。四氟化氙XeF₄是一个众所周知的例子。两个孤对电子占据轴向位置，四个氟原子以90°角位于同一平面内。
8. Lone Pair – Bonding Pair Repulsion and Bond Angles | 孤对电子–成键电子排斥与键角
The general order of repulsion strength is: lone pair‑lone pair > lone pair‑bonding pair > bonding pair‑bonding pair. This explains why bond angles decrease as the number of lone pairs increases. A lone pair is held closer to the nucleus and is more diffuse, occupying a larger volume of space around the central atom. Consequently, it pushes bonding pairs together, reducing the angle between them.
排斥力的一般顺序为：孤对‑孤对 > 孤对‑成键 > 成键‑成键。这就解释了为什么键角随着孤对电子数增加而减小。孤对电子更靠近原子核且更为弥散，在中心原子周围占据更大的空间体积。因此，它将成键电子对推挤在一起，减小了它们之间的角度。
ngle decreases from 109.5° to 107° to 104.5° as the number of lone pairs goes from 0 to 1 to 2. Similarly, in PF₅ the axial and equatorial angles are 90° and 120°, but in SF₄ the seesaw shape shows squeezed angles. This concept is frequently examined in IB questions that ask students to compare and explain bond angles.
在CH₄ → NH₃ → H₂O系列中，随着孤对电子数从0增加到1再到2，键角从109.5°降至107°，再降至104.5°。类似地，PF₅中的轴向和赤道角分别为90°和120°，而SF₄的跷跷板形则表现出被压缩的角度。这一概念在IB考题中经常出现，要求学生比较并解释键角。
9. Influence of Multiple Bonds on Molecular Shape | 多重键对分子形状的影响
Multiple bonds count as one electron domain, but they contain a higher electron density than single bonds. This increased electron density exerts a larger repulsive force on neighbouring electron pairs. As a result, bond angles adjacent to a double or triple bond are often slightly smaller than predicted for an ideal geometry, while the angles opposite the multiple bond may be larger.
多重键计作一个电子域，但它们所含的电子密度比单键更高。这种增加的电子密度对相邻的电子对施加更大的排斥力。因此，紧邻双键或三键的键角往往略小于理想几何构型的预测值，而与多重键相对的角度则可能更大。
In formaldehyde, H₂C=O, the carbon atom has three electron domains (two single C– bonds and one C=O double bond). The ideal trigonal planar angle is 120°, but the actual H–C–H angle is about 116°, while the H– about 122°. The double bond squashes the single bonds together. In phosgene, COCl₂, the Cl–C– gle is around 111°, even smaller because of the larger chlorine atoms and the double bond.
在甲醛H₂C=O中，碳原子有三个电子域（两个C–H单键和一个C=O双键）。理想的平面三角形角度为120°，但实际上H–C–H角约为116°，而H–C=O角约为122°。双键将单键挤压到一起。在光气COCl₂中，Cl–C–Cl角约为111°，由于较大的氯原子和双键的共同作用，角度更小。
10. VSEPR Applied to Polyatomic Ions | VSEPR在多原子离子中的应用
The VSEPR procedure works identically for ions once the Lewis structure has accounted for the overall charge. For a negatively charged ion, extra electrons are added; for a positively charged ion, electrons are removed. For example, the sulfate ion SO₄²⁻ has 32 valence electrons; the Lewis structure gives sulfur with four equivalent S–O bonds and no lone pairs – four domains, tetrahedral, bond angle 109.5°. Similarly, NO₃⁻ (nitrate) has three bonding domains, trigonal planar; NO₂⁻ (nitrite) has two bonding domains and one lone pair, bent with an angle around 115°.
一旦路易斯结构计入了总电荷，VSEPR步骤对离子的处理与分子完全相同。对于带负电荷的离子
Published by TutorHao | IB Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图