Question

Why radioactivity is a nuclear phenomenon?

Answer 1

Radioactivity must be a nuclear phenomenon for the following reasons:
There are three kinds of radioactive decay particles and all of them carry the clue about their origin.

1. Alpha Rays : Alpha radiation is made of alpha particles which are positively charged and are heavy. When examined these particle were found to be Helium-4 nucleus. The configuration of two protons and two neutrons seem to have exceptional stability and so when bigger nucleii disintegrate they seem to be disintegrating in such units. Clearly protons and neutrons are constituents of nucleus. Thus alpha radiation makes it obvious that they originate from the atomic nucleus.

2. Beta Rays : Beta radiation is made of beta particles that are either positively (`\beta^{+}` decay) or negatively (`\beta^{-}` decay) charged. When examined carefully they were found to be positrons, in the case of `\beta^{+}` decay and electrons , in the case of `\beta^{+}` decay.
   The electron could be from outside the nucleus but the clue for their nuclear origin comes from the change that happens to the nuclear charge after the beta decay. After a nucleus undergoes a `\beta^{-}` decay, emitting an electron, it is found that the atomic number of the nuclus goes up by one. This is a clear indication that the electron is the byproduct of some interaction involving the nucleons.

3. Gamma Rays : Gamma radiation are made of particles that are charge neutral. When examined it was revealed to be extremely high energetic electromagnetic radiation with energies in the range of MeV . Electronic rearrangement can give photons in the energy range of few eV s. So the gamma rays cannot be due to electronic rearrangements. However the energy levels of the nucleons in atomic nuclei are in the range of MeV s. So they must be of nuclear origin.

Answer 2

radioactivity is a nuclear phenomenon since it occurs due to nuclear excitation
it caused in element having no of proton <= 82
n/p = ranges from 0to 1