Radioactive Substances

Radioactivity is the spontaneous emission of radiation from the nucleus of an unstable atom. Some atoms have unstable nuclei, which means they emit radiation in the form of alpha particles, beta particles, or gamma rays. These atoms are called radioactive substances.

There are several radioactive substances, but in the given options, the only radioactive substance is tritium (option A). Let's understand why tritium is radioactive and the other options are not.

1. Tritium (option A)
Tritium is a radioactive isotope of hydrogen. It has two neutrons in its nucleus, unlike the common hydrogen isotope, protium, which has no neutrons. The presence of extra neutrons makes tritium unstable, leading to its radioactivity. Tritium undergoes beta decay, where a neutron in the nucleus is converted into a proton, emitting a high-energy electron (beta particle) and a neutrino. The resulting nucleus is helium-3.

2. Protium (option B)
Protium is the most common isotope of hydrogen and does not have any neutrons in its nucleus. It is stable and not radioactive.

3. Chlorine (option C)
Chlorine is an element with several isotopes, but none of them are radioactive. The stable isotopes of chlorine include chlorine-35 and chlorine-37, which have 17 and 20 neutrons, respectively.

4. Deuterium (option D)
Deuterium is another isotope of hydrogen and is often referred to as "heavy hydrogen." It contains one neutron in its nucleus. Deuterium is stable and not radioactive.

Summary
In summary, tritium is the only radioactive substance among the given options. It is an isotope of hydrogen with two neutrons in its nucleus, making it unstable and prone to radioactive decay. Protium, chlorine, and deuterium, on the other hand, are stable isotopes and not radioactive.