Why molecules have shapes
Electrons around an atom push each other apart. Add atoms and lone pairs to the centre of this molecule and watch that push decide its shape.
A water molecule is bent at 104.5°. Carbon dioxide is a straight line. Methane is a tetrahedron, and sulfur hexafluoride an octahedron. None of these shapes is arbitrary: each follows from one physical fact. The pairs of electrons around a central atom repel one another, so they spread out as far as they can, and the atoms attached to them have to go where the electrons point.
That idea, called valence shell electron pair repulsion (VSEPR), lets you predict the shape of a molecule from a drawing of its electrons, before you have seen a single measurement. The three chapters below build it up: first the shapes themselves and why lone pairs squeeze bond angles, then the Lewis structures that tell you how many electron pairs there are, then polarity, which is where shape starts to matter for everything from why water dissolves salt to why carbon dioxide does not stick to itself.
How this is made and checked
Every drawing on this site is computed, not drawn by hand. A small geometry engine finds the arrangement of electron domains by letting them repel on a sphere, builds Lewis structures by the textbook procedure for a checked list of molecules, and adds bond dipoles as vectors. Measured bond angles and dipole moments come from the NIST Computational Chemistry Comparison and Benchmark Database, each traceable to its original measurement. The engine is tested against exact geometry and published structures, and an independent reviewer checks every number against its source. Every claim is listed with its source, and every simplification is named next to the figure that makes it. More about how it is checked.