VSEPR theory
/ VES-per or V-S-E-P-R /
Once you know which atoms are bonded to which (from a Lewis structure), how do you predict the actual three-dimensional shape of the molecule? VSEPR theory gives a wonderfully simple answer: the pairs of electrons around a central atom repel one another, so they spread out as far apart in space as they can. Arrange the electron pairs to minimise their crowding, and you have the molecule's shape.
The recipe is to count the regions of electron density around the central atom — each single bond, double bond, triple bond or lone pair counts as one region — and then place those regions at the angles that keep them maximally apart. Two regions go linear (180 degrees), three go trigonal planar (120 degrees), four go tetrahedral (about 109.5 degrees), five go trigonal bipyramidal, and six go octahedral. Lone pairs occupy regions too, and they take up more room than bonding pairs (a lone pair is held by only one nucleus, so it spreads out), which squeezes the bond angles a little smaller than the ideal.
VSEPR is the everyday tool for predicting molecular geometry in inorganic chemistry, and it is impressively reliable for main-group species: it correctly predicts that water is bent, ammonia is pyramidal, methane is tetrahedral, SF6 is octahedral, and XeF4 is square planar. It is a model based on electron-pair repulsion, not a rigorous theory, and it can stumble for some transition-metal and heavy-element compounds, but for the molecules you draw with Lewis structures it is hard to beat for speed and accuracy.
Methane CH4 has four bonding pairs around carbon and no lone pairs; pushed as far apart as possible, they point to the corners of a tetrahedron with 109.5-degree angles. Ammonia NH3 has three bonds plus one lone pair around nitrogen; the lone pair pushes the bonds together, giving a pyramidal shape with H-N-H angles of about 107 degrees.
Four electron regions go tetrahedral; a lone pair (NH3) squeezes the angles smaller.
VSEPR predicts the arrangement of electron regions; the named shape, however, refers only to the positions of the atoms, ignoring the lone pairs. Four regions on water is tetrahedral electronically, but the molecule is called bent.