VSEPR theory and molecular shape

VSEPR theory (valence shell electron pair repulsion) predicts the shapes of molecules. Its idea is simple: the electron pairs around the central atom repel each other and arrange themselves as far apart as possible. A multiple bond is treated as a single super pair.

With no lone pairs on the central atom, the shape follows the number of bond pairs:

Lone pairs take up more room than bond pairs, because they are held by only one nucleus. The repulsions are in the order: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. So lone pairs squeeze the bonds closer. NH₃ (one lone pair) has an angle of 107°, and H₂O (two lone pairs) 104.5°, both less than the 109.5° of a perfect tetrahedron.

With lone pairs, start from the parent shape and name the shape by the atoms only:

• 3 bond pairs + 1 lone pair: trigonal pyramidal (NH₃).
• 2 bond pairs + 2 lone pairs: bent (H₂O).
• 4 bond pairs + 1 lone pair: see-saw (SF₄).
• 3 bond pairs + 2 lone pairs: T-shaped (ClF₃).
• 5 bond pairs + 1 lone pair: square pyramidal (BrF₅); an octahedron with one corner taken away.
• 4 bond pairs + 2 lone pairs: square planar (XeF₄).

In a trigonal bipyramid, lone pairs always go to the equatorial positions (round the middle). There they meet fewer of the strong 90° repulsions than they would at the top or bottom.