A Lewis structure shows connectivity and valence-electron placement, but molecules are three-dimensional. Valence-shell electron-pair repulsion (VSEPR) theory predicts the local arrangement of atoms by treating bonds and lone pairs around a central atom as regions of electron density that arrange to reduce repulsion.

Count each single, double or triple bond as one electron-density region, and count each lone pair as one region.

The basic electron-domain geometries are:

• 2 regions: linear, approximately $180^\circ$;
• 3 regions: trigonal planar, approximately $120^\circ$;
• 4 regions: tetrahedral, approximately $109.5^\circ$;
• 5 regions: trigonal bipyramidal;
• 6 regions: octahedral.

Molecular geometry names only the positions of atoms, so lone pairs can change the molecular shape while the electron-domain geometry remains the same. For example, $NH_3$ has four electron regions around nitrogen: three N-H bonds and one lone pair. Its electron-domain geometry is tetrahedral, but its molecular geometry is trigonal pyramidal.

For $H_2O$, two O-H bonds and two lone pairs also give four electron regions. The molecular geometry is therefore bent rather than tetrahedral.

Lone pairs generally repel more strongly than bonding regions, so real bond angles can deviate from the ideal values.

VSEPR is a predictive model for many main-group molecules. It supplies the missing three-dimensional structure needed to reason about molecular dipoles, intermolecular interactions and chemical reactivity.