What effect does a lone pair have on the geometry of a molecule?

A lone pair of electrons significantly influences molecular geometry due to its spatial requirements and repulsive interactions with bonding pairs of electrons. Lone pairs occupy more space around the central atom compared to bonding pairs because they are not shared with another atom. This extra repulsion causes a distortion in the geometry of the molecule, pushing the bonding pairs closer together and thus altering the ideal bond angles.

In many common geometries, like tetrahedral or trigonal bipyramidal, the presence of lone pairs results in modifications of the angles between the bonded atoms. For instance, in a tetrahedral geometry with one lone pair, the molecular shape becomes trigonal pyramidal, which manifests as smaller bond angles due to the increased repulsive force from the lone pair.

This ability to exert a strong influence on the arrangement of bonded atoms makes the understanding of lone pairs crucial when predicting the three-dimensional structures of molecules. Thus, the correct answer reflects the reality that lone pairs push against bonding pairs, modifying the expected bond angles and the overall geometry.