Ammonia Molecule

A pyramid with a lone pair on top

Ammonia, NH₃, is a nitrogen atom bonded to three hydrogens. Nitrogen has five outer electrons: three go into the bonds and the other two form a lone pair. That gives four electron pairs around the nitrogen, which spread out towards the corners of a tetrahedron, just as in methane. But only three corners hold atoms, so the shape we see is a trigonal pyramid, with the nitrogen at the top and the three hydrogens forming the base.

Why 107° and not 109.5°

The lone pair isn’t shared with another atom, so it sits closer to the nitrogen and spreads out more than a bonding pair does. It pushes the three N–H bonds slightly closer together, and the H–N–H angle shrinks to about 106.7°. Water, with two lone pairs, is squeezed even further, to 104.5°: a neat sequence from methane (no lone pairs) through ammonia (one) to water (two).

The umbrella flip

The pyramid can turn inside out, like an umbrella in the wind. The nitrogen passes through the plane of the hydrogens, the molecule is momentarily flat with 120° angles, and then it is a pyramid pointing the other way. This inversion has to climb an energy barrier of about 24 kJ/mol. Remarkably, ammonia doesn’t need to climb it: thanks to quantum mechanics the molecule tunnels through the barrier, flipping back and forth about 24 billion times a second. That frequency, about 24 GHz in the microwave range, was used in the first maser and in one of the first atomic clocks.

The lone pair at work

The lone pair makes ammonia a base: it can grab a hydrogen ion to form the ammonium ion, NH₄⁺. With four bonding pairs and no lone pair, ammonium is a perfect tetrahedron with 109.5° angles. The lone pair also makes ammonia polar and lets it form hydrogen bonds, which is why it dissolves so readily in water.

For the curious: the geometry

For three equal bonds of length d at an angle θ to each other, the hydrogens lie on a circle of radius ρ = d·√((1 − cos θ)/1.5) around the axis, and the nitrogen sits h = √(d² − ρ²) above their plane. With d = 1.012 Å and θ = 106.7° that gives h ≈ 0.38 Å. In the simulation the atoms are joined by springs fitted to ammonia’s four measured vibrations (3337, 950, 3444 and 1627 cm⁻¹). Because those springs only care about bond lengths and angles, the inverted pyramid is just as comfortable as the original, and the flat shape in between costs about 17 kJ/mol, somewhat less than the real 24 kJ/mol. The vibrations are slowed down about 40 trillion times.

Safety

Ammonia gas is irritating and toxic at high concentrations, and concentrated solutions burn skin and eyes. Never mix ammonia cleaners with bleach: the mixture gives off toxic gases. Please read our disclaimer.