Final answer:
Electron group geometry considers all electron pairs (bonding and lone pairs), while molecular geometry considers only the atoms' placements, leading to differences when lone pairs are present.
Step-by-step explanation:
It is possible for the electron group geometry to differ from the molecular geometry due to the presence of lone pairs. The electron group geometry considers both the bonded electron pairs and the lone electron pairs around a central atom. In contrast, the molecular geometry only considers the positioning of the atoms and not the lone electron pairs. This differentiation is important in understanding how molecules are shaped.
For example, if you have a molecule with four bonding pairs and no lone pairs, its electron group geometry and molecular geometry would both be tetrahedral. However, if there's one lone pair and three bonding pairs, the electron group geometry would still be tetrahedral, but the molecular geometry would be trigonal pyramidal. The presence of the lone pairs affects the spatial arrangement of the atoms due to repulsion forces, causing the actual shape of the molecule to be different.
Therefore, the answer to the student's question is C: Electron group geometry can differ from molecular geometry when there are lone pairs.