Three isotopes of hydrogen are
a. Protium (`._(1)^(1)H`: Its atomic mass is `1.008 123 am u`. In `1s` nucleus consists of one proton and one electron in `1s` energy level. It constitues `99.985%` of natural hydrogen.
b. Deuterium (`._(2)^(1)H`or `D`) or heavy hydrogen: its atomic mass is `2.014` amu. an atom of deuterium has one proton and one neutron in its nucleus, `0.015%` exists in natural hydrogen.
c. Tritium(`._(3)^(1)H` or `T`): Its atomic mass is `3.01` amu. an atom of tritium has one proton and two neutrons is its nucleus. tritium atoms are unstable and are radioactive whereas the former two are non-radioactive. It is found in heavy water. heavy water is used to control the nuclear reaction in nuclear reactor and helps to slow down the speed of neutrons.