VSEPR theory
/ VESS-per /
Blow up a few balloons and tie them together at their necks — they automatically push apart as far as they can, taking a predictable shape. Electron groups around an atom behave just like those balloons: being negatively charged, they repel one another and spread out to stay as far apart as possible. VSEPR theory uses exactly this idea to predict the shape of a molecule.
Precisely, VSEPR stands for Valence Shell Electron Pair Repulsion. The rule is: count the groups of electrons around a central atom — each bond (single, double, or triple counts as one group) and each lone pair — then arrange those groups to minimize repulsion. Two groups give a linear shape, three a trigonal plane, four a tetrahedron, and so on. Lone pairs push a bit harder than bonds, so they squeeze the bond angles slightly.
VSEPR matters because molecular shape governs polarity, smell, drug action, and reactivity, yet VSEPR predicts it from a simple electron count with no heavy math. The honest caveat is that it is a rule of thumb, not a derivation from quantum mechanics: it gives good shapes and approximate angles, but it does not explain why bonds form or predict exact angles, and it occasionally fails for unusual molecules.
Water has four electron groups around the oxygen: two bonds to hydrogen and two lone pairs. They arrange roughly tetrahedrally, but the two lone pairs push harder and bend the H–O–H angle down to about 104.5° instead of the ideal 109.5° — which is exactly why water is a bent, polar molecule.
Lone pairs bend water's angle below the ideal tetrahedral 109.5°.
Count electron groups, not atoms, and remember lone pairs. The shape name you report (e.g. 'bent', 'pyramidal') describes only the positions of the atoms, but the lone pairs still occupy space and set the geometry — leaving them out is the most common VSEPR mistake.