Technetium

Mendeleev's table left holes, and most of them were filled within a generation: gallium in 1875, scandium in 1879, germanium in 1886. Element 43, sitting between molybdenum and ruthenium, refused.

It was claimed repeatedly. Names proposed for it over the decades included davyum, lucium and nipponium, and all were withdrawn or reassigned. The most substantial claim came in 1925, when Walter Noddack, Ida Tacke and Otto Berg in Berlin announced that they had found both element 75 and element 43 in platinum ores and columbite, naming them rhenium and masurium. Rhenium was real and they had it. Masurium could not be reproduced by anyone, including themselves, and the quantity of element 43 that could conceivably have been present in their samples is far below what their X-ray method could have detected. The claim is not accepted.

The reason for the difficulty was not experimental. Element 43 has no stable isotope at all, and effectively none of it exists on Earth. This is a genuine oddity, since it sits in the middle of the table surrounded by perfectly stable neighbours. The explanation lies in a rule derived by Josef Mattauch in 1934 concerning which pairs of neighbouring isobars can both be stable, and it accounts for the two gaps below bismuth, technetium and promethium, and for nothing else.

Carlo Perrier and Emilio Segre closed it in 1937. Segre had visited Berkeley and asked Ernest Lawrence for discarded parts of the cyclotron; Lawrence sent him a molybdenum deflector foil that had been bombarded with deuterons. Working in Palermo, Perrier and Segre separated from it a new radioactive substance whose chemistry matched the predictions for element 43. They named it technetium, from the Greek tekhnetos, artificial, because it was the first element made rather than found.

Technetium's absence from the Earth is not an absence from the universe. In 1952 Paul Merrill, observing S-type red giant stars at Mount Wilson, identified technetium absorption lines in their spectra.

The significance is hard to overstate. Technetium's longest-lived isotopes survive a few million years, which is nothing at all on a stellar timescale. Its presence in the atmosphere of a star means it is being made there, now, and dredged to the surface. Before Merrill, the idea that stars manufacture the heavy elements was a theory; after him it had direct observational support, and the great synthesis papers on stellar nucleosynthesis followed within a few years.

The element's terrestrial career is medical, and it is enormous. Technetium-99m, a metastable nuclear state identified by Emilio Segre and Glenn Seaborg in 1938, decays by emitting a single gamma ray of convenient energy, with a half-life of about six hours: long enough to image a patient, short enough to clear quickly afterwards. It can be attached to a wide range of carrier molecules that concentrate in bone, heart muscle, thyroid, kidney or tumour tissue.

It is supplied by a device called a technetium generator, in which molybdenum-99 decays into technetium-99m and the product is washed out with saline as required, so that a hospital can draw fresh doses for a week from a single delivery. Technetium-99m is used in tens of millions of diagnostic scans a year worldwide, the great majority of all nuclear medicine procedures.

An element that does not occur naturally on this planet, was announced only in 1937 and was disputed for a century before that, is now among the most administered radioactive substances in medicine. Nothing traditional attaches to it, and nothing could.