Isotope(s) of hydrogen which emits low energy β⁻ particles with t 1/2 value > 12 years is/are

Solution

Chemistry ma’am · 3 steps

Of hydrogen's three isotopes, only tritium is radioactive, so ask which one has too many neutrons.

1. Check stability. Protium (¹₁H, no neutron) and deuterium (²₁H, one neutron) are both stable. Neither decays, so neither emits anything.

   Isotope	Neutrons : protons	Behaviour
   Protium, ¹₁H	0 : 1	stable
   Deuterium, ²₁H	1 : 1	stable
   Tritium, ³₁H	2 : 1, neutron-rich	β⁻ emitter, t_(1/2) ≈ 12.3 years

2. Tritium decays. Tritium has two neutrons for one proton, which is too many. One neutron turns into a proton and gives out an electron, the β⁻ particle, so tritium becomes helium-3.

   ³₁H → ³₂He + ⁰₋₁e

3. So. The β⁻ particles from tritium are very weak, with a maximum of about 18.6 keV, and its half-life is about 12.3 years. The answer is (b), tritium.

The idea to keep. A nucleus with too many neutrons fixes itself by β⁻ decay, turning a neutron into a proton. For hydrogen, only tritium is neutron-rich enough to be radioactive.

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