VSEPR Theory
Valence Shell Electron Pair Repulsion Theory (Model)
KEY POINTS:
1. VSEPR theory – states that the repulsion forces between valence electron pairs surrounding an atom
cause them to be oriented as far apart as possible. Molecules are classified according to the number of
electron pairs (shared electron pairs and unshared electron pairs) surrounding a central atom.
2. AB2E where, “A” represents the central atom; it is usually a nonmetal or the least electronegative
atom; most other atoms are bonded off the central atom. “B” represents atoms bonded to the central
atom (subscript tells how many). “E” represents unshared electron pairs on the central atom (subscript
tells how many). Unshared electron pairs are treated as any other atom in terms of shape but the
repulsion force is stronger making the geometry asymmetric (unshared pair forces bonded pairs closer
together). Molecules containing carbon normally have carbon as the central atom.
3. All molecules with one atom bonded to the central atom or diatomic molecules are linear.
4. All symmetrical molecules are nonpolar even if their individual bonds are polar since their symmetry
cancels out their polarity (no net displacement of charge). In order for a molecule to be symmetrical
and nonpolar, the following conditions must be met:
1. all atoms bonded to the central atom must be the same; and,
2. there can be no unshared electron pairs on the central atom as determined by the Lewis
structure.
5. All nonsymmetrical (asymmetric) molecules are polar if individual bonds are polar, and nonpolar if
individual bonds are nonpolar. In order for a molecule to be nonsymmetrical, only one of the two
following conditions must be met:
1. all atoms bonded to the central atom are not the same; and/or,
2. there is at least one unshared electron pair on the central atom as determined by the Lewis
structure.
6. Molecular shape (geometry) is determined by the number of shared and unshared pairs on the central
atom.
7. Multiple bonds (double and triple) are treated as single bonds/shared pairs.
8. Shape Summary: (assuming all bonded atoms are the same).
a) AB linear, may be polar or nonpolar (includes diatomic molecules; where A=B i.e. A2)
a) AB2, linear, nonpolar.
b) AB2E, bent (triangular), 115o.
c) AB3, triangular planar, nonpolar, 120o.
d) AB4, tetrahedral, 109.5o, nonpolar.
e) AB3E, triangular pyramidal, 107.3o.
f) AB2E2, bent, 104.5o
g) AB5, triangular bipyramidal, nonpolar
h) AB6, octahedral, nonpolar.