Predicting the Shapes of Molecules with Lone Pairs on the Central Atom

The NH3 molecule has four electron groups

(one lone pair and three bonding pairs).

• If we look only at the electrons, the electron geometry—the geometrical arrangement of the electron groups—is tetrahedral.
• The molecular geometry—the geometrical arrangement of the atoms—is trigonal pyramidal.
• The lone pair exerts its influence on the bonding pairs.
• The bond angles in NH3 are actually a few degrees smaller than the ideal tetrahedral angles (for reasons covered elsewhere).