极性分子Chemical polarity

同种原子之间的键（如 H–H、Cl–Cl）是非极性键。电负性差越大，键的极性越强：H–F 的极性大于 H–Cl。电负性差很大时，电子几乎完全转移，形成离子键；因此离子键和非极性共价键可以看作键极性的两个极端。

Bonds between identical atoms, such as H–H and Cl–Cl, are non-polar. The larger the electronegativity difference, the more polar the bond: H–F is more polar than H–Cl. When the difference is very large, the electron is essentially transferred and an ionic bond forms, so non-polar covalent and ionic bonds are the two extremes of bond polarity.

分子的极性取决于键的极性和分子形状。含极性键的分子如果高度对称，键偶极相互抵消，仍是非极性分子，如 CO₂（直线形）、BF₃（平面三角形）、CH₄ 和 CCl₄（正四面体）。键偶极不能抵消的分子是极性分子，如 H₂O（V 形）、NH₃（三角锥形）、CHCl₃。

Molecular polarity depends on both bond polarity and shape. A molecule with polar bonds is still non-polar if it is symmetrical enough for the bond dipoles to cancel, as in CO₂ (linear), BF₃ (trigonal planar), CH₄ and CCl₄ (tetrahedral). Where they do not cancel the molecule is polar, as in H₂O (bent), NH₃ (pyramidal) and CHCl₃.

极性分子之间除色散力外还有偶极–偶极作用，沸点往往较高。“相似相溶”：极性物质易溶于水等极性溶剂，非极性物质易溶于己烷等非极性溶剂。键的极性也影响反应性：C–X 键中碳带部分正电荷，易受亲核试剂进攻。

Polar molecules have dipole–dipole forces as well as dispersion forces, so they tend to boil higher. Like dissolves like: polar substances dissolve in polar solvents such as water, and non-polar substances in non-polar solvents such as hexane. Bond polarity also affects reactivity: the carbon of a C–X bond is partially positive and is attacked by nucleophiles.