Technetium

Category: transition metal Atomic number: 43 Colour: silvery grey Melting point: 2,157°C (3,915°F) Boiling point: 4,265°C (7,709°F) First identified: 1937

This is the substance that completes the story of Mendeleev’s original four missing elements. After the discovery of scandium, gallium and germanium, there were many and varied attempts to identify and isolate element 43, which he had called eka-manganese. But these all failed until 1937, when the Italian scientists Carlo Perrier and Emilio Segrè at the University of Palermo in Sicily discovered it in an unexpected way. Segrè had visited the particle accelerator at Berkeley in America, and its creator Ernest Lawrence sent him a piece of molybdenum foil that had been bombarded with deuterons (nuclei from deuterium cells containing one proton and one neutron) in the cyclotron.

The scientists managed to isolate two radioactive isotopes of a new element, which they named technetium. This was a controversial discovery, as the creation of ‘artificial’ elements in this way was seen as a kind of cheating at the time, and the element was not widely recognized. We now know that all isotopes of the element are radioactive, which means that it is mainly formed in nuclear reactions in stars (and its relatively short half-life means that any technetium present at the formation of the Earth would have decayed and disappeared long ago).

However, the advances made in subatomic physics during the Second World War and the discovery of plutonium, which is similarly artificial, led to a revision in opinion: ‘The Making of the Missing Chemical Elements’, by Professor F.A. Paneth, was a key paper that persuaded the scientific community that there should be no distinction made between artificial elements and those that are naturally found, and also that the original discoverer of any isotope of an element should be allowed to name it. Perrier and Segrè soon issued a dignified response, suggesting for their discovery the name technetium, from the Greek for ‘artificial’, and it was finally accepted as a genuine element.

Ironically, natural reserves of technetium were discovered in 1972 – geologists found that there had been a spontaneous nuclear reaction long ago inside uranium deposits beneath Gabon in Africa, and there was a small amount of technetium still present.

The technetium produced today is gathered from spent nuclear fuel rods and is predominantly used (in the form of ‘technetium-99m’) for medical imaging: this isotope will bond with cancer cells and so can be used to identify the locations of tumours within the body.