1.7 : VSEPR理论

The valence shell electron-pair repulsion, or VSEPR, theory assumes that electron groups involved in single bonds, multiple bonds, or lone pairs repel each other and try to stay at the maximum possible distance from each other.  

The molecular geometry is dictated by the arrangement of various electron groups around the central atom.

There are five basic molecular shapes: linear for two bonding electron groups, trigonal planar for three, tetrahedral for four, trigonal bipyramidal for five, and octahedral for six.

While predicting the molecular geometry, remember that lone pair–lone pair repulsions are greater than lone pair–bonding pair and bonding pair–bonding pair repulsions.

Consider the Lewis structures of methane, ammonia, and water. In each, the central atom is surrounded by four electron groups.

In methane, the four bonding electron pairs are arranged tetrahedrally with an ideal H–C–H angle of 109.5°.

In ammonia, the nitrogen atom has three bonding pairs and one lone pair.

The lone pair of electrons occupies a larger space than the bonding pairs. This is because a lone pair is bound to only one nucleus, whereas a bonding electron group is shared by two nuclei.

The H–N–H bond angles are smaller than the expected tetrahedral angle of 109.5°, as observed in methane. This compression of the bond angle is attributed to the repulsive force exerted by a lone pair on the adjacent bonding electron groups.

The arrangement of electron pairs is called electron-pair geometry. The molecular geometry describes the arrangement of the atoms, and differs from the electron-pair geometry. The electron-pair geometry for ammonia is tetrahedral, whereas the molecular shape is trigonal pyramidal.

A water molecule also has four electron groups around the central atom. The electron pair geometry is also tetrahedral with two bonding electron groups and two lone pairs.

The greater repulsion exerted by two lone pairs further compresses the H–O–H bond angle in water molecules. It is much smaller than the ideal tetrahedral bond angle, and the molecular geometry is bent.