Why is technetium radioactive although the other transition elements (except certain lanthanides and actionoids) are not radioactive?
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The cores (nuclei) of atoms consist of two kinds of particles, protons and neutrons. The stability of the nucleus is a balancing act between the strong nuclear force binding them together and the electromagnetic force trying to push the protons apart from each other, since they are all positively charged.
As the number of protons increases, you need more and more neutrons to “space out” the protons enough to not have them fly apart due to the positive charge. However, the strong nuclear force is very short range, which means at some point, the nucleus will break apart as well. That’s why all known elements past lead are radioactive.
Technetium has 43 protons, which is an odd number. Nuclei with an odd number of protons are generally less stable than nuclei with an even number. The same goes for the neutrons. It just so happens that there is no stable isotope for this number of protons. You can’t add half a neutron to the nucleus to balance everything out perfectly.
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