An atom is mostly empty space around a tiny centre. To talk about that centre we need a new unit.

• A typical atom is about 10⁻¹⁰ m across. That is 0.1 nanometres, or one ångström.
• A nucleus is between about 10⁻¹⁵ m and 10⁻¹⁴ m across. The unit for this scale is the femtometre (fm): 1 fm = 10⁻¹⁵ m. A single proton is a bit under 2 fm across, and a big nucleus such as lead is roughly 14 fm across.

So the nucleus is smaller than the atom by a factor of roughly 10⁴ to 10⁵. Put the other way round, the ratio of nuclear size to atom size is about 10⁻⁵. If you blew a nucleus up to the size of a pea (about 1 cm), its atom would be somewhere between 100 m and 1 km across. That is anything from a football pitch to a small town, with a pea in the middle.