The story of Neptunium’s discovery

From staring at foldable chairs to going on to invent shopping carts, we are moving over to the discovery of an element in the actinide series. Neptunium, with the atomic number 93, was the first synthetic transuranium element (atomic number greater than 92, that of uranium) of the actinide series to be discovered.

The discovery was announced on June 8, 1940 by Edwin M. McMillan and Philip H. Abelson, who were working at the University of California at Berkeley.

McMillan was born in 1907 to parents, both of whom were of English and Scottish descent.

After spending his formative years in California, he went on to obtain his B.Sc. degree from the California Institute of Technology. A M.Sc. and Ph.D. later, McMillan was studying nuclear reactions and their products, apart from helping in the design and construction of cyclotrons and similar equipments.

The discovery of fission of uranium had created a stir worldwide. McMillan was no exception and he began experimenting with the fission of uranium.

He encountered two components in his experiments, one with a half-life of 25 minutes and the other with roughly two days.

While McMillan was able to speculate that the component with the 25 minute half-life was an isotope of uranium, the other component intrigued him. Through further experimentation, McMillan was able to conclude that a decay product of 239U by beta decay was producing atoms of a new element.

In May 1940, McMillan began collaborating with Abelson, who was serving as an assistant physicist. Together they were able to use a oxidation-reduction cycle to create a series of precipitations of the 2.3-day activity component. Using this, they were able to prove that it is an isotope of element 93.

McMillan named the element neptunium, after Neptune, the planet beyond Uranus. The oxidation-reduction cycle technique used for this went on to form the basis of transuranic chemistry.

Nobel Prize

In 1941, McMillan was involved in the isolation of another new element along with Glenn Seaborg, Joseph Kennedy and Arthur Wahl. They named this element plutonium, after Pluto, the planet beyond Neptune. For his discoveries in the field of transuranium elements, McMillan shared the Nobel Prize in Chemistry in 1951 with Seaborg.

McMillan also came up with the idea of “phase stability” in 1945. It was this idea that led to the development of synchrotron and synchro-cyclotron machines. These devices have made important researches possible as they help artificially accelerate particles to high energy levels.

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The discovery of fission of uranium had created a stir worldwide. McMillan was no exception.

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