Final answer:
Molecular geometries are different from electron geometries because molecular geometry only describes the arrangement of atoms, whereas electron geometry includes all areas where electrons are, including lone pairs. The VSEPR theory explains how lone pairs alter electron group geometry, causing a difference between both geometries.
Step-by-step explanation:
The reason why molecular geometries differ from electron geometries lies in the concept of Valence Shell Electron Pair Repulsion (VSEPR) theory. Electron group geometry considers all regions of electron density, including bonding pairs of electrons that form bonds between atoms, and nonbonding electron pairs, also known as lone pairs, which do not participate in bonding but still repel other electron pairs. On the other hand, molecular geometry describes the arrangement of only the atoms in a molecule, disregarding the lone pairs. When there are no lone pairs on the central atom, the electron group geometry and the molecular geometry will be identical. However, the presence of lone pairs can alter the electron group geometry, making it different from the molecular structure.
Molecular structure is three-dimensional and better represents a molecule's physical and chemical properties by describing the molecular bonds in terms of distances, angles, and spatial arrangements. Bond angles and bond distances are crucial parameters for understanding molecular shapes, with distances often measured in Ångstroms or picometers. Molecular geometry significantly impacts the physical properties and chemical reactivity of a molecule and differs from electron geometry due to the space occupied by lone pairs.