Mysterious missing element

By Associate Professor Allan Blackman

This article was orignally published in the Otago Daily Times on Thursday 20 September 2007.

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The universe is constructed from only 117 different types of atom, each of which is called an element. The elements are usually classified according to their atomic number, which is equal to the number of positively-charged protons in the nucleus of an atom of that element. For example, the atomic number of hydrogen is 1, meaning an atom of hydrogen contains a single proton in its nucleus, while the nucleus of an atom of uranium, (atomic number 92) contains 92 protons.

Some of you will be familiar with the Periodic Table of the Elements. This is a listing of all the elements in order of increasing atomic number and is usually credited to the 19th century Russian chemist, Dmitri Mendeleev, who coincidentally died 100 years ago this year. The Periodic Table is one of the most important chemical discoveries of all time, but despite this, Mendeleev was not awarded the Nobel prize (he apparently missed out by a single vote in 1906). In the late 19th century, when Mendeleev proposed his periodic table, not all of the elements were known, and he left gaps in his table to accommodate these. By 1925, virtually all of the first 92 elements had been discovered, but, despite the best efforts of many research groups, a stubborn gap remained in the periodic table at element 43.

The mysterious element 43 was finally discovered 70 years ago this year, and it has the distinction of being the first 'artificial' element to be prepared by mankind (one hesitates to call it the first manmade element, as it's a little un-PC). It was named technetium (pronounced tek-nee-seeum), from the Greek technikos, meaning artificial, to reflect its origins. Technetium is unusual in that it doesn't occur naturally on earth, and so those scientists who were looking for it over 70 years ago were on a bit of a fruitless mission! The reason that technetium cannot be found on earth is that all of its isotopes are radioactive. The most stable isotope of technetium has a half life of around 4 million years – this means that exactly half of a sample of this isotope will undergo decay every 4 million years. This might seem a long time, but when you consider that the earth is about 4 billion years old, any technetium that existed when the earth was formed will long since have decayed away.

In 1937, Carlo Perrier and Emilio Segrè, carried out experiments in Sicily which confirmed that they had prepared technetium by bombarding another element, molybdenum, with deuterium nuclei (deuterium is an isotope of hydrogen). And for this quite extraordinary discovery – the creation of an element – they didn't get the Nobel prize (Segrè actually received it later for different work).

So why should we worry about the 70th anniversary of an esoteric chemical element which doesn't exist on earth? It so happens that one isotope of technetium, called technetium-99m, has ideal properties for use in medical imaging applications. When this isotope decays, it emits a gamma ray, which can be detected using a special camera. By binding this technetium isotope to molecules which target particular parts of the body, detailed images of specific organs or tumours can be obtained. Its short half-life (6 hours) means that it decays rapidly, thereby minimising the patient's exposure to radioactivity. However, this also means that it cannot be stored for any length of time and a continuous supply is necessary. New Zealand obtains weekly supplies of the potentially lifesaving technetium-99m thanks to the Australian nuclear reactor in Sydney, where it is prepared.

It just goes to show how apparently 'academic' research can end up having unexpected beneficial applications.