why tritium is more radioactive than other isotopes of hydrogen?
An element is radioactive if its atomic nuclei are unstable. The nucleus contains protons and neutrons; the electromagnetic force pushes the protons apart because they have the same charge, but the strong nuclear force pulls the protons and neutrons together. A nucleus is only stable if it has the right balance of protons and neutrons, and doesn't have any extra energy.
Tritium is more radioactive than other isotopes of hydrogen because it contain an unstable nucleus due to the presence of 1 proton and 2 neutrons.