定义Definition
键角是分子中同一原子与另外两个原子形成的两个化学键之间的夹角，如 H₂O 中 H–O–H 键角为 104.5°。键角和键长共同决定分子的空间构型。
A bond angle is the angle between two bonds from the same atom to two other atoms, such as the H–O–H angle of 104.5° in water. Bond angles and bond lengths together fix a molecule's shape.
典型数值Typical values
直线形 180°（CO₂、BeCl₂）；平面三角形 120°（BF₃、乙烯中的碳）；正四面体 109.5°（CH₄）；三角锥形约 107°（NH₃）；V 形约 104.5°（H₂O）；三角双锥 90° 和 120°（PCl₅）；八面体 90°（SF₆）。
Linear 180° (CO₂, BeCl₂); trigonal planar 120° (BF₃, the carbons of ethene); tetrahedral 109.5° (CH₄); trigonal pyramidal about 107° (NH₃); bent about 104.5° (H₂O); trigonal bipyramidal 90° and 120° (PCl₅); octahedral 90° (SF₆).
影响因素What affects it
价层电子对互斥理论认为排斥强弱为：孤对–孤对 > 孤对–成键 > 成键–成键，所以孤对电子越多，键角越小（CH₄ 109.5° > NH₃ 107° > H₂O 104.5°）。中心原子电负性降低或配位原子电负性增大也会使键角减小，如 NH₃ 107° 而 PH₃ 约 93.5°。键角可由 X 射线衍射、电子衍射和微波光谱测定。
VSEPR theory ranks repulsion lone–lone > lone–bond > bond–bond, so more lone pairs mean smaller angles (CH₄ 109.5° > NH₃ 107° > H₂O 104.5°). A less electronegative central atom or more electronegative ligands also close the angle: NH₃ is 107° but PH₃ about 93.5°. Angles are measured by X-ray and electron diffraction and microwave spectroscopy.
年表Timeline
- 1874
范特霍夫提出碳的四面体结构，键角约 109.5°
van 't Hoff proposes the tetrahedral carbon with angles of about 109.5°
- 1957
吉莱斯皮与尼霍姆用电子对互斥解释键角
Gillespie and Nyholm explain bond angles by electron-pair repulsion