Final answer:
Protium, deuterium, and tritium are called isotopes of hydrogen because they have the same number of protons but different numbers of neutrons.
Step-by-step explanation:
Isotopes of Hydrogen include protium, deuterium, and tritium. These isotopes are called isotopes because they have the same number of protons (1) but different numbers of neutrons. Protium has no neutrons, deuterium has one neutron, and tritium has two neutrons in its nucleus. These isotopes share the same chemical properties but may have different physical properties due to the differences in their atomic masses.