定义Definition
超价分子是中心主族元素原子周围的价电子数按路易斯结构计算超过 8 个的分子或离子，如 PCl₅（10 个）、SF₆（12 个）、XeF₄ 和 I₃⁻。这类化合物只出现在第三周期及以后的元素中。
A hypervalent molecule or ion has a main-group central atom that, by its Lewis structure, has more than eight valence electrons around it, as in PCl₅ (10), SF₆ (12), XeF₄ and I₃⁻. Such compounds are formed only by elements from period 3 onwards.
为什么第二周期元素不能超价Why period 2 elements cannot
第二周期原子半径小，周围容纳不下五六个配位原子，因此有 PCl₅ 而没有 NCl₅，有 SF₆ 而没有 OF₆。
Period 2 atoms are too small to fit five or six atoms around them, so PCl₅ exists but NCl₅ does not, and SF₆ exists but OF₆ does not.
成键的解释How the bonding is explained
中学教材常说中心原子的 3d 轨道参与成键（sp³d、sp³d² 杂化）。现代计算表明 d 轨道贡献很小，更合理的描述是三中心四电子键：例如 XeF₂ 中 F–Xe–F 共用一个 p 轨道上的成键作用，电子密度偏向电负性大的配位原子。
School texts often say that empty 3d orbitals take part (sp³d and sp³d² hybridisation). Modern calculations show the d orbitals contribute little; a better description is three-centre four-electron bonding, as along F–Xe–F in XeF₂, with electron density pushed towards the electronegative outer atoms.
年表Timeline
- 1969
穆舍提出“超价分子”一词
Musher introduces the term hypervalent molecule