What is isotope notation?

tells us several things: = A; = Z; element symbol = X

The of an element include all of those with the same number of protons or atomic number but with different mass numbers which is a change in the number of neutrons.

Here is an example of the isotopes of H and C:

The H isotopes are shown pictorially rather than in notation to show exactly what the difference in each isotope means. We have H with its one proton, then deuterium with the proton and an added neutron. The third isotope tritium has an additional neutron.

Each extra neutron changes the mass number of the isotope. In notation this is shown in the A position as we can see with the C isotopes. Each mass number increases by one as a neutron is added.