Molecular Geometry and VSEPR Theory: Predicting Shapes
By ChemistryIQ Team · February 22, 2026
What Is VSEPR Theory?
VSEPR stands for Valence Shell Electron Pair Repulsion. The core idea is simple: electron pairs around a central atom repel each other and arrange themselves as far apart as possible in 3D space. This arrangement determines the molecule's shape. To use VSEPR, you need a correct Lewis structure first. From the Lewis structure, count the number of electron groups (bonding pairs and lone pairs) around the central atom. Double and triple bonds each count as ONE electron group.
The Five Basic Electron Geometries
2 electron groups → Linear (180° angles). Example: CO2. 3 electron groups → Trigonal planar (120° angles). Example: BF3. 4 electron groups → Tetrahedral (109.5° angles). Example: CH4. 5 electron groups → Trigonal bipyramidal (90° and 120° angles). Example: PCl5. 6 electron groups → Octahedral (90° angles). Example: SF6. These are the electron geometries — the arrangement of ALL electron groups including lone pairs.
Molecular Shape vs Electron Geometry
Molecular shape describes the arrangement of ATOMS only (not lone pairs). If all electron groups are bonding pairs, the molecular shape matches the electron geometry. But when lone pairs are present, the molecular shape is different from the electron geometry because lone pairs are invisible in the 3D structure. Key examples with 4 electron groups (tetrahedral electron geometry): 4 bonding, 0 lone pairs → Tetrahedral (CH4). 3 bonding, 1 lone pair → Trigonal pyramidal (NH3). 2 bonding, 2 lone pairs → Bent (H2O).
How Lone Pairs Affect Bond Angles
Lone pairs occupy more space than bonding pairs because they are held closer to the nucleus and spread out more. This extra repulsion compresses the bond angles between bonding pairs. In CH4 (no lone pairs), bond angles are exactly 109.5°. In NH3 (one lone pair), bond angles compress to about 107°. In H2O (two lone pairs), bond angles compress further to about 104.5°. The ranking of repulsion strength: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.
Applying VSEPR: Step-by-Step
Step 1: Draw the correct Lewis structure. Step 2: Count electron groups around the central atom (bonding pairs + lone pairs). Remember: double/triple bonds = 1 group. Step 3: Determine the electron geometry from the number of groups. Step 4: Determine the molecular shape by considering only the positions of atoms. Step 5: Predict approximate bond angles, adjusting for lone pair compression. Need to verify your VSEPR predictions? ChemistryIQ analyzes molecular structures from a photo and identifies geometry, bond angles, and polarity.
Polarity and Molecular Shape
Molecular shape determines whether a molecule is polar or nonpolar. A molecule is nonpolar if its bond dipoles cancel out due to symmetry (linear with identical atoms, trigonal planar, tetrahedral with identical substituents). A molecule is polar if the bond dipoles don't cancel — this happens when the shape is asymmetric (bent, trigonal pyramidal) or when different atoms create unequal dipoles. CO2 is linear and nonpolar (dipoles cancel). H2O is bent and polar (dipoles don't cancel).
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Common questions about molecular geometry and vsepr theory
One. In VSEPR theory, a double bond (or triple bond) counts as a single electron group because all the electrons in that bond point in the same direction from the central atom.
Draw the molecular shape, add arrows pointing from less electronegative to more electronegative atoms. If the arrows cancel out (symmetric arrangement), the molecule is nonpolar. If they don't cancel, the molecule is polar.
With 5 total electron groups (trigonal bipyramidal electron geometry), 3 bonding pairs and 2 lone pairs gives a T-shaped molecular geometry. The lone pairs occupy the equatorial positions where they have more room.