Isotopes of a given element share the same characteristics, differing only in the neutron count within the nucleus.
The atomic number remains unchanged, whereas the mass number varies.
In this particular instance, protium, deuterium, and tritium represent isotopes of hydrogen.
Protium is 1H, or Hydrogen-1, which has no neutrons.
Deuterium is 2H, or Hydrogen-2, containing one neutron.
Tritium is 3H, or Hydrogen-3, consisting of two neutrons.