The Shapes of Molecules: VSEPR Theory
The shape (geometry) of a molecule influences physical and chemical properties, including melting point, boiling point, and reactivity
Valence Shell Electron Pair Repulsion (VSEPR) Theory: based on Lewis structures and the principles that:
Valence electron pairs repel each other
The geometry around the central atom will minimize the repulsion
VSEPR Nomenclature
A = central atom
X = bonding atom
E = lone pair
If a molecule has 2 or more resonance structures, the VSEPR model can be applied to any one of them
If there’s more than one central atom in a molecule, consider the bonding about each atom independently
Steric number = number of atoms bonded to central atom + number of lone pairs
Used to predict geometries
Formula Type | SN | Geometry | Bond angle |
---|---|---|---|
AX₂ | 2 | Linear | 180° |
AX₃ | 3 | Trigonal planar | 120° |
AX₄ | 4 | Tetrahedral | 109.5° |
AX₅ | 5 | Trigonal bipyramidal | 120° (equatorial)90° (axial) |
AX₆ | 6 | Octahedral | 90° |
Electrons in bonds have less “spatial distribution that lone pairs, meaning:
Electrons in bonds take up less space
Lone-pair electrons can be anywhere
Lone pair electrons take up more space and therefore experience more repulsion
Repulsive forces for molecules with lone pairs
Lone pair/lone pair repulsion (most repulsion) -> lone pair/bonding pair repulsion -> bonding pair/bonding pair repulsion (least repulsion)
In molecules with lone-pair electrons, angles between bonded atoms tend to be smaller
When atomic size increases, lone pairs occupy larger spatial volumes, so angles between bonded atoms tend to be even smaller
Formula Type | SN | Geometry | Bond angle |
---|---|---|---|
AX₂E | 3 | Bent | Less than 120° |
AX₃E | 4 | Trigonal pyramidal | Less than 109.5° |
AX₂E₂ | 4 | Bent | Less than 109.5° |
AX₄E | 5 | See-saw | Less than 120° (equatorial)Less than 90° (axial) |
AX₃E₂ | 5 | T-shaped | Less than 90° |
AX₂E₃ | 5 | Linear | 180° - not less than because there’s no other way to minimize repulsion |
AX₅E | 6 | Square pyramidal | Less than 90° |
AX₄E₂ | 6 | Square planar | 90° |
AX₃E₃ | 6 | T-shaped | Less than 90° |
AX₂E₄ | 6 | Linear | 180° |
The shape (geometry) of a molecule influences physical and chemical properties, including melting point, boiling point, and reactivity
Valence Shell Electron Pair Repulsion (VSEPR) Theory: based on Lewis structures and the principles that:
Valence electron pairs repel each other
The geometry around the central atom will minimize the repulsion
VSEPR Nomenclature
A = central atom
X = bonding atom
E = lone pair
If a molecule has 2 or more resonance structures, the VSEPR model can be applied to any one of them
If there’s more than one central atom in a molecule, consider the bonding about each atom independently
Steric number = number of atoms bonded to central atom + number of lone pairs
Used to predict geometries
Formula Type | SN | Geometry | Bond angle |
---|---|---|---|
AX₂ | 2 | Linear | 180° |
AX₃ | 3 | Trigonal planar | 120° |
AX₄ | 4 | Tetrahedral | 109.5° |
AX₅ | 5 | Trigonal bipyramidal | 120° (equatorial)90° (axial) |
AX₆ | 6 | Octahedral | 90° |
Electrons in bonds have less “spatial distribution that lone pairs, meaning:
Electrons in bonds take up less space
Lone-pair electrons can be anywhere
Lone pair electrons take up more space and therefore experience more repulsion
Repulsive forces for molecules with lone pairs
Lone pair/lone pair repulsion (most repulsion) -> lone pair/bonding pair repulsion -> bonding pair/bonding pair repulsion (least repulsion)
In molecules with lone-pair electrons, angles between bonded atoms tend to be smaller
When atomic size increases, lone pairs occupy larger spatial volumes, so angles between bonded atoms tend to be even smaller
Formula Type | SN | Geometry | Bond angle |
---|---|---|---|
AX₂E | 3 | Bent | Less than 120° |
AX₃E | 4 | Trigonal pyramidal | Less than 109.5° |
AX₂E₂ | 4 | Bent | Less than 109.5° |
AX₄E | 5 | See-saw | Less than 120° (equatorial)Less than 90° (axial) |
AX₃E₂ | 5 | T-shaped | Less than 90° |
AX₂E₃ | 5 | Linear | 180° - not less than because there’s no other way to minimize repulsion |
AX₅E | 6 | Square pyramidal | Less than 90° |
AX₄E₂ | 6 | Square planar | 90° |
AX₃E₃ | 6 | T-shaped | Less than 90° |
AX₂E₄ | 6 | Linear | 180° |