When a nucleus in an atom undergoes a radioactive decay, the electronic energy levels of the atom:
 

Tritium is an isotope of hydrogen whose nucleus triton contains 2 neutrons and 1 proton. Free neutrons decay into $\mathrm{p}+\overline{\mathrm{e}}+\overline{\mathrm{\nu }}$. If one of the neutrons in Triton decays, it would transform into He³ nucleus. This does not happen. This is because;