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Lessons / Bonding and shape / GCSE and A-level

Why is a water molecule bent?

The A-level picture: VSEPR, counting electron pairs, and shapes up to six pairs.

  1. A raindrop2 mm
  2. A film of water1 µm
  3. A few molecules1 nm
  4. One molecule0.3 nm
H–O–H angle: 104.5°
What the simulation shows, in words

A molecule drawn in three dimensions, turning slowly, with lone pairs shown as purple clouds. Choose carbon dioxide (linear, 180°), boron trifluoride (trigonal planar, 120°), methane (tetrahedral, 109.5°), ammonia (trigonal pyramidal, about 107°), water (bent, 104.5°), phosphorus pentachloride (trigonal bipyramidal, 120° and 90°), sulfur hexafluoride (octahedral, 90°) or xenon tetrafluoride (square planar, 90°). The readout gives the numbers of bonding and lone pairs.

Counting electron pairs

To find the shape, count the electron pairs around the central atom.

  1. Take the number of outer electrons of the central atom.
  2. Add one for each atom joined to it by a single bond.
  3. For a negative ion add the charge; for a positive ion take it away.
  4. Divide by two. That is the number of pairs.
  5. The number of bonded atoms is the number of bonding pairs. The rest are lone pairs.

Water: 6 + 2 = 8, so 4 pairs, 2 bonding and 2 lone. The ammonium ion, NH₄⁺: 5 + 4 − 1 = 8, so 4 pairs, all bonding, and it is tetrahedral at 109.5°, just like methane.

The basic shapes

Pairs Bonding : lone Shape Angle
2 2 : 0 linear 180°
3 3 : 0 trigonal planar 120°
4 4 : 0 tetrahedral 109.5°
4 3 : 1 trigonal pyramidal about 107°
4 2 : 2 bent (non-linear) about 104.5°
5 5 : 0 trigonal bipyramidal 120° and 90°
6 6 : 0 octahedral 90°
6 4 : 2 square planar 90°

Repulsion goes in this order: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. A common rule of thumb is that each lone pair closes the angle by about 2.5°, which takes 109.5° to about 107° and then 104.5°. The measured angles for ammonia (106.7°) and water (104.5°) fit it, but it is a rule of thumb, not a law.

Multiple bonds and more than eight

A double or triple bond points in one direction, so it counts as one region of electrons. Carbon dioxide has two double bonds and no lone pairs on carbon, so it is linear.

Atoms from period 3 onwards can have more than eight electrons around them. Phosphorus in PCl₅ has five bonding pairs. They can’t all be equally spaced, so three sit in a flat triangle at 120° and two point up and down at 90°. Sulfur in SF₆ has six, pointing along the axes. In XeF₄ the two lone pairs take opposite positions, so the four fluorines lie in a square.

Where VSEPR stops working

VSEPR is a counting rule. It works best for small molecules with a central atom from period 2. Further down, it gets weaker. Hydrogen sulfide is a good warning: sulfur has two bonding pairs and two lone pairs, just like oxygen in water, yet the H–S–H angle is only about 92°. Explaining that needs the orbital picture of bonding, which goes beyond A-level.

Safety

Several molecules here are toxic or corrosive, among them hydrogen sulfide and phosphorus pentachloride. This page is about their shapes only.