Glenn Seaborg and the actinide series

Who was Glenn Seaborg?

Glenn Theodore Seaborg (1912–1999) was born in Ishpeming, Michigan, and grew up in Southern California. He earned his PhD in chemistry at the University of California, Berkeley, in 1937 and spent most of his career there, working alongside physicists who used Ernest Lawrence’s cyclotrons to create new radioactive isotopes.

Making plutonium

In 1940 Edwin McMillan and Philip Abelson at Berkeley made neptunium, element 93, the first element heavier than uranium. Later that year Seaborg, McMillan, Joseph Kennedy and Arthur Wahl bombarded uranium with deuterons and produced element 94, which they identified chemically early in 1941 and named plutonium, after Pluto. In 1941 Seaborg and his colleagues also showed that plutonium-239 splits when struck by slow neutrons, which made it a candidate fuel for nuclear reactors and weapons. During World War II Seaborg moved to the Metallurgical Laboratory at the University of Chicago, where he led the work on separating plutonium for the Manhattan Project.

A puzzle with elements 95 and 96

At the time, periodic tables placed thorium, protactinium and uranium in the main body of the table, below hafnium, tantalum and tungsten, as if they were transition metals. By that logic, elements 95 and 96 should behave like iridium and platinum. Experiments based on that assumption failed to find them.

The actinide concept

In 1944 Seaborg proposed a different idea. The heavy elements from actinium onward, he suggested, form a family of their own, filling an inner set of electron orbitals (the 5f orbitals) just as the lanthanides fill the 4f orbitals. If that were true, element 95 should resemble europium and element 96 gadolinium. Searching with that chemistry in mind, his group identified both elements in 1944, and they were later named americium and curium. Because of wartime secrecy, the discovery was first revealed in November 1945, when Seaborg answered a child’s question on a radio quiz show a few days before the official announcement.

Seaborg published the actinide concept in 1945, along with a periodic table that placed the actinides in a separate row beneath the lanthanides. He later recalled that colleagues had warned him the idea could ruin his scientific reputation.

Changing the shape of the table

The actinide concept moved thorium, protactinium and uranium out of the transition metals and gave the periodic table its familiar modern look, with two rows of f-block elements printed below the main body. It also proved to be a reliable guide. Seaborg’s team used it to plan the searches that produced berkelium in 1949 and californium in 1950, and it correctly placed the end of the series at lawrencium, element 103, with element 104 returning to the main table below hafnium.

The Nobel Prize and seaborgium

Seaborg shared the 1951 Nobel Prize in Chemistry with Edwin McMillan for their discoveries in the chemistry of the transuranium elements. He took part in the discovery of plutonium, americium, curium, berkelium, californium and mendelevium, among others, and later chaired the U.S. Atomic Energy Commission from 1961 to 1971.

In 1974 a team from Berkeley and Lawrence Livermore National Laboratory made element 106, and the discoverers proposed calling it seaborgium. After a long international debate over the names of the heaviest elements, IUPAC approved seaborgium in 1997, the first time an element had been officially named after a living person. Seaborg died two years later, in 1999.