Yes, that's right hydrogen can be radioactive. No, this doesn't mean 3 hydrogen atoms per molecule. Instead it is the number of isotopes.

If you know what sub-atomic particles are then this should make sense. An isotope is determined by how many neutrons there are in the nucleus plus the number of protons. Protons of an element will always be the same because they determine the elements atomic number. This is where they appear on the periodic table.

You probably guessed how you represent the number of neutrons in an atom. It is completely different. It would be 3H. Where 3 is in superscript (the small number going up) and is also the sum of the protons and neutrons.

Because the number of protons never changes then 1H has one proton and 0 neutrons because hydrogens atomic number is one. Another name for 3H is tritium which is the only isotope to receive a name different from the element. Tritium naturally occurs in the environment and is used in hydrogen bombs.

There are other radioactive isotopes which brings me to my challenge for you. How many other naturally occuring radioactive isotopes (commonly abbreviated as radioisotope) can you find?