The radioactive isotope of hydrogen is ________.

Hydrogen has three naturally occurring isotopes: protium, deuterium, and tritium. Protium (hydrogen-1) is the most abundant and stable isotope, consisting of one proton and no neutrons [1]. Deuterium (hydrogen-2), also known as heavy hydrogen, is a stable, non-radioactive isotope containing one proton and one neutron [2]. Tritium (hydrogen-3) is the only naturally occurring radioactive isotope of hydrogen. It consists of one proton and two neutrons in its nucleus [2]. Due to its unstable nature, tritium undergoes radioactive decay with a half-life of approximately 12.3 years, eventually transforming into helium-3. While other unstable isotopes (4H to 7H) have been synthesized in laboratory settings, they do not occur naturally [2]. Hydronium (H3O+) is not an isotope but a cation formed when hydrogen ions combine with water molecules [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Activity 2.9 > p. 23
2. [2] https://www.sciencedirect.com/topics/engineering/hydrogen-isotopes