杂化轨道理论Orbital hybridisation

一个 s 轨道与 n 个 p 轨道组合，得到 n+1 个等价的 spⁿ 杂化轨道。sp 杂化轨道夹角 180°（直线形），sp² 为 120°（平面三角形），sp³ 为 109.5°（四面体）。杂化轨道一端肥大，与其他原子轨道重叠更多，因此成键更强。未参与杂化的 p 轨道可侧面重叠形成 π 键。

Combining one s orbital with n p orbitals gives n+1 equivalent spⁿ hybrids. sp hybrids lie at 180° (linear), sp² at 120° (trigonal planar) and sp³ at 109.5° (tetrahedral). A hybrid orbital is enlarged on one side, so it overlaps more with orbitals on other atoms and forms stronger bonds. Unhybridized p orbitals can overlap sideways to form π bonds.

碳的基态电子排布为 2s²2p²，只有两个未成对电子，却在甲烷中形成四个完全等同的 C–H 键。为解释这一矛盾，鲍林在 1931 年提出：碳先把一个 2s 电子激发到 2p，再将 2s 与三个 2p 轨道混合成四个等价的 sp³ 轨道，指向正四面体顶点。斯莱特同年发表了相近的方向性价键处理。

Carbon's ground state, 2s²2p², has only two unpaired electrons, yet methane has four identical C–H bonds. To resolve this, Pauling proposed in 1931 that carbon promotes a 2s electron to 2p and then mixes the 2s and three 2p orbitals into four equivalent sp³ orbitals pointing to the corners of a tetrahedron. Slater published a closely related treatment of directed valence the same year.

理论随后扩展到 sp²、sp 以及含 d 轨道的 sp³d、sp³d² 杂化，用于解释乙烯、乙炔、PCl₅、SF₆ 等分子。后来的量子化学计算表明，主族元素超价分子中 d 轨道的贡献很小，sp³d 等说法已被三中心四电子键等模型取代。本特规则进一步指出，电负性大的取代基倾向与 p 成分较多的杂化轨道成键。

The idea was extended to sp², sp and to d-containing sp³d and sp³d² hybrids to explain ethylene, acetylene, PCl₅ and SF₆. Later quantum-chemical work showed that d orbitals contribute little in main-group hypervalent molecules, so sp³d-type descriptions have given way to models such as the three-centre four-electron bond. Bent's rule added that electronegative substituents prefer hybrids richer in p character.

杂化轨道直观地说明了有机分子的立体结构和键角，是有机化学教学的核心工具。但它不能解释分子的磁性（如 O₂ 的顺磁性）、电子光谱和光电子能谱中能级的分裂；这些需要分子轨道理论。杂化只是一种数学描述，并非真实发生的物理过程。

Hybridisation gives an intuitive account of the shapes and bond angles of organic molecules and is a core teaching tool. It cannot explain magnetism (such as the paramagnetism of O₂), electronic spectra or the energy-level splitting seen in photoelectron spectra; those need molecular orbital theory. Hybridisation is a mathematical description, not a physical process that actually takes place.