Final answer:
Expanded valence shell molecules, such as PCl5 and PF5, can have more than eight electrons in their valence shell and typically feature central atoms with access to d orbitals. These molecules have geometries like trigonal bipyramidal, as predicted by the VSEPR theory.
Step-by-step explanation:
Expanded valence shell molecules are those in which the central atom can have more than eight electrons in its valence shell. This phenomenon occurs primarily with central atoms located in the third period or beyond of the periodic table that have access to empty d orbitals.
The valence shell electron-pair repulsion (VSEPR) theory helps predict the molecular geometry of these types of molecules. For instance, PCl5 has a trigonal bipyramidal geometry with three chlorine atoms equatorially attached to the phosphorous, and two chlorine atoms axially attached, reflecting the arrangement of five electron pairs aiming to minimize repulsion between them.