Molecular geometry

From Lewis structure to shape

Lewis structure first

Begin with valence electrons and a Lewis structure. Around the central atom, each bond counts as one electron domain—even a double or triple bond—and each lone pair counts as one domain.

The notation AX_nE_m summarizes the central atom A, n bonded atoms X and m lone pairs E. VSEPR then predicts a low-repulsion arrangement of those domains.

VSEPR is a model of electron-density regions, not a picture of electrons as fixed balls. Its power comes from capturing enough of the repulsion physics to predict many main-group shapes quickly.

Electron-domain geometry versus molecular shape

H₂O also starts from four domains, but two are lone pairs. Its molecular shape is bent.

Lone pairs change angles

Lone-pair electron density is concentrated near one nucleus rather than shared between two nuclei, so it occupies more angular space around the central atom. The qualitative repulsion order is:

This explains the progression CH₄ 109.5°, NH₃ about 107°, H₂O about 104.5°. The measured angles show that “tetrahedral electron geometry” does not require every molecule to display the ideal tetrahedral bond angle.

Beyond four electron domains

Five and six domains

A trigonal bipyramid has axial and equatorial positions. Axial sites have three 90° interactions, while an equatorial site has only two. A lone pair therefore prefers an equatorial position, giving SF₄ a seesaw shape and ClF₃ a T shape.

Six domains form an octahedral arrangement. Removing one atom position for a lone pair gives square pyramidal; two opposite lone pairs can leave a square-planar molecular shape, as in XeF₄.

Mini VSEPR explorer

Multiple bonds and real bond angles

A double or triple bond counts as one VSEPR domain because it occupies one direction from the central atom, but its electron density is greater than that of a single bond and can repel neighboring domains more strongly.

Real bond angles are measured quantities, not exact outputs of a simple domain count. Substituent size, multiple bonds, electronegativity and lone-pair character can all shift them away from ideal values.

Structural methods such as gas electron diffraction, rotational spectroscopy and X-ray or neutron diffraction provide experimental geometries.

Reading and using molecular shape

Geometry and molecular polarity

Bond dipoles are vectors. Symmetry can make polar bonds cancel, as in linear CO₂ or trigonal-planar BF₃. In bent H₂O or trigonal-pyramidal NH₃, the bond dipoles do not cancel.

Shape also controls steric accessibility and orbital approach, so molecular geometry influences reaction pathways, intermolecular interactions and molecular recognition.

Worked route from Lewis structure to shape

The logic is always the same: count domains first, assign domain geometry, then hide the lone-pair positions when naming molecular shape.

Common shapes and examples

Seeing a 3D molecule on a 2D page

Structural drawings use conventions to encode depth. A normal line lies roughly in the page, a solid wedge points toward the viewer, and a hashed wedge points away. These symbols do not represent special bond types; they represent orientation in three-dimensional space.

A tetrahedral carbon cannot be represented faithfully as four bonds separated by 90° on paper. The wedge–dash convention reminds us that the real angle is about 109.5°.

Hybridization labels such as sp³ can be useful later, but they are not required to run the VSEPR prediction. Geometry should first be understood from electron domains; orbital models can then explain bonding in more detail.

Real molecules and model limits

Limits of VSEPR

VSEPR works best for many main-group molecules. It does not predict bond energies, spectra or magnetism, and it can fail for transition-metal complexes because partially filled d orbitals introduce ligand-field effects absent from the simple repulsion model.

Heavy p-block hydrides can also show bond angles far from tetrahedral expectations. A model is most useful when its limits are explicit.

Local geometry and molecular conformation are different

VSEPR predicts the local arrangement around one central atom. A larger molecule can contain several such centers and still adopt many overall conformations because single bonds can rotate.

Ethane, for example, is approximately tetrahedral around each carbon, but rotation about the C–C bond produces staggered and eclipsed conformations. The local carbon geometry does not change; the relative orientation of the two tetrahedral groups does.

In rings and biomolecules, conformational preferences can dominate three-dimensional shape. Steric strain, torsional strain, hydrogen bonding and solvent effects then act in addition to local VSEPR geometry.

Geometry is measured, not merely drawn

Microwave spectroscopy can determine gas-phase rotational constants and infer bond lengths and angles. X-ray diffraction maps electron density in crystals, while neutron diffraction is especially useful for locating light nuclei such as hydrogen. Electron diffraction is another route for gas-phase structures.

These measurements are why values such as the 104.5° H–O–H angle are physical data rather than decorative numbers attached to a model. VSEPR succeeds when it reproduces the broad pattern of those observations and fails where its assumptions are too simple.

Exercises

SO₂

Two S–O bonding domains and one lone pair surround S. Predict domain geometry and molecular shape.

SF₄

Why does its lone pair occupy an equatorial site?

XeF₄

Four bonds and two lone pairs surround Xe. Predict the molecular shape.