范德华力Van der Waals force

即使在非极性分子中，电子云也在不停运动，某一瞬间分布不均匀，产生瞬时偶极。这个瞬时偶极使相邻分子的电子云发生极化，产生诱导偶极，两者相互吸引。由于这种作用随时出现、随时消失，又随时重新出现，总效果是一种净吸引力。

Even in a non-polar molecule the electron cloud is constantly moving, and at any instant it may be unevenly spread, creating an instantaneous dipole. This dipole polarises the electron cloud of a neighbouring molecule, inducing a dipole, and the two attract. Because such dipoles keep appearing, vanishing and reappearing, the overall effect is a net attraction.

分子中电子越多（相对分子质量越大），电子云越容易变形（极化率越大），色散力越强，所以稀有气体和卤素单质的沸点随原子序数增大而升高。分子间接触面积越大，作用越强：直链烷烃的沸点高于其支链异构体。

The more electrons a molecule has (larger Mr), the more easily its electron cloud is distorted (higher polarisability) and the stronger the dispersion forces; this is why the boiling points of the noble gases and halogens rise with atomic number. Larger contact area also strengthens them: straight-chain alkanes boil higher than their branched isomers.

稀有气体能被液化、固态碘升华时吸热，都说明非极性粒子之间存在范德华力。石墨层与层之间靠范德华力结合，所以层间容易滑动。壁虎脚掌上的大量细毛与墙面之间的范德华力足以支撑其体重。

That noble gases can be liquefied, and that iodine absorbs heat when it sublimes, both show van der Waals forces act between non-polar particles. The layers of graphite are held together by van der Waals forces, so they slide over one another easily. The van der Waals forces between the many fine hairs on a gecko's feet and a wall can support its weight.