Final answer:
Tritium is radioactive due to its unstable nucleus, while deuterium is not radioactive because its nucleus is more stable. Deuterium, on the other hand, is not radioactive because its nucleus is more stable with just one proton and one neutron.
Step-by-step explanation:
Tritium is radioactive because it has an unstable nucleus. It contains one proton and two neutrons, which gives it a mass number of 3.016.
This makes tritium heavier than deuterium, which has a mass number of 2.014.
Because tritium has an extra neutron, its nucleus is less stable and tends to decay, releasing radiation in the process.
Deuterium, on the other hand, is not radioactive because its nucleus is more stable with just one proton and one neutron.