Re: What's radioactivity?How do changes in the energy levels take place?

Greetings!

You are right in stating that energy emission is common to radioactivity, fluorescence etc. The difference is that the former is a phenomenon which happens due to changes in the structure of the nucleus of an atom, wehereas the latter have to do with the electrons in the atom's electron shells. Furthermore, the energies involved are several orders of magnitude different: nuclear transitions can take place at gamma ray energies of MeV, whereas atomic transitions start essentially at zero energy and can reach tens, maybe a hundred keV (one eV [electron volt] is the energy that an electron gets when it is accelerated by a voltage difference of 1V).

Actually, fluorescence and phosphorescence are not explained as chemical reactions. Chemical reactions are changes in the way how different atoms are bound together (or split apart). Fluorescence and phosphorescence are, as I have already mentioned, explained as processes which change the arrangement of electrons in an atom's or molecule's shells.

I have used the term `changes in the arrangement' or `structure' of nuclei/shells. This is actually quite a poor description of what is really going on. The laws of quantum mechanics tell us, for example, that an atomic nucleus might exist in a state that is metastable, i.e. it is doomed to change its structure sooner or later because this is energetically more favourable. This means that by such a change it is possible for it to arrive in a state that has less energy than the one before. The `excess energy' is then radiated off in the form of particles (photons, electrons, alpha particles or positrons). The processes uccuring in atomic shells and which lead to phosphorescence and fluorescence are quite similar to this, but, as described above, happen at much lower energies and so the emitted radiation is photons only.

As to artificial radioactivity: I will assume that you want to know how radioactive elements are produced that do not usually occur in nature. Such elements are usually formed by bombarding stable elements with accelerator beams and by irradiating them in different types of reactors. One of the most common artificial radioactive elements is ⁹⁹Te (Technetium). It is a byproduct of nuclear fission in reactors and is used mainly for medical diagnostics. Actually, the element which is the actual source for ⁹⁹Tc is ⁹⁹Mo (Molybdenum), which decays into ⁹⁹Tc with a half-life of about 66 hours (Tc has a very short half-life of about six hours). ⁹⁹Mo is one of many possible fragments of nuclear fission, where Uranium nuclei are hit by neutrons and split into two lighter parts. The artificial element must consequently be extracted from the used nuclear `fuel' rods by chemical processes.

Hope that helps,
Georg.