Molecular Shapes & Why Water Is Bent
Electron groups repel and push as far apart as they can. The shape that results decides whether a molecule is polar — and water's shape decides most of biology.
Builds on: 1.2 Covalent Bonds: Sharing Pairs
One rule: electron groups repel
Molecules are three-dimensional objects with definite shapes, and you can predict those shapes from a single idea called VSEPR (Valence Shell Electron Pair Repulsion). It says: groups of electrons around a central atom all repel each other, so they arrange themselves as far apart as possible.
A “group” is either a bond (single, double or triple — each counts once) or a lone pair: a pair of valence electrons on the central atom that is not bonded to anything. Count the groups, and geometry does the rest:
| Groups | Arrangement | Angle |
|---|---|---|
| 2 | linear | 180° |
| 3 | trigonal planar | 120° |
| 4 | tetrahedral | 109.5° |
Methane, CH₄, has four bonds and no lone pairs, so the hydrogens sit at the four corners of a tetrahedron at 109.5°. Not flat — a genuinely three-dimensional shape, and the reason organic chemistry is so rich.
Lone pairs push harder
A lone pair is held by only one nucleus, so it spreads out more and pushes harder than a bonding pair. And when you name a shape, you name what you can see — the atoms — not the invisible lone pairs.
Take the three neighbours in period 2, each with four electron groups:
- Methane CH₄: 4 bonds, 0 lone pairs → tetrahedral, 109.5°.
- Ammonia NH₃: 3 bonds, 1 lone pair → the pair presses down, giving a trigonal pyramid at 107°.
- Water H₂O: 2 bonds, 2 lone pairs → two pairs press down, giving a bent molecule at 104.5°.
The same underlying tetrahedral arrangement every time; the shape you observe changes because some corners are occupied by things you cannot see.
Polar bonds do not always make a polar molecule
This is the payoff, and the part most often got wrong. A molecule’s overall polarity depends on both its bond dipoles and its shape, because dipoles are vectors — they can cancel.
Carbon dioxide, O=C=O, has two strongly polar bonds. But the molecule is linear, so the two pulls point in exactly opposite directions and cancel perfectly. CO₂ is nonpolar despite its polar bonds.
Water, H₂O, has two polar bonds too — but it is bent at 104.5°, so the pulls do not cancel. They add to give a strong net dipole pointing from the hydrogens towards the oxygen. Water is a strongly polar molecule.
Because water is bent and polar, it dissolves salts and sugars, it climbs tree trunks, it has a huge heat capacity that stabilises the climate, and its solid form is less dense than its liquid, so ice floats and lakes freeze from the top down. Had oxygen lacked those two lone pairs, water would be a linear, nonpolar gas at room temperature — and none of that, including you, would be here.
⚗️ Lab — Shape & Polarity Explorer
Five molecules, drawn with their lone pairs and their net dipole.
- Step CH₄ → NH₃ → H₂O and watch the lone pairs squeeze the bond angle down each time.
- Compare CO₂ and H₂O: both have two polar bonds, but only one is a polar molecule. Shape decides.
- Hide the lone pairs. The shape suddenly looks arbitrary — that is why VSEPR counts what you cannot see.