The unit for the amount of a
substance is the **mole**. It is defined as the amount of a substance that contains the same
number of particles (atoms or molecules) as the number of atoms in 12 g of the isotope
carbon-12. This number is known as the **Avogadro constant** (N_{A} or L) and is 6.02 x 10^{23}
particles per mole.

The ratio of the mass of one mole of the substance to one twelfth
of the mass of one mole of carbon-12 is called the **relative molecular mass** of the substance -
it is 32 for oxygen, 2 for hydrogen and so on.

A knowledge of the Avogadro constant
enables us to calculate the number of molecules in any mass of a substance and therefore to
get an idea of the size of one molecule.

For example, a drop of water of volume 1.0
cm^{3} has a mass of 1 g. The relative molecular mass of water is 18, and therefore this
drop of water must contain 6.02x10^{23}/18 = 3.34x10^{22} molecules.

Avogadro's
constant is the number of particles in a mole of the substance. This number is always the
same.

So in:

1 mole of hydrogen (2 g) there are 6.02x10^{23} molecules
(hydrogen exists as H_{2})

1 mole of oxygen (32 g) there are 6.02x10^{23} molecules
(oxygen exists as O_{2})

1 mole of copper (63 g) there are 6.02x10^{23} atoms

1
mole of uranium 235 (235 g) there are 6.02x10^{23} atoms

For example if we
have 2 kg of uranium in a fuel rod we have 2000/235 = 8.51 moles and this contains
8.51x6.02x10^{23} = 5.12x10^{23} atoms and so 5.12x10^{23} uranium
nuclei.

The average volume of a water molecule must therefore be 2.99x10^{-
23} cm^{3}, and if we assume the molecules to be spherical the diameter of a water molecule
is about 2x10^{-8 }cm or 2x10^{-10} m, a result confirmed by X-ray
diffraction.