Edexcel A-Level Chemistry Topic 2.8: Shapes, Polarity and Intermolecular Forces | Edexcel A-Level化学专题2.8：分子形状、极性与分子间作用力

📚 Edexcel A-Level Chemistry Topic 2.8: Shapes, Polarity and Intermolecular Forces | Edexcel A-Level化学专题2.8：分子形状、极性与分子间作用力

In Edexcel A-Level Chemistry, Topic 2.8 brings together three ideas that students often study separately: the shapes of molecules, whether a molecule is polar, and the intermolecular forces that hold molecules together. These ideas are not isolated facts; they form a chain. The shape of a molecule determines how its bond dipoles combine, and the resulting polarity decides which type of intermolecular force is possible. This article explains each link in that chain and gives you a clear method for exam questions on shapes, polarity and physical properties.

在Edexcel A-Level化学中，专题2.8把三个经常被分开学习的概念串联起来：分子的形状、分子是否具有极性，以及把分子维系在一起的分子间作用力。这些概念并不是孤立的知识点，而是一条逻辑链。分子的形状决定了键偶极如何叠加，而由此产生的分子极性又决定了可能存在哪种类型的分子间作用力。本文解释这条逻辑链中的每一个环节，并为你提供解答有关形状、极性和物理性质考题的清晰方法。

1. The key idea: electron-pair repulsion | 核心思想：电子对排斥

The shape of a molecule or ion is not random. It is controlled by the number of electron pairs in the outer shell of the central atom. These electron pairs are negatively charged, so they repel each other and move as far apart as possible. This is the basis of valence-shell electron-pair repulsion theory, usually called VSEPR theory.

分子或离子的形状并不是随机的。它由中心原子最外层的电子对数目控制。这些电子对带负电，因此它们相互排斥，并尽可能远离彼此。这就是价层电子对互斥理论的基础，通常称为VSEPR理论。

In VSEPR theory, both bonding pairs and lone pairs count as electron clouds around the central atom. A bonding pair is shared between two nuclei, while a lone pair is attached to only the central atom. The total number of electron clouds determines the basic shape.

在VSEPR理论中，成键电子对和孤电子对都算作中心原子周围的电子云。成键电子对在两个原子核之间共享，而孤电子对只属于中心原子。电子云的总数决定了基本形状。

2. Bonding pairs vs lone pairs | 成键电子对与孤电子对

Lone pairs repel more strongly than bonding pairs. This is because a lone pair is held closer to the central nucleus and spreads out more in space. A bonding pair is pulled between two positive nuclei, so its electron cloud is less diffuse and causes less repulsion.

孤电子对的排斥作用比成键电子对更强。这是因为孤电子对离中心原子核更近，并在空间中占据更大的范围。成键电子对被两个正原子核吸引，因此其电子云较不弥散，产生的排斥作用也较小。

The repulsion order is therefore: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. This ordering explains why the bond angle in water H₂O is 104.5°, not the tetrahedral 109.5°.

因此，排斥作用的顺序为：孤电子对–孤电子对 > 孤电子对–成键电子对 > 成键电子对–成键电子对。这个顺序解释了为什么水 H₂O 的键角是104.5°，而不是正四面体的109.5°。

3. Common shapes and bond angles | 常见形状与键角

The table below summarises the shapes required for Edexcel A-Level Chemistry. The first column gives the number of bonding pairs and lone pairs around the central atom.

下表总结了Edexcel A-Level化学要求掌握的分子形状。第一列给出中心原子周围的成键电子对数和孤电子对数。

Bonding pairs / lone pairs	Shape	Bond angle	Example
2 / 0	Linear	180°	BeCl₂, CO₂
3 / 0	Trigonal planar	120°	BF₃
2 / 1	V-shaped / bent	≈ 117–118°	SO₂
4 / 0	Tetrahedral	109.5°	CH₄, NH₄⁺
3 / 1	Pyramidal	107°	NH₃
2 / 2	V-shaped / bent	104.5°	H₂O
5 / 0	Trigonal bipyramidal	90°, 120°	PCl₅
6 / 0	Octahedral	90°	SF₆

You should be able to name the shape, state the bond angle, and draw a 3D representation using wedges and dashed lines where Edexcel requires it.

你应当能够说出形状名称、说明键角，并在Edexcel要求时使用楔形线和虚线画出三维表示。

4. A reliable method for predicting shape | 预测分子形状的可靠方法

Use this four-step method for any molecule or ion. First, count the number of outer-shell electrons on the central atom. Second, add one electron for each atom bonded to the central atom. Third, add or subtract electrons for any charge. Fourth, divide the total by two to get the number of electron pairs. Then identify how many pairs are bonding pairs and how many are lone pairs.

对于任何分子或离子，都可以使用这个四步法。第一，计算中心原子最外层的电子数。第二，每个与中心原子成键的原子加上一个电子。第三，根据电荷加上或减去电子。第四，将总数除以2，得到电子对数。然后判断其中有多少是成键电子对，多少是孤电子对。

For example, in NH₃, nitrogen has 5 outer electrons. Three hydrogen atoms contribute 3 electrons. There is no charge. The total is 8, so there are 4 electron pairs. Since nitrogen bonds to three hydrogen atoms, there are 3 bonding pairs and 1 lone pair. This gives a pyramidal shape with a bond angle of about 107°.

例如，在NH₃中，氮有5个外层电子。三个氢原子贡献3个电子。没有电荷。总数为8，因此有4对电子。由于氮与三个氢原子成键，所以有3对成键电子和1对孤电子。这给出三角锥形，键角约为107°。

5. Electronegativity and bond polarity | 电负性与键的极性

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. When two atoms in a bond have different electronegativities, the bonding pair is pulled towards the more electronegative atom. This creates a polar bond with a permanent dipole.

电负性是一个原子在共价键中吸引成键电子的能力。当成键的两个原子具有不同的电负性时，成键电子对会被拉向电负性较大的原子。这产生具有永久偶极的极性键。

We represent this using partial charges. The more electronegative atom carries a partial negative charge, written δ⁻, and the less electronegative atom carries a partial positive charge, written δ⁺. For example, in hydrogen chloride HCl, chlorine is more electronegative than hydrogen, so chlorine is δ⁻ and hydrogen is δ⁺.

我们用部分电荷表示这一点。电负性较大的原子带部分负电荷，写作δ⁻；电负性较小的原子带部分正电荷，写作δ⁺。例如，在氯化氢HCl分子中，氯比氢的电负性大，因此氯为δ⁻，氢为δ⁺。

6. Molecular polarity: when polar bonds combine | 分子极性：极性键的叠加

A molecule with polar bonds is not always a polar molecule. The shape of the molecule decides whether the individual bond dipoles cancel out. If the dipoles pull in opposite directions with equal strength, the molecule is non-polar. If they do not cancel, the molecule has a permanent dipole and is polar.

含有极性键的分子并不总是极性分子。分子的形状决定了各键偶极是否相互抵消。如果各偶极以相等的强度向相反方向拉伸，则分子是非极性的。如果它们没有抵消，分子就具有永久偶极，属于极性分子。

Carbon dioxide CO₂ is linear: the two C=O bond dipoles point in exactly opposite directions, so they cancel. Carbon dioxide is therefore non-polar. Water H₂O is V-shaped, so the two O–H bond dipoles do not cancel; they combine to give a net dipole. Water is polar.

二氧化碳CO₂是直线形：两个C=O键偶极指向完全相反的方向，因此相互抵消。所以二氧化碳是非极性分子。水H₂O是V形，两个O–H键偶极不抵消；它们叠加产生净偶极。因此水是极性分子。

7. The three types of intermolecular force | 三种分子间作用力

Intermolecular forces are attractions between separate molecules. They are much weaker than covalent or ionic bonds, but they control properties such as boiling point, melting point and solubility. Edexcel requires you to know three types: London dispersion forces, permanent dipole–dipole forces and hydrogen bonds.

分子间作用力是不同分子之间的吸引力。它们比共价键或离子键弱得多，但控制着沸点、熔点和溶解度等性质。Edexcel要求你掌握三种类型：伦敦色散力、永久偶极–偶极力和氢键。

Every molecule has London dispersion forces. Only polar molecules have permanent dipole–dipole forces. Hydrogen bonding occurs only in molecules where hydrogen is directly bonded to nitrogen, oxygen or fluorine.

所有分子都存在伦敦色散力。只有极性分子才存在永久偶极–偶极力。氢键只出现在氢直接与氮、氧或氟成键的分子中。

8. London dispersion forces | 伦敦色散力

London dispersion forces arise from temporary fluctuations in the electron cloud of a molecule. At any instant, one side of the molecule may have slightly more electron density than the other. This creates a temporary dipole, which can induce a dipole in a neighbouring molecule.

伦敦色散力来源于分子电子云的瞬时涨落。在任意瞬间，分子的一侧可能比另一侧具有略多的电子密度。这产生瞬时偶极，并能在相邻分子中诱导出偶极。

The strength of London forces increases with the number of electrons in the molecule. Larger molecules have more electrons and more diffuse electron clouds, so their temporary dipoles are stronger. This explains why boiling points increase down Group 17 from fluorine to iodine.

伦敦力的强度随分子中电子数的增加而增强。较大的分子有更多的电子和更弥散的电子云，因此其瞬时偶极更强。这解释了为什么第17族从氟到碘沸点逐渐升高。

9. Permanent dipole–dipole forces and hydrogen bonding | 永久偶极–偶极力与氢键

Permanent dipole–dipole forces act between polar molecules. The δ⁺ end of one molecule is attracted to the δ⁻ end of a neighbouring molecule. These forces are stronger than London forces for molecules of similar size, but they are still much weaker than covalent bonds.

永久偶极–偶极力作用于极性分子之间。一个分子的δ⁺端与相邻分子的δ⁻端相互吸引。对于大小相近的分子，这些力比伦敦力强，但仍远弱于共价键。

Hydrogen bonding is the strongest intermolecular force required at A-Level. It is a special dipole–dipole interaction that occurs when hydrogen is bonded to nitrogen, oxygen or fluorine. The lone pair on the N, O or F atom is attracted to the highly polarised H atom on another molecule.

氢键是A-Level要求掌握的最强分子间作用力。它是一种特殊的偶极–偶极相互作用，当氢与氮、氧或氟成键时发生。N、O或F原子上的孤电子对被另一个分子上高度极化的氢原子吸引。

A hydrogen bond is represented as a dashed line: X–H···Y, where X and Y are N, O or F. The strongest hydrogen bonds occur in water, ammonia and hydrogen fluoride.

氢键用虚线表示：X–H···Y，其中X和Y为N、O或F。最强的氢键出现在水、氨和氟化氢中。

10. Linking forces to physical properties | 分子间作用力与物理性质的联系

The boiling point of a molecular substance depends on the strength of the intermolecular forces that must be overcome. A substance with stronger intermolecular forces has a higher boiling point because more energy is required to separate the molecules.

分子物质的沸点取决于必须克服的分子间作用力的强度。分子间作用力越强，沸点越高，因为分离分子需要更多的能量。

Water has a much higher boiling point than methane CH₄, even though both are small molecules. Methane has only London forces, while water has London forces, dipole–dipole forces and hydrogen bonds. The hydrogen bonds make the difference.

水比甲烷CH₄的沸点高得多，尽管两者都是小分子。甲烷只有伦敦力，而水同时具有伦敦力、偶极–偶极力和氢键。氢键造成了这种差异。

11. Worked example: H₂O, CO₂ and NH₃ | 解题示例：H₂O、CO₂和NH₃

For each molecule, identify the shape, the polarity, and the main intermolecular force. Water H₂O has 4 electron pairs: 2 bonding pairs and 2 lone pairs. Its shape is V-shaped, its bond angle is 104.5°, and it is polar because the two O–H dipoles do not cancel. Its strongest intermolecular force is hydrogen bonding.

对于每个分子，确定其形状、极性和主要分子间作用力。水H₂O有4对电子：2对成键电子和2对孤电子。其形状为V形，键角为104.5°，并且具有极性，因为两个O–H偶极不抵消。其最强的分子间作用力是氢键。

Carbon dioxide CO₂ has 2 electron pairs around carbon, so it is linear with a bond angle of 180°. The two C=O dipoles cancel, so CO₂ is non-polar. Its only intermolecular force is London dispersion forces.

二氧化碳CO₂的碳周围有2对电子，因此是直线形，键角为180°。两个C=O偶极抵消，所以CO₂是非极性分子。它唯一的分子间作用力是伦敦色散力。

Ammonia NH₃ has 4 electron pairs: 3 bonding pairs and 1 lone pair. Its shape is pyramidal, its bond angle is 107°, and it is polar because the three N–H dipoles combine to give a net dipole. Its strongest intermolecular force is hydrogen bonding.

氨NH₃有4对电子：3对成键电子和1对孤电子。其形状为三角锥形，键角为107°，并且具有极性，因为三个N–H偶极叠加产生净偶极。其最强的分子间作用力是氢键。

12. Exam tips for Edexcel Topic 2.8 | Edexcel专题2.8的考试技巧

In shape questions, always state the shape name, the bond angle, and the number of bonding and lone pairs. If the question asks for a diagram, draw the 3D shape clearly with wedges and dashed lines, and label the bond angle.

在形状题中，一定要写出形状名称、键角以及成键电子对和孤电子对的数目。如果题目要求画图，请清晰地画出三维形状，使用楔形线和虚线，并标注键角。

In polarity questions, do not say that a molecule with polar bonds is automatically polar. Always comment on the shape and whether the dipoles cancel. Use the terms ‘symmetrical’ or ‘asymmetrical’ carefully, because symmetry alone is not enough unless all the surrounding atoms are identical.

在极性题中，不要认为含有极性键的分子就一定是极性分子。一定要结合形状说明偶极是否抵消。谨慎使用“对称”或“不对称”这些术语，因为除非周围原子完全相同，否则仅靠对称性并不充分。

In intermolecular force questions, name the strongest force present, not just any force. For hydrogen bonding, check that hydrogen is directly bonded to N, O or F. For comparing boiling points, refer to the type of intermolecular force and the number of electrons where London forces are involved.

在分子间作用力题中，要指出存在的最强作用力，而不只是任意一种作用力。对于氢键，要确认氢直接与N、O或F成键。在比较沸点时，要说明分子间作用力的类型，并在涉及伦敦力时提及电子数。

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