• A. 2
  • B. 3 ✓
  • C. 4
  • D. 5


Hydrogen, the first element on the periodic table, has three naturally occurring isotopes. These isotopes are sometimes denoted as 1H2H, and 3H. Among these isotopes, 1H and 2H are stable, while 3H is radioactive with a half-life of 12.32 years.

  1. Protium (1H):
    • Protium is the most common hydrogen isotope, with an abundance of over 99.98%. It consists of only a single proton in its nucleus, making it stable.
    • The proton in protium has never been observed to decay, but some theoretical predictions suggest that proton decay might occur with an extremely long half-life.
  2. Deuterium (2H):
    • Deuterium is known as hydrogen-2 and contains one proton and one neutron in its nucleus.
    • It is the other stable hydrogen isotope and makes up about 0.0026–0.0184% of hydrogen samples on Earth.
    • Deuterium is not radioactive and is commonly used in various applications such as heavy water production and nuclear fusion research.
  3. Tritium (3H):
    • Tritium is known as hydrogen-3 and contains one proton and two neutrons in its nucleus.
    • It is a radioactive isotope with a half-life of 12.32 years and occurs naturally in trace amounts due to cosmic ray interactions.
    • Tritium finds applications in nuclear weapons, fusion research, and as a radioactive tracer.

Therefore, hydrogen has a total of three isotopes: protium (1H), deuterium (2H), and tritium (3H.

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