Why is a water molecule bent?

Count the pairs

An oxygen atom has six electrons in its outer shell. In water it shares one with each hydrogen atom, and each hydrogen shares one back. That makes two bonding pairs. The other four outer electrons of oxygen sit in two lone pairs, which belong to oxygen alone.

So the oxygen is surrounded by four pairs of electrons. Each pair is a cloud of negative charge, and like charges repel.

Push them apart

Four clouds that repel each other get as far apart as they can. On paper that looks like a cross, 90° apart. But molecules are three-dimensional, and in three dimensions four things get furthest apart by pointing to the corners of a tetrahedron, 109.5° apart. Methane, CH₄, is exactly that shape.

Water has the same four pairs, but two of them are lone pairs. A lone pair is held closer to the oxygen and spreads out more, so it pushes harder than a bonding pair. The two lone pairs squeeze the two bonds closer together, from 109.5° to 104.5°.

We describe a molecule’s shape by where its atoms are, not its lone pairs, which you can’t see. With only the three atoms to go by, water is bent.

Why one small angle matters so much

Oxygen pulls the shared electrons towards itself, so each O–H bond is polar, with the oxygen slightly negative and the hydrogens slightly positive. Because the molecule is bent, the two hydrogens are on the same side. The whole molecule has a positive end and a negative end: it is polar.

If water were straight, the two bonds would point in opposite directions and cancel out, as they do in carbon dioxide. That is exactly why CO₂ is not polar even though its bonds are.

Polarity is behind a lot of what makes water unusual. It is why water dissolves salt (see the salt lesson), why its molecules stick together so strongly, and why it is a liquid at room temperature when molecules of a similar size, like methane, are gases.