Build molecules and predict their 3D geometry using VSEPR theory.
VSEPR theory (Valence Shell Electron Pair Repulsion) is one of the most powerful and widely applicable models in introductory chemistry, used to predict the three-dimensional geometry of molecules from a simple count of electron pairs. It forms the foundation of AP Chemistry Unit 2 and appears regularly in USNCO and IChO problems.
The central principle of VSEPR is that electron pairs — both bonding pairs and lone pairs — repel each other and arrange themselves to minimize that repulsion. The result is a predictable geometry: 2 electron pairs give linear, 3 give trigonal planar, 4 give tetrahedral, 5 give trigonal bipyramidal, and 6 give octahedral arrangements of electron pairs.
The molecular geometry (the shape described by the atoms alone) differs from the electron geometry when lone pairs are present on the central atom. Water (H₂O) has four electron pairs (2 bonding, 2 lone), giving a tetrahedral electron geometry — but the molecular geometry is bent, with a bond angle of approximately 104.5° rather than the ideal 109.5°. This is because lone pairs occupy more space than bonding pairs and compress the bonding angles.
Ammonia (NH₃) provides another classic example: three bonding pairs and one lone pair give a trigonal pyramidal molecular geometry with approximately 107° bond angles. Contrast this with methane (CH₄), which has four bonding pairs and no lone pairs, giving perfect tetrahedral geometry with exactly 109.5° angles.
Polarity follows directly from geometry. A molecule with polar bonds is nonpolar overall if the bond dipoles cancel due to symmetry (CO₂ is linear and nonpolar; H₂O is bent and polar). Understanding polarity from geometry is essential for predicting intermolecular forces and physical properties.
This VSEPR tool lets you build any molecule, add lone pairs, and immediately see the predicted 3D geometry with bond angles — making abstract molecular shapes concrete and memorable for any exam.