Why is technetium radioactive and in the middle of stable elements on the periodic table?

Answer 1

There is actually no beautiful answer to this question. The simple truth is, the laws of quantum chromodynamics dictate how a given number of protons and neutrons attract each other, and the laws of quantum electrodynamics dictate how the protons repel each other, and these terms add together to give a binding energy of the nucleus, similar to the atomization energy of a molecule. This is potential energy. There is a law which states that all systems seek the lowest possible potential energy. It turns out that for all possible combinations of 43 protons with a number of neutrons, there turns out to be a different combination of protons, neutrons, and maybe one or more electrons, positrons, and neutrinos which has a lower potential energy. This is not exactly "coincidence" the way that it would be coincidence if you met a long-lost friend at the subway, but it is the case that technetium's instability is simply a side-effect of the exact values of coupling constants and masses which arise in the fundamental equations of physics. Promethium's instability is similarly inexplicable; as is that of tungsten.

Answer 2

Really, i do not know why, but remark that promethium element 61 is in the same case . All isotopes are radioactive