* Isotopes: Isotopes of an element have the same number of protons (defining the element) but differ in the number of neutrons. This results in different atomic masses.
* Hydrogen: Normal hydrogen (protium) has 1 proton and 0 neutrons (mass number 1).
* Hydrogen-3 (Tritium): A hydrogen atom with a mass of 3 has 1 proton and 2 neutrons. This is called tritium.
In summary: Tritium is an isotope of hydrogen because it has the same number of protons (1) but a different number of neutrons (2) than the common hydrogen isotope, protium.
