Count the things around the central atom — bonds and lone pairs alike — and put them as far apart as they will go. Two is a line, three a flat triangle, four a tetrahedron at 109.5°, six an octahedron.
Electron pairs are all negative, so they repel; the arrangement that survives is the one where they are furthest apart. It needs no quantum mechanics and no computer, and it gets the shape of most small molecules right on the back of an envelope.
Wherever a lone pair is involved. A lone pair is held by one nucleus rather than shared between two, so it spreads wider and pushes harder than a bond: ammonia closes to 107.8° and water to 104.5° rather than both sitting at 109.5°. The rule tells you the shape and then lies to you by a degree or two about the angle.
Estimate with the rule, then check it against the calculator that models it properly.
Open Molecules →