偶极矩Dipole moment

两个电负性不同的原子成键时，电子对偏向电负性大的一方，产生键偶极。分子的偶极矩是各键偶极的矢量和，因此取决于分子形状：CO₂ 是直线形，两个 C=O 键偶极大小相等、方向相反，相互抵消，分子偶极矩为零；H₂O 为 V 形，键偶极不能抵消，偶极矩为 1.85 D。CCl₄、BF₃ 含极性键但对称，也是非极性分子。

When two atoms of different electronegativity bond, the electron pair is pulled towards the more electronegative one, creating a bond dipole. A molecule's dipole moment is the vector sum of its bond dipoles, so it depends on shape: linear CO₂ has two equal C=O dipoles pointing opposite ways that cancel, giving zero; bent H₂O does not cancel and has 1.85 D. CCl₄ and BF₃ contain polar bonds but are symmetrical, so they are non-polar too.

HF 1.83 D，HCl 1.08 D，HBr 0.82 D，HI 0.44 D，随卤素电负性减小而减小。NH₃ 1.47 D，H₂O 1.85 D，CH₃Cl 1.87 D，丙酮 2.88 D。H₂、Cl₂、CH₄、苯的偶极矩为零。

HF 1.83 D, HCl 1.08 D, HBr 0.82 D and HI 0.44 D, falling as the halogen's electronegativity falls. NH₃ 1.47 D, H₂O 1.85 D, CH₃Cl 1.87 D, propanone 2.88 D. H₂, Cl₂, CH₄ and benzene have zero dipole moment.

有偶极矩的分子是极性分子，分子之间存在永久偶极–偶极作用，沸点通常高于相对分子质量相近的非极性分子。极性分子在电场中会定向排列；用带电的棒靠近细水流，水流会偏转，而己烷液流不偏转。偶极矩还可帮助判断分子构型，例如区分顺-1,2-二氯乙烯（有偶极矩）和反式异构体（偶极矩为零）。

Molecules with a dipole moment are polar; they attract each other by permanent dipole–dipole forces and usually boil higher than non-polar molecules of similar Mr. Polar molecules line up in an electric field: a charged rod deflects a thin stream of water but not of hexane. Dipole moments also help establish structure, for example distinguishing cis-1,2-dichloroethene (polar) from the trans isomer (zero dipole moment).