A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular geometry by assuming that electron groups (bonding pairs and lone pairs) around a central atom arrange to minimize repulsion. The number of electron groups determines electron geometry: 2 → linear, 3 → trigonal planar, 4 → tetrahedral, 5 → trigonal bipyramidal, 6 → octahedral. Lone pairs repel more strongly than bonding pairs, compressing bond angles and distinguishing molecular geometry (atom positions only) from electron geometry (all groups).
Work through a large variety of molecules systematically: count all electron groups, determine electron geometry, identify lone pairs, then name molecular geometry. Build physical models to develop 3D intuition. Compare H₂O (bent, 104.5°) vs. CH₄ (tetrahedral, 109.5°) to see the effect of lone pairs on angles.
Once you have drawn a Lewis structure for a molecule, VSEPR theory lets you predict its three-dimensional shape using one simple principle: electron groups repel each other and arrange themselves as far apart as possible around a central atom. An electron group is any region of electron density — a single bond, a double bond, a triple bond, or a lone pair all count as one group each. This is the critical counting rule: a C=O double bond is one electron group, not two, because the electrons in both bonds occupy roughly the same region of space.
Start by counting the total number of electron groups around the central atom. Two groups push to opposite sides of the atom, giving a linear arrangement (180° apart). Three groups spread into a trigonal planar shape (120° angles). Four groups adopt a tetrahedral arrangement (109.5° angles). Five and six groups give trigonal bipyramidal and octahedral geometries, respectively. This count gives you the electron geometry — the arrangement of all electron groups, whether they contain atoms or not.
The distinction between electron geometry and molecular geometry is where most students stumble. Molecular geometry describes only where the atoms are — lone pairs are invisible to experimental shape-determination methods. Water (H₂O) has four electron groups around oxygen (two bonding pairs and two lone pairs), so its electron geometry is tetrahedral. But since we only "see" the two O–H bonds, its molecular geometry is bent. Ammonia (NH₃) also has tetrahedral electron geometry (three bonds plus one lone pair), but its molecular geometry is trigonal pyramidal. The name changes because removing a vertex from a tetrahedron gives a pyramid, not a flat triangle.
Lone pairs also compress bond angles below the ideal values. A lone pair spreads out more than a bonding pair (there is no second nucleus to confine it), so it exerts greater repulsion on neighboring groups. In methane (CH₄), with four identical bonding pairs, angles are a perfect 109.5°. In ammonia, the lone pair pushes the three N–H bonds slightly closer together to about 107°. In water, two lone pairs compress the H–O–H angle further to about 104.5°. This predictable compression lets you refine angle estimates beyond the ideal geometry and explains trends across related molecules.
To apply VSEPR systematically to any molecule: (1) draw the Lewis structure, (2) count electron groups around the central atom, (3) determine the electron geometry from the count, (4) identify how many groups are lone pairs, and (5) name the molecular geometry based on the positions of atoms only. This procedure works for molecules with expanded octets (like PCl₅ or SF₆) just as well as for simple cases. The shapes you predict here become essential for determining molecular polarity — a topic that depends entirely on knowing the geometry first.