Théorie VSEPR

VSEPR theory assumes that electron groups—including single bonds, multiple bonds, and lone pairs—repel each other and arrange themselves to maximize distance and minimize repulsions. The molecular geometry around a central atom is dictated by how these electron groups position themselves to stay as far apart as possible.

VSEPR theory predicts five basic shapes: linear for two electron groups, trigonal planar for three, tetrahedral for four, trigonal bipyramidal for five, and octahedral for six electron groups. Each shape represents the optimal arrangement that minimizes electron pair repulsion around the central atom.

Lone pairs occupy more space than bonding pairs because they are bound to only one nucleus, whereas bonding pairs are shared by two nuclei. This causes lone pair–lone pair repulsions to be greater than bonding pair repulsions, compressing bond angles. In ammonia and water, lone pairs reduce H–N–H and H–O–H angles below the ideal tetrahedral angle of 109.5°.

Electron-pair geometry describes the arrangement of all electron groups—both bonding pairs and lone pairs—around the central atom. Molecular geometry describes only the positions of atoms, not electrons. These differ when lone pairs are present. For example, ammonia has tetrahedral electron-pair geometry but trigonal pyramidal molecular geometry.

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 repulsive forces than bonding pairs, compressing the H–O–H bond angle significantly below 109.5°, resulting in a bent molecular geometry.

In trigonal bipyramidal electron-pair geometry, the three equatorial positions have more available space due to 120° bond angles compared to the two axial positions. Lone pairs preferentially occupy the equatorial positions because the larger space reduces repulsion. When two lone pairs and four bonding pairs are arranged octahedrally, the lone pairs position 180° apart, creating a square planar molecular structure.

Methane has four bonding electron pairs and no lone pairs around carbon, so all electron groups repel equally, creating an ideal tetrahedral geometry with H–C–H angles of 109.5°. Ammonia has three bonding pairs and one lone pair; the lone pair's stronger repulsion compresses the H–N–H angles to less than 109.5°, demonstrating how lone pairs distort ideal geometries.