What VSEPR theory says
The idea behind VSEPR — Valence Shell Electron Pair Repulsion — is simple: negative charges repel, so the groups of electrons around a central atom push each other as far apart as they can. Whatever 3D arrangement keeps those groups farthest apart is the shape the molecule takes. To use it you only need one thing: an accurate Lewis structure that tells you how many electron groups surround the central atom. Pick any molecule in the tool above to see its predicted shape, bond angle, and hybridization.
Electron domains vs. bonding regions
The number VSEPR actually cares about is the count of electron domains: every bond plus every lone pair on the central atom. A key rule — a double or triple bond still counts as one domain, because the extra electrons share the same direction in space. So the formula is:
electron domains = bonding regions + lone pairs on the central atom
For carbon dioxide (CO₂) the central carbon has two double bonds and no lone pairs, giving 2 domains and a linear, 180° shape. For water (H₂O) the oxygen has 2 bonds plus 2 lone pairs, giving 4 domains. The domains aim at a tetrahedron, but only the two hydrogen atoms are visible, so the molecular shape is bent, not tetrahedral.
How lone pairs change the shape
This is the heart of the topic. Three molecules all have four electron domains, yet their shapes differ because of lone pairs:
- CH₄ (methane): 4 bonds, 0 lone pairs → tetrahedral, 109.5°
- NH₃ (ammonia): 3 bonds, 1 lone pair → trigonal pyramidal, 107°
- H₂O (water): 2 bonds, 2 lone pairs → bent, 104.5°
A lone pair is held by just one atom, so it billows outward and pushes harder than a bonding pair (which is pinned between two nuclei). Each lone pair you add squeezes the remaining bonds a little closer together, shaving the angle down step by step. The geometry name describes only the atoms you can see, even though the lone pairs are doing the steering.
From domains to the angle
Once you know the domain count, the ideal angles follow a fixed menu: 2 domains → 180°, 3 → 120°, 4 → 109.5°, 5 → 90° and 120° (trigonal bipyramidal), 6 → 90° (octahedral). Lone pairs then nudge the observed angle below the ideal. The central atom’s hybridization tracks the same count — sp for 2 domains, sp² for 3, sp³ for 4 — which is why valence electrons and the Lewis dot picture feed straight into the shape. Shape, in turn, decides whether bond dipoles cancel, which sets a molecule’s overall polarity.
Using this with a class
Have students draw the Lewis structure first, count the domains out loud, and predict the shape before clicking a molecule. Compare CH₄, NH₃, and H₂O side by side to watch the angle shrink as lone pairs appear. The widget is free to embed on your own site or LMS using the snippet below.