This is the mass number A. It shows the number of nucleons (neutrons and protons in the nucleus). It is made up of the atomic number Z and the neutron number N:
A = Z + N
Using the example of helium:
Helium has a mass number A of 4 and the atomic number Z 2, so the atom has 2 neutrons in its nucleus.
A = Z + N
4 = 2 + 2
