σ键Sigma bond

多种轨道组合都能形成 σ 键：H₂ 中是两个 1s 轨道重叠（s–s）；HCl 中是 H 的 1s 与 Cl 的 3p 重叠（s–p）；Cl₂ 中是两个 3p 轨道沿键轴重叠（p–p）；CH₄ 中是 C 的 sp³ 杂化轨道与 H 的 1s 重叠。

Many orbital combinations give σ bonds: in H₂ two 1s orbitals overlap (s–s); in HCl the H 1s overlaps a Cl 3p (s–p); in Cl₂ two 3p orbitals overlap along the axis (p–p); in CH₄ carbon's sp³ hybrids overlap the H 1s orbitals.

σ 键的重叠程度大，一般比 π 键牢固。由于电子云绕键轴对称，原子可以绕 σ 键自由旋转，所以乙烷等分子存在不同的构象而不形成可分离的异构体。两个原子之间只能有一个 σ 键，其余的键（双键中的第二个、三键中的第二和第三个）都是 π 键。

σ bonds involve extensive overlap and are generally stronger than π bonds. Because the electron density is symmetrical about the axis, atoms can rotate freely about a σ bond, so molecules such as ethane have different conformations rather than separable isomers. Two atoms can share only one σ bond; any further bonds (the second of a double bond, the second and third of a triple bond) are π bonds.

数 σ 键时，每个单键、双键、三键各算一个 σ 键。例如乙烯 C₂H₄ 有 5 个 σ 键（4 个 C–H、1 个 C–C）和 1 个 π 键；乙炔 C₂H₂ 有 3 个 σ 键和 2 个 π 键；苯 C₆H₆ 有 12 个 σ 键，6 个 π 电子形成离域 π 键。

When counting, each single, double or triple bond contributes one σ bond. Ethene, C₂H₄, has five σ bonds (four C–H, one C–C) and one π bond; ethyne, C₂H₂, has three σ and two π bonds; benzene, C₆H₆, has twelve σ bonds, with six π electrons in a delocalised π system.