THE STABILITY OF ISOTOPES
3 marks · NormalA student claims that all isotopes of hydrogen are equally stable. Use your knowledge of isotopes and nuclear forces to explain why this statement is not correct.
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The stability of hydrogen isotopes varies. Protium (hydrogen-1) is stable because it has the right balance of protons and neutrons (no neutrons, just one proton). Deuterium (hydrogen-2) is also stable, with one proton and one neutron. However, tritium (hydrogen-3) has two neutrons and one proton, and is unstable as the extra neutron makes the nucleus unstable, so it decays by beta emission. Thus, not all isotopes of hydrogen are stable.
Also accept: Reference to stability and decay of tritium
Common mistakes: Assuming all isotopes of any element behave identically; Ignoring the existence of radioactive isotopes
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Marking points
B1 · 1 mark · States that not all hydrogen isotopes are stable.
M1 · 1 mark · Mentions stability of protium and deuterium.
M2 · 1 mark · Explains tritium is unstable and decays.
Worked solution
Not all hydrogen isotopes are stable. Hydrogen-1 (protium) and hydrogen-2 (deuterium) are stable, but hydrogen-3 (tritium) is unstable due to having too many neutrons for one proton; tritium undergoes beta decay.