Which of the following is unstable?

The correct answer is Option (3) → Tritium

Option (1) Hydrogen-1 — Incorrect. Its nucleus is a single proton, and a proton is stable. Hydrogen-1 makes up about 99.985% of natural hydrogen.

Option (2) Deuterium — Incorrect. Its nucleus has one proton and one neutron, and it is stable. A free neutron is unstable, but it is stable inside the nucleus. Deuterium occurs in natural hydrogen.

Option (3) Tritium — Correct. The tritium nucleus, with one proton and two neutrons, is unstable. For this reason tritium does not occur naturally and is produced artificially in laboratories.

NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… The other two isotopes of hydrogen are called deuterium and tritium. Tritium nuclei, being unstable, do not occur naturally and are produced artificially in laboratories.”