The total number of isotopes of hydrogen and number of radioactive...

Hydrogen is the first element in the periodic table and has the atomic number one. Those elements which have the same atomic number but a different mass number are called isotopes. There are three isotopes of hydrogen namely, protium $H 11$, deuterium $H 12$ or $D$ and lastly tritium $H 13$ or $T$. The isotopes are different because of the different number of neutrons present in them.

Isotopes of Hydrogen:



Among all three isotopes only Tritium is radioactive.