Which of the following forms of hydrogen emits low energy β⁻ particles?

Solution

Chemistry ma’am · 3 steps

Find the hydrogen nucleus with too many neutrons: only tritium has that, so only tritium beta-decays.

1. Why β⁻ decay happens. A nucleus emits a β⁻ particle when it has too many neutrons for its protons. One neutron changes into a proton, and an electron is thrown out.

   Form	Protons	Neutrons	Stable?
   Protium ¹₁H	1	0	yes
   Proton H⁺	1	0 (bare nucleus)	yes
   Deuterium ²₁H	1	1	yes
   Tritium ³₁H	1	2	no, radioactive

   ³₁H → ³₂He + ⁰₋₁e + ν̄

2. Why low energy. Tritium and helium-3 differ only slightly in mass, so little energy is released. The β⁻ particles have at most about 18.6 keV, among the weakest of any beta emitter. Tritium's half-life is about 12.3 years.
3. So. Only tritium, ³₁H, emits low-energy β⁻ particles. The answer is (a).

The idea to keep. Too many neutrons for the protons means β⁻ decay, because a neutron turns into a proton. Among hydrogen's forms, only tritium (1p, 2n) is unstable this way.

Answer from the NTA final answer key. Solution by PrepWiser.