VSEPR and Molecular Geometry

Electrons repel each other, so groups of electrons around a central atom - whether a bonding pair or a lone pair - push each other away and settle into the arrangement that keeps them as far apart as possible. That arrangement is the molecule's electron domain geometry. Count the domains: 2 gives linear (180 degrees), 3 gives trigonal planar (120 degrees), 4 gives tetrahedral (109.5 degrees), 5 gives trigonal bipyramidal (90 and 120 degrees), and 6 gives octahedral (90 degrees). These are the ideal angles when every domain is an identical bonding pair.

Lone pairs change the picture. A lone pair belongs to only one nucleus, so its electron cloud spreads out wider and closer to the central atom than a bonding pair, which is stretched taut between two nuclei. That wider cloud repels neighboring domains more strongly, squeezing the bond angles below the ideal. Start from the Lewis structure: NH3 has four domains around nitrogen, three N-H bonds and one lone pair, so the electron domain geometry is tetrahedral, but with one corner occupied by a lone pair instead of an atom, the observed molecular geometry is trigonal pyramidal and the H-N-H angle shrinks to about 107 degrees.

This is why CH4 (109.5 degrees, no lone pairs), NH3 (about 107 degrees, one lone pair), and H2O (104.5 degrees, two lone pairs) all have four electron domains but increasingly compressed angles as lone pairs replace bonding pairs. Molecular geometry, the shape you actually see from the atoms, differs from electron domain geometry whenever lone pairs are present, and that distinction carries straight into polarity: a bent or pyramidal shape with polar bonds gives water and ammonia a permanent dipole moment, setting up everything that unit covers next about intermolecular forces.