Nos. 95 & 96

An element is a substance each of whose atoms contains the same number of electrons. Until recently, scientists thought there were 92 elements, ranging from hydrogen (with one electron circling round its nucleus) to uranium (with 92). All the intervening numbers had been accounted for. So the chemists sat back, feeling that their long search for elements had been completed.

They sat up again with a start in 1940, when University of California scientists produced a new, "synthetic" element (neptunium) by bombarding uranium with neutrons from a cyclotron. Neptunium has 93 electrons, which meant that the list of known elements was growing at the heavy end. It grew some more that same year when Dr. Glenn T. Seaborg and co-workers synthesized plutonium, which has 94 electrons.

Last week Dr. Seaborg, speaking at Chicago, made another momentous announcement. By bombarding uranium and plutonium with high-energy helium ions, he produced two more elements: Nos. 95 and 96. He told little more about them, for like all atomic scientists, Dr. Seaborg still has G-men breathing down his neck. But all elements heavier than uranium are supposedly unstable. Numbers 95 and 96 will certainly be of interest to atomic bombardiers.

When the Manhattan Project decided to produce plutonium in quantity for making atomic bombs, almost nothing was known about its chemical properties. It existed only in sub-microscopic quantities. By August 1942, the Metallurgical Laboratory at Chicago had isolated a weighable amount. It wasn't much. Major General Leslie R. Groves, military head of the bomb project, was shown the entire world supply. He peered hard, and remarked, "I don't see anything."

The speck which the General could not see was plenty for the chemists. Working with solutions measured in microlitres (7,000ths of a teaspoon), they accurately determined plutonium's chemical properties. Then they devised a complex process for separating it from the fiercely radioactive by-products of the uranium-plutonium pile at Hanford, Washington. The pile produced at least ico different byproducts. Most are unstable isotopes of familiar elements (the same periodic numbers but different atomic weights).

Eventually the chemists learned so much about plutonium that they decided to use their new techniques in looking for it in nature. In pitchblende, that mineral Pandora's box of exploding elements, they found unmistakable traces: one part in 100,000,000,000,000. This was the first time that the discovery of a man-made element had led to its identification in a natural ore.*

*Helium was found on earth after the spectroscope found it in the sun's atmosphere.

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