Isotopes and radioactivity: which of the following hydrogen isotopes is intrinsically radioactive by nature?

Introduction / Context:
Hydrogen has three isotopes of interest: protium (H-1), deuterium (H-2), and tritium (H-3). Recognizing which isotope is radioactive is important in fusion research, radiolabeling, and environmental monitoring.

Given Data / Assumptions:

• We compare stability of hydrogen isotopes.
• Terminology: “heavy hydrogen” is deuterium.
• Helium is included as a distractor and is a different element entirely.

Concept / Approach:
Protium (H-1) and deuterium (H-2) are stable. Tritium (H-3) is radioactive, undergoing beta decay to helium-3 with a half-life of about 12.3 years. Therefore, among the options, only tritium is intrinsically radioactive.

Step-by-Step Solution:
List isotopes: H-1 (stable), H-2 = deuterium/heavy hydrogen (stable), H-3 = tritium (radioactive).Identify the radioactive one: tritium.Eliminate helium and duplicate naming of deuterium (“heavy hydrogen”).

Verification / Alternative check:
Standard nuclear data handbooks confirm tritium's beta decay to He-3, widely used in fusion research and self-luminous devices (with strict safety controls).

Why Other Options Are Wrong:

• Helium: Different element; stable isotopes He-3 and He-4.
• Deuterium/heavy hydrogen: Stable.
• Protium: Stable.

Common Pitfalls:
Confusing “heavy hydrogen” with tritium; heavy hydrogen is deuterium, which is stable.

Final Answer:
Tritium