A very good question. The answer is not straightforward and has to do with the so called shell-model of the nucleus. You probably know that the electrons in an atom tend to group into shells, s, p, d... etc with corresponding occupancy numbers of 2, 8, 18... etc. This is a consequence of the Pauli Principle which states that no two identical spin 1/2 Fermions can occupy the same state.
Likewise, nuclei have a shell structure.
Unfortunately, since the nucleus contains both neutrons and protons and in addition the nuclear forces being rather complicated in comparison to the electromagnetic forces binding electrons to the nucleus, the calculations are rather daunting. A Nobel Prize was awarded to Wigner, Gopert-Mayer, and Jensen in 1963 for pioneering work in this field. A result of their calculations revealed that there are certain magic numbers
of nucleons: 2, 8, 20, 28, 50, 82, 126 which are more tightly bound than the next higher number, hence more stable. The upshot of all this is that Technetium is not favored with its 43 protons. The most stable of the isotopes is Te98
with a half-life of 4.2 million years.
(published on 01/25/11)