An eye for an i #411 Children

All about element No. 85

Electron shell diagram for astatine, the 85th element in the periodic table.   | Photo Credit: Greg Robson/Wikimedia Commons

At any given point of time, there is only about 25 g of astatine that occurs naturally on our planet. If that doesn’t wow you, then it’ll help to nudge you along by mentioning that that is less than two tablespoons of naturally occurring astatine on Earth at any given instant!

When there is so little of something around, it surely is difficult to find it – even when its existence has been revealed. For astatine, it was the periodic table of elements created by Russian chemist Dmitri Mendeleev in 1869 that predicted its existence. It would be over 70 years later that the element is first successfully discovered.

Properties predicted

Mendeleev’s periodic table predicted properties of what was then an unknown element. It was to fill in the blank space left for element number 85 on the periodic table. Positioned right below iodine in the halogen group, Mendeleev called it eka-iodine.

Among the first claims for the discovery of this element came in November 1931. A physicist with controversial methods, American Fred Allison at the Alabama Polytechnic Institute said that he found the element. He even called it alabamine (after Alabama), but as his results couldn’t be replicated and his equipment and methods were shown to be faulty, his claim bit the dust.

Breakout of WWII

Two groups next came close to discovering the element in mineral samples in the 1930s. Horia Hulubei and Yvetter Cauchois were researchers at the Sorbonne in Paris and they used a high-resolution X-ray apparatus to analyse mineral samples. They believed they detected the as-yet-undiscovered element, but World War II put paid to their research.

The other group to suffer a similar fate was headed by Swiss chemist Walter Minder. After observing the radioactivity of radium, Minder suggested that it appeared to have another element present. The chemical tests that he undertook suggested properties like iodine for the element, but he was never able to categorically make a claim.

Produced, finally!

The first recognised discovery of astatine finally came about in 1940. It wasn’t identified in nature, but instead was successfully produced at the University of California Berkeley by researchers Dale R. Coson, Kenneth Ross Mackenzie and Emilio Segre.

Physicist Emilio Segre was among the group of scientists who finally produced astatine. Segre, who was also involved in the discovery of technetium, won the 1959 Nobel Prize in Physics for the discovery of antiproton, a subatomic particle.

Physicist Emilio Segre was among the group of scientists who finally produced astatine. Segre, who was also involved in the discovery of technetium, won the 1959 Nobel Prize in Physics for the discovery of antiproton, a subatomic particle.   | Photo Credit: Nobel foundation/ Wikimedia Commons

Using a particle accelerator, this group of scientists bombarded bismuth with alpha particles to produce an isotope of astatine. Noting that the element produced was both highly radioactive and unstable, they named it astatine – derived from the Greek word astatos meaning unstable.

Even though they reported their discovery, they weren’t able to continue their research much further. This was owing to the demands of World War II, which diverted all the resources devoted to the study of radioactive materials towards the making of nuclear weapons.

Found in nature

Astatine was found in nature for the first time a few years later by Austrian physicist Berta Karlik and her assistant Traude Bernert. While the war was still raging, Karlik was able to identify astatine as an intermediate in radioactive decay chains.

With news not flowing freely during the war, Karlik was under the impression that they were the first to discover element number 85, and they even reported their results along with a name for the element. When made aware of the results from the Berkeley group, Karlik continued to study astatine and was able to expand on the subject of decay chains that form the element.

Astatine, with the symbol At, is the heaviest-known halogen, and is also the least reactive and most metallic within the halogen group. Decades have now passed since astatine was finally discovered, but the element continues to be steeped in mystery. With the longest-known isotope having a half-life of eight hours and only tiny amounts of the element ever produced, studying the element hasn’t been easy.

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Printable version | Jan 24, 2021 9:42:30 AM | https://www.thehindu.com/children/element-no-85/article32989543.ece

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