Molecule shapes (VSEPR)

Methane's bonds are 109.5° apart, ammonia's 107° and water's 104.5°. Each has four electron pairs around the centre.

Tie four balloons together at their necks and they settle into a shape on their own: they point to the corners of a tetrahedron, as far from each other as they can get. Electron pairs around an atom do something very similar. They all carry negative charge, they repel each other, and they spread out to minimise that repulsion. The shape of a molecule follows from where the pairs end up.

This idea is called valence shell electron pair repulsion, VSEPR for short, and it was set out by Ronald Gillespie and Ronald Nyholm in 1957. To use it, count the regions of electron density around the central atom. A single, double or triple bond each counts as one region, because all its electrons point the same way. A lone pair, a pair of electrons that belongs to the atom but is not used in a bond, also counts as one region.

Two, three and four regions

Two regions point in opposite directions, 180° apart, giving a linear molecule. Carbon dioxide, O=C=O, has two double bonds and no lone pairs on carbon, so it is a straight line. Three regions spread out in a flat triangle at 120°, trigonal planar. Each carbon in ethene has three regions (two C-H bonds and the C=C double bond), and the whole molecule is flat; the measured H-C-H angle is 117.6°, close to the ideal. Four regions point to the corners of a tetrahedron at 109.5°. Methane is the textbook example.

Lone pairs squeeze the angle

Ammonia and water also have four regions around the central atom, but some are lone pairs. Nitrogen in NH₃ has three bonds and one lone pair; oxygen in H₂O has two bonds and two lone pairs. A lone pair is held by only one nucleus, so it spreads out closer to that atom and pushes harder on the neighbouring bonds than a bonding pair does. The bonds are squeezed together. In ammonia the H-N-H angle closes from 109.5° to about 107°; in water, with two lone pairs, the H-O-H angle closes further to 104.5°.

The shape name describes where the atoms are, not the lone pairs. Ammonia is called trigonal pyramidal, a low pyramid with nitrogen at the top. Water is called bent. Turn on "Lone pairs" in the model and you can see the invisible fourth corner of ammonia's tetrahedron, and the two that complete water's. The model draws them as yellow lobes with two small dots for the electrons; real lone pairs are clouds of charge without sharp edges.

Bigger molecules have more than one centre, and you apply the same rules to each one. In ethane every carbon has four single bonds, so each carbon is tetrahedral and the molecule zigzags in three dimensions. In methanol, CH₃OH, the carbon is tetrahedral and the oxygen, with two bonds and two lone pairs, is bent like the oxygen in water. Benzene is the opposite case: each of its six carbons has three regions, so all twelve atoms lie flat in a hexagon with 120° angles. Pick each of them in the model and look at it edge on to check.