Labeling of isotopes

Isotopes are labeled by prefixing the atomic number of the element and the mass number of the isotope to the respective symbol. The atomic number refers to the number of protons in the atomic nucleus, the mass number refers to the sum of protons and neutrons.

The example regards an isotope of hydrogen (symbol: H). In this case, the atomic nucleus contains one proton (atomic number 1) and one neutron (mass number 2).