📚 Intermolecular Forces | 分子间力
Intermolecular forces are the attractive or repulsive interactions between separate molecules. They determine whether a substance is a gas, liquid or solid at room temperature, and they explain key physical trends such as boiling point, melting point, solubility and surface tension. Although these forces are much weaker than the covalent or ionic bonds within a molecule, they are essential for understanding the macroscopic behaviour of molecular substances.
分子间力是独立分子之间的吸引或排斥相互作用。它们决定一种物质在室温下是气体、液体还是固体，并能解释沸点、熔点、溶解度和表面张力等关键物理性质的递变规律。虽然这些力比分子内的共价键或离子键弱得多，但它们是理解分子型物质宏观行为的关键。
In A-Level chemistry, you need to distinguish clearly between intramolecular forces (the bonds inside a molecule) and intermolecular forces (the forces between molecules). You should also be able to name the main types of intermolecular force, compare their relative strengths and use them to predict physical properties.
在 A-Level 化学中，你必须清楚区分分子内力（分子内部的化学键）和分子间力（分子之间的作用力）。你还需要能够说出分子间力的主要类型、比较它们的相对强度，并用它们预测物理性质。
1. Intramolecular vs Intermolecular Forces | 分子内力与分子间力
Intramolecular forces are the chemical bonds that hold atoms together inside a molecule or compound. They include covalent bonds, ionic bonds and metallic bonds. For example, in a water molecule the two O–H covalent bonds are intramolecular. Intermolecular forces act between separate molecules, such as the hydrogen bonds between neighbouring water molecules.
分子内力是把原子结合在分子或化合物内部的化学键，包括共价键、离子键和金属键。例如，在水分子中，两个 O–H 共价键属于分子内力。分子间力则作用在独立分子之间，例如相邻水分子之间的氢键。
The distinction matters because a substance can be heated to boil a molecular liquid without breaking its covalent bonds. When water boils at 100 °C, the H₂O molecules remain intact; only the hydrogen bonds between water molecules are overcome.
这种区分很重要，因为加热分子型液体至沸腾时不需要断裂其共价键。水在 100 °C 沸腾时，H₂O 分子本身保持完整；被克服的只是水分子之间的氢键。
2. Types of Intermolecular Forces | 分子间力的类型
The three main types of intermolecular force you must know for Cambridge A-Level are London dispersion forces, permanent dipole-dipole forces and hydrogen bonding. All particles experience London dispersion forces, but only polar molecules have permanent dipole-dipole forces, and only molecules with N–H, O–H or F–H bonds can form hydrogen bonds.
剑桥 A-Level 要求掌握的三种主要分子间力是伦敦色散力、永久偶极-偶极力和氢键。所有粒子之间都存在伦敦色散力；只有极性分子才有永久偶极-偶极力；只有含有 N–H、O–H 或 F–H 键的分子才能形成氢键。
London forces arise from instantaneous dipoles, permanent dipole-dipole forces arise from the attraction between opposite partial charges on polar molecules, and hydrogen bonds are a special strong dipole-dipole attraction involving N, O or F with a hydrogen atom.
伦敦色散力来源于瞬时偶极；永久偶极-偶极力来源于极性分子上相反部分电荷之间的吸引；氢键则是一种特殊的强偶极-偶极吸引，涉及 N、O 或 F 与氢原子。
3. London Dispersion Forces | 伦敦色散力
London dispersion forces, also called instantaneous dipole-induced dipole forces, exist between all atoms and molecules. They are caused by the random movement of electrons. At any moment, electrons may be unevenly distributed, creating a temporary dipole with δ⁺ and δ⁻ regions. This temporary dipole can induce a complementary dipole in a neighbouring particle, leading to a weak attraction.
伦敦色散力又称瞬时偶极-诱导偶极力，存在于所有原子和分子之间。它们由电子的随机运动引起。在任一时刻，电子可能分布不均匀，形成具有 δ⁺ 和 δ⁻ 区域的瞬时偶极。这个瞬时偶极可以诱导邻近粒子产生互补偶极，从而产生微弱的吸引力。
The strength of London forces increases with the number of electrons and the size of the electron cloud. Larger, more polarizable molecules have stronger London forces. This explains why boiling points increase down Group 0 from He to Rn and down Group 17 from F₂ to I₂.
伦敦色散力的强度随着电子数目和电子云尺寸的增大而增强。体积越大、极化率越高的分子，其伦敦色散力越强。这解释了为什么第 0 族从 He 到 Rn、第 17 族从 F₂ 到 I₂ 沸点逐渐升高。
Shape also matters: linear isomers with more surface contact have stronger London forces than branched isomers with less contact. For example, pentane has a higher boiling point than 2,2-dimethylpropane despite the same molecular formula and number of electrons.
分子形状也很重要：具有更多表面接触的直链异构体比支链异构体具有更强的伦敦色散力。例如，戊烷的沸点高于 2,2-二甲基丙烷，尽管两者的分子式和电子数相同。
4. Permanent Dipole-Dipole Forces | 永久偶极-偶极力
Permanent dipole-dipole forces occur between polar molecules. A polar molecule has a permanent separation of charge due to a difference in electronegativity between bonded atoms. For example, HCl is polar because chlorine is more electronegative than hydrogen: H δ⁺–Cl δ⁻. Adjacent HCl molecules align so that δ⁺ on one molecule attracts δ⁻ on another.
永久偶极-偶极力存在于极性分子之间。极性分子由于成键原子电负性差异而具有永久电荷分离。例如，HCl 是极性分子，因为氯比氢电负性更强：H δ⁺–Cl δ⁻。相邻的 HCl 分子会以 δ⁺ 端吸引另一个分子 δ⁻ 端的方式取向排列。
H δ⁺–Cl δ⁻
Compare chloromethane and ethane of similar molar mass: chloromethane is polar, so it has both London forces and permanent dipole-dipole forces, giving it a higher boiling point than non-polar ethane.
比较摩尔质量相近的氯甲烷和乙烷：氯甲烷是极性分子，因此它既有伦敦色散力又有永久偶极-偶极力，所以其沸点高于非极性的乙烷。
5. Hydrogen Bonding | 氢键
Hydrogen bonding is not a true chemical bond but a particularly strong intermolecular attraction. It occurs when a hydrogen atom is covalently bonded to nitrogen, oxygen or fluorine and is attracted to a lone pair of electrons on an N, O or F atom in another molecule.
氢键不是真正的化学键，而是一种特别强的分子间吸引力。当一个氢原子与氮、氧或氟共价结合，并被另一个分子中 N、O 或 F 原子上的孤对电子吸引时，就形成氢键。
The hydrogen atom carries a high δ⁺ charge because N, O and F are very electronegative. The lone pair on the neighbouring N/O/F carries a δ⁻ region. The small size of hydrogen allows close approach, making the attraction strong.
由于 N、O 和 F 的电负性很强，氢原子带有较高的 δ⁺ 电荷。邻近 N/O/F 上的孤对电子形成 δ⁻ 区域。氢原子体积小，可以更接近孤对电子，使吸引力更强。
H–O····H–O
6. Key Requirements for Hydrogen Bonding | 形成氢键的关键条件
For a molecule to be a hydrogen bond donor, it must contain a hydrogen atom bonded directly to N, O or F. For it to be a hydrogen bond acceptor, it must have an available lone pair on N, O or F. Water, ammonia and hydrogen fluoride can both donate and accept hydrogen bonds.
一个分子要作为氢键供体，必须含有直接与 N、O 或 F 相连的氢原子。要作为氢键受体，必须在 N、O 或 F 上具有可用的孤对电子。水、氨和氟化氢既可以提供氢键，也可以接受氢键。
Common mistakes: a molecule with C–H bonds cannot form hydrogen bonds because carbon is not electronegative enough. Similarly, H bonded to Cl cannot form hydrogen bonds, even though HCl is polar.
常见误区：含有 C–H 键的分子不能形成氢键，因为碳的电负性不够强。同样，与 Cl 相连的氢也不能形成氢键，即使 HCl 是极性分子。
Ethanol CH₃CH₂OH forms hydrogen bonds through its –OH group. Methoxymethane CH₃OCH₃ has an oxygen atom but no O–H bond, so it cannot donate a hydrogen bond despite having lone pairs.
乙醇 CH₃CH₂OH 通过其 –OH 基团形成氢键。甲醚 CH₃OCH₃ 虽然含有氧原子，但没有 O–H 键，因此尽管有孤对电子，也不能作为氢键供体。
7. Comparing Strengths | 强度比较
The order of strength is: London dispersion forces < permanent dipole-dipole forces < hydrogen bonding << covalent or ionic bonding. This huge gap explains why molecular substances have much lower melting and boiling points than giant covalent or ionic compounds.
强度顺序为：伦敦色散力 < 永久偶极-偶极力 < 氢键 << 共价键或离子键。这个巨大差距解释了为什么分子型物质的熔点和沸点远低于巨型共价化合物或离子化合物。
| Interaction | Typical strength / kJ mol⁻¹ | Found in |
|---|---|---|
| London dispersion | 0.5 – 10 | all atoms and molecules |
| Permanent dipole-dipole | 2 – 10 | polar molecules |
| Hydrogen bonding | 10 – 40 | molecules with N–H, O–H or F–H |
| Covalent bond | 150 – 500 | within molecules |
| Ionic bond | 400 – 4000 | ionic lattices |
A hydrogen bond is roughly 1/10 to 1/20 the strength of a covalent O–H bond, but it is sufficient to create strong intermolecular networks in substances like water and ice.
氢键的强度大约只有共价 O–H 键的 1/10 到 1/20，但足以在水和冰等物质中形成牢固的分子间网络。
8. Effect on Boiling and Melting Points | 对沸点和熔点的影响
A higher boiling point indicates stronger intermolecular forces because more energy is needed to separate molecules from the liquid phase into the gas phase. For molecular substances, compare the types and strengths of intermolecular forces before explaining a trend.
沸点越高，说明分子间力越强，因为需要更多能量将分子从液相分离到气相。对于分子型物质，在解释递变规律前，应先比较分子间力的类型和强度。
For the hydrogen halides, the boiling point trend is HCl (-85 °C) < HBr (-66 °C) < HI (-35 °C), but HF (+20 °C) is highest. HCl, HBr and HI increase because more electrons
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