In nuclear physics, a puzzle has long been nagging: why among elements lighter than bismuth, only technetium (No. 43) and promethium (No. 61) have no stable isotope at all? All their neighbors in the periodic table can boast at least one long-lived variant, but these two seem to fall out of the general pattern. A group of researchers approached the problem as a search for 'gaps' in the sequence of the lightest odd-mass nuclei. They decomposed the masses of isobars (nuclei with the same number of particles but different charges) into two components: a smooth parabola and sharp jumps between odd protons and odd neutrons.
The smooth part is well described by standard nuclear models that account for shell structure. But the jumps are the key to the solution. In the region of technetium and promethium, they turned out to be positive and large enough. As a result, the sequence of the lightest isobars seems to 'skip over' them.
Analysis of nuclear shell filling showed that the observed stepwise change in masses is associated with a special type of interaction — the tensor effect in the proton-neutron pair. This effect depends on the mutual orientation of the spins (intrinsic angular momenta) of the particles and the shape of their orbits. In the end, nature seems to say: 'Here, at numbers 43 and 61, stability has no place.' Thus, tensor forces, usually unnoticeable against the background of other nuclear interactions, have manifested themselves in pure form.
Technetium became the first artificially produced element: it was synthesized in 1937, and its name comes from the Greek 'technetos' (artificial).
In Stanislaw Lem's novel 'Fiasco', technetium is mentioned as a rare element used in technologies of the far future.