分子间作用力Intermolecular force

色散力：电子云的瞬时不均匀产生瞬时偶极，并诱导相邻分子产生偶极，所有分子之间都存在，电子越多越强。偶极–偶极作用：极性分子（如 HCl）的永久偶极之间相互吸引。氢键：H 与 N、O、F 直接相连时，带部分正电荷的 H 与另一分子中 N、O、F 的孤对电子之间的强烈吸引。

Dispersion forces: momentary unevenness in the electron cloud creates an instantaneous dipole that induces a dipole in a neighbour; they act between all molecules and grow with the number of electrons. Dipole–dipole forces: attraction between the permanent dipoles of polar molecules such as HCl. Hydrogen bonds: when H is bonded directly to N, O or F, the strong attraction between that partially positive H and a lone pair on N, O or F of another molecule.

一般而言，氢键（约 5–40 kJ mol⁻¹）强于偶极–偶极作用，偶极–偶极作用又常强于同等大小分子间的色散力；共价键键能通常为 150–500 kJ mol⁻¹ 以上。但对大分子，色散力可以超过其他作用：碘 I₂ 是非极性分子，常温却为固体。

In general hydrogen bonds (about 5–40 kJ mol⁻¹) are stronger than dipole–dipole forces, which are usually stronger than dispersion forces between molecules of similar size; covalent bond energies are typically 150–500 kJ mol⁻¹ or more. For large molecules, however, dispersion forces can dominate: non-polar I₂ is a solid at room temperature.

分子物质熔化或沸腾时只需克服分子间作用力，所以熔沸点较低。作用力越强，沸点越高、挥发性越小、黏度越大。“相似相溶”也源于分子间作用力：极性溶质易溶于极性溶剂，非极性溶质易溶于非极性溶剂。冰的密度小于水、DNA 双螺旋的配对都与氢键有关。

Melting or boiling a molecular substance only overcomes intermolecular forces, so the temperatures are low. Stronger forces mean higher boiling points, lower volatility and higher viscosity. "Like dissolves like" also stems from intermolecular forces: polar solutes dissolve in polar solvents and non-polar solutes in non-polar ones. The low density of ice and the base pairing in DNA both depend on hydrogen bonds.