The mole fraction is one way of expressing the relative concentration of a given species. The mole fraction, x_i of species i is defined as the number of moles of i divided by the total number of moles in the system:

x_i = n_i/N

where n_i is the number of moles of species i and N is the total number of moles in the system.

Since the number of moles of a species is directly proportional to the number of atoms or molecules of that species through a constant of proportionality, Avogadro's number N_A, the mole fraction is really the same as the number fraction for the given species. In other words the mole fraction of species i, refers to the number of atoms or molecules of i divided by the total number of atoms or molecules in the system.