定义Definition
色散力是瞬时偶极与诱导偶极之间的吸引作用，存在于所有原子和分子之间，是非极性分子之间唯一的分子间作用力。原子或分子中电子数越多、电子云越易变形，色散力越强。
London dispersion forces are attractions between instantaneous dipoles and the dipoles they induce. They act between all atoms and molecules and are the only intermolecular force between non-polar molecules. The more electrons and the more easily distorted the electron cloud, the stronger they are.
成因Origin
电子云在不断运动，某一瞬间电子分布可能偏向一侧，使原子或分子产生瞬时偶极。这个瞬时偶极使相邻粒子的电子云发生偏移，产生方向相配的诱导偶极，两者相互吸引。瞬时偶极不断出现和消失，但相邻粒子的偶极总是协同变化，所以平均下来是净吸引力。
The electron cloud is always moving, and at any instant it may be lopsided, giving the atom or molecule an instantaneous dipole. This shifts the electron cloud of a neighbour, inducing a matching dipole, and the two attract. The instantaneous dipoles keep appearing and vanishing, but neighbouring dipoles stay in step, so on average there is a net attraction.
影响因素与实例What affects them, with examples
电子越多，色散力越强：稀有气体的沸点 He −269 °C、Ne −246 °C、Ar −186 °C、Kr −153 °C、Xe −108 °C 依次升高；卤素单质常温下 F₂、Cl₂ 为气体，Br₂ 为液体，I₂ 为固体。分子间接触面积越大，色散力越强：正戊烷的沸点（36 °C）高于 2,2-二甲基丙烷（10 °C）。
More electrons mean stronger dispersion forces: the noble-gas boiling points rise in turn, He −269 °C, Ne −246 °C, Ar −186 °C, Kr −153 °C, Xe −108 °C, and at room temperature F₂ and Cl₂ are gases, Br₂ a liquid and I₂ a solid. Greater contact area also strengthens them: pentane boils at 36 °C, 2,2-dimethylpropane at 10 °C.
重要性Importance
色散力单个很弱，但在大分子中累加起来可以很大，长链烷烃、聚乙烯的物理性质主要由色散力决定。许多教材所说的“范德华力”即指色散力。对极性分子而言，色散力也常常是分子间作用力中贡献最大的部分，例如 HCl 与 HI 的沸点差异主要来自色散力。
Each interaction is weak, but in large molecules they add up to a lot; the physical properties of long-chain alkanes and polyethylene are governed mainly by dispersion forces. Many textbooks mean dispersion forces when they say van der Waals forces. Even for polar molecules dispersion is often the largest contribution: the higher boiling point of HI compared with HCl comes mainly from dispersion.
年表Timeline
- 1930
伦敦用量子力学推导出色散力，并指出其与距离的六次方成反比
London derives dispersion forces from quantum mechanics, showing they fall off as the sixth power of distance
- 1937
伦敦发表关于分子间力一般理论的综述
London publishes a review of the general theory of molecular forces