Technetium – facts and properties

Position in the periodic table

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Occurrence and extraction

Technetium does not occur naturally on Earth in any useful amount. All its isotopes are radioactive and decay, even the longest-lived ones, whose half-lives are millions of years. That is short compared with the age of the Earth, so any supply from its formation has long since vanished. Only tiny traces keep forming anew when uranium in ores undergoes spontaneous fission.

In his periodic table Mendeleev had left a gap below manganese and called the missing element eka-manganese. For a long time people searched for it in ores in vain. It was found in 1937 in Palermo, Sicily, and not in nature but in the laboratory: the physicist Ernest Lawrence in California had bombarded molybdenum foil with deuterons in a cyclotron and sent it to the physicist Emilio Segrè and the mineralogist Carlo Perrier. They found the new, radioactive element in it. It was the first one that a human being made artificially.

That technetium nevertheless exists in the universe became clear later: its spectral lines were found in certain red giant stars. There it must be forming freshly, which is evidence that stars build elements.

Properties and reactions

Technetium is in period 5 and group 7, below manganese. Its electron configuration is [Kr] 4d⁵ 5s² and its electronegativity is 1.9. The metal has a density of 11 g/cm³, melts at 2157 °C and boils at 4265 °C. The atomic mass is listed as 98 u. The value stands for the long-lived isotope technetium-98, because there is no natural mixture with a fixed atomic mass. The metal is silver-grey and can be studied only in small amounts because it radiates.

Chemically technetium resembles rhenium, its heavier neighbour in the group, more than manganese. It is most stable with oxidation number plus seven, in the pertechnetate ion TcO₄⁻. Pertechnetate dissolves readily in water and is colourless. The metal tarnishes slowly in moist air.

Most of it comes from nuclear reactors. When uranium is split, technetium-99 forms as a fission product, making up several percent of the fission products in spent fuel. Its half-life is around two hundred thousand years, it radiates weakly, and in final storage it counts as a substance that can move easily in water.

Uses in medicine

In everyday life one does not meet technetium, but in medicine one does. Technetium-99m is an excited state with a half-life of about six hours. On dropping to the ground state it emits gamma rays, which a detector measures from outside, but hardly any particle radiation that would burden tissue strongly. This allows a very gentle look at how an organ works.

The isotope is obtained from molybdenum-99, which forms from uranium in research reactors. Molybdenum-99 decays to technetium-99m with a half-life of just under three days. Hospitals therefore receive a device, the technetium generator, from which the freshly formed technetium is separated as needed. It is bound to a carrier substance that collects in the organ being studied, and is given by injection. A camera then records where the radiation comes from. In this way bones are checked for inflammation or tumour spread, along with blood flow in the heart muscle, the thyroid and the kidneys. Worldwide there are many millions of examinations a year, and technetium-99m is the most widely used medical radioisotope.

Because of the short half-life, technetium-99m cannot be stored and has to be delivered continually. Supply therefore depends on a few research reactors, and shortages have occurred when they were out of service.

Quick question

What is the atomic number of technetium (Tc)?

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