Why do Lewis structures not always show a molecule’s actual three-dimensional shape?

Lewis structures do not always show a molecule’s actual three-dimensional shape because they are primarily intended as a topological portrayal of bonding rather than a geometric one.1

While Lewis structures effectively identify bonding and lone pairs of electrons, they do not inherently account for the spatial arrangement of atoms. The primary reasons for this limitation include:2

• Focus on Connectivity: Lewis structures focus on how atoms are connected by electron pairs and the presence of lone pairs, rather than the physical orientation of those atoms in space.12
• Lack of Geometric Prediction: In its basic form, the theory does not suggest the specific three-dimensional shapes that molecules adopt.2
• Requirement for Additional Theory: To determine actual molecular geometry, chemists use VSEPR theory, which builds upon the Lewis approach. VSEPR theory posits that electron pairs repel one another and will arrange themselves to minimize these repulsions, thereby dictating the molecule's three-dimensional shape.13

Ultimately, while Lewis structures are a useful rule of thumb for understanding chemical bonding, they must be supplemented by models like VSEPR or quantum mechanical theories to accurately predict or rationalize molecular geometry.132