极性共价键Polar covalent bond

电负性差越大，键越有极性。同种原子之间（H–H、Cl–Cl）是非极性键；C–H 键（差 0.35）极性很小；O–H（差约 1.24）、H–F（差约 1.78）极性很强。电负性差很大时电子几乎完全转移，形成离子键，二者之间没有截然的界限。

The greater the electronegativity difference, the more polar the bond. Bonds between identical atoms (H–H, Cl–Cl) are non-polar; C–H (difference 0.35) is only slightly polar; O–H (about 1.24) and H–F (about 1.78) are strongly polar. With a very large difference the electron is essentially transferred and the bond is ionic; there is no sharp boundary between the two.

极性键使分子可能成为极性分子，但还取决于分子形状：CO₂ 含两个极性 C=O 键却是非极性分子。键的极性决定反应活性：卤代烷 C–X 键中碳带 δ+，易受亲核试剂进攻；羰基碳带 δ+，发生亲核加成。

Polar bonds can make a molecule polar, but shape matters: CO₂ has two polar C=O bonds yet is non-polar. Bond polarity governs reactivity: the δ+ carbon of a C–X bond in a halogenoalkane is attacked by nucleophiles, and the δ+ carbonyl carbon undergoes nucleophilic addition.