Final answer:
Electron-pair geometry considers both bonding and lone electron pairs to determine the overall shape, while molecular structure considers the arrangement of atoms to define the actual shape of the molecule. They differ when lone pairs are present, as lone pairs can alter the distribution of atoms and thus the molecular shape.
Step-by-step explanation:
The electron-pair geometry and the molecular structure (or molecular geometry) are terms that describe the three-dimensional arrangement of atoms in a molecule, but they focus on different aspects. The electron-pair geometry considers the spatial arrangement of all electron pairs (bonding and lone pairs) around the central atom, which dictates the overall shape.
In contrast, the molecular structure only takes into account the spatial arrangement of atoms, not lone electron pairs, which defines the actual shape of the molecule. This means the electron-pair geometry and molecular structure will coincide when there are no lone pairs, but will differ when lone pairs affect the distribution of atoms.
It's important to minimize repulsions between areas of high electron density, which is why molecules adopt specific shapes. This understanding is key when predicting how atoms in a molecule are arranged (arrangement of atoms), how this impacts properties like polarity (dipole moment), and how they influence molecular interactions.