The Valence Shell Electron Repulsion (VSEPR) model can predict the structure of most molecules and polyatomic ions which the central atom is a non-metal.
The VSEPR model works for some structures in which the central atom is a metal.
Pairs of electrons (in bonds and in lone pairs), called groups, repel each other.
Because electrons repel each other electrostatically, the most stable arrangement of electron groups (the one with the lowest energy) is the one that minimizes repulsion.
Groups are positioned around the central atom in a way that produces the molecular structure with the lowest energy.
The molecule or polyatomic ion is given an AXmEn designation. A is the central atom, X is a bonded atom, E is a nonbonding valence electron group, and m and n are integers.
The number of groups is equal to the sum of m and n.
The molecular geometry around the central atom is the arrangement of the bonded atoms in a molecule or polyatomic ion.
2 groups around the central atom results in a linear geometry with a bond angle of 180 degrees.
3 groups around the central atom results in a trigonal plane geometry with a bond angle of 120 degrees.
4 groups around the central atom results in a tetrahedron geometry with a bond angle of 109.5 degrees.
5 groups around the central atom results in a trigonal bipyramid geometry with a bond angle of 90 and 120 degrees.
6 groups around the central atom results in a octahedron geometry with a bond angle of 90 degrees.
7 groups around the central atom results in a pentagonal bipyramid geometry with a bond angle of 90 and 72 degrees.
8 groups around the central atom results in a square antiprism geometry with a bond angle of 70.5, 99.6, and 109.5 degrees.
