VSEPR理論と孤立電子対の効果

Ammonia has a lone pair of electrons on the central nitrogen atom, while methane has only bonding pairs. The lone pair occupies more space than bonding pairs because it is bound to only one nucleus rather than shared by two. This lone pair-bonding pair repulsion compresses the H-N-H bond angles below the ideal tetrahedral angle of 109.5°, creating a trigonal pyramidal molecular shape instead of tetrahedral.

Electron-pair geometry describes the arrangement of all electron groups around a central atom, including both bonding pairs and lone pairs. Molecular geometry describes only the positions of atoms, excluding lone pairs. When no lone pairs are present, these geometries are identical. However, when lone pairs exist, electron-pair geometry and molecular geometry differ, as lone pairs occupy space but are not part of the molecular structure.

According to VSEPR theory, electron-pair repulsions follow this order from greatest to least: lone pair-lone pair repulsion is strongest, followed by lone pair-bonding pair repulsion, and bonding pair-bonding pair repulsion is weakest. This hierarchy explains why lone pairs occupy larger regions of space than bonding electrons and why their placement significantly affects molecular geometry and bond angles.

In trigonal bipyramidal electron-pair geometry, axial positions are surrounded by 90° bond angles, while equatorial positions have more space available due to 120° bond angles. Lone pairs prefer equatorial positions because these spacious regions can more easily accommodate the larger lone pairs, minimizing repulsive forces and stabilizing the molecular structure.

Water has four electron groups around the central oxygen atom: two bonding pairs and two lone pairs. The electron-pair geometry is tetrahedral, but the two lone pairs exert greater repulsion than bonding pairs, further compressing the H-O-H bond angle well below the ideal tetrahedral angle of 109.5°. This results in a bent molecular geometry with significantly reduced bond angles.

Xenon difluoride has a trigonal bipyramidal electron-pair geometry with three lone pairs and two bonding pairs. All three lone pairs occupy equatorial positions to minimize repulsion, leaving only the two fluorine atoms in axial positions. This arrangement results in a linear molecular geometry with the two Xe-F bonds aligned at 180°.

Xenon tetrafluoride has an octahedral electron-pair geometry with six electron groups: four bonding pairs and two lone pairs. The two lone pairs occupy opposite sides of the octahedron, positioned 180° apart. This arrangement minimizes lone pair-lone pair repulsion while leaving the four fluorine atoms in a square planar configuration around the central xenon atom.