Two groups arrange linearly; three arrange trigonal-planar; four arrange tetrahedrally. Lone pairs occupy regions too, so removing them from the shape name changes the molecular shape. In water, four groups give bent atom positions.

The model has a domain of validity. Explain its assumptions before using it in a changed context, then compare your prediction with evidence.

• Electron groups spread out. Molecular shape names the positions of atoms, while electron geometry also counts lone pairs.
• Electron geometry and molecular shape match only when there are no central lone pairs.