Radioactive metal m.p. 641 ºC; 1186 ºF b.p. 3232 ºC; 5850 ºF density (α modification) 19.84 g/cc (25 ºC); 1238.57 pound/cubic foot (77 ºF)	memory peg
	1940-41 Glenn T. Seaborg and co-workers, Berkeley, Calif., USA
	Pluto, planet, discovered 1930 and named after Πλουτων (ploutōn), the Greek god of the underworld.

First prepared in 1940-41 Glenn T. Seaborg (1912-1999), Edwin M. McMillan, Joseph W. Kennedy, and Arthur C. Wahl at the Berkeley Laboraty of the University of California by bombardement of Uranium with deuterons.

The second element following Uranium is named after the second planet after Uranus: Pluto, discovered in 1930 and named after Πλουτων [ploutōn], the Greek god of the underworld. About the naming, Glenn T. Seaborg said in 1996:

"In that first report [March 21, 1942], we decided to name the element Plutonium, just like Uranium is named after Uranus, Neptunium by McMillan and Abelson after Neptune, we decided to name it Plutonium. We should have named it plutium, but we liked Plutonium better. It just sounded better. And the symbol obviously should have been Pl, but we liked Pu better so we gave it the symbol Pu." (on-line).

The report was held secret until after the World War II when it was published in the Journal of the American Chemical Society in 1948. This is where the names Plutonium and Neptunium were first revealed.

False transuranic elements (#93-97)

Element #94 has got in 1934-38 the preliminary name Eka-Osmium by Otto Hahn, Lise Meitner and Fritz Strassmann in Germany, who thought they had found traces of several transuranium elements. In December of 1938, Hahn and Strassman found out that these radioactivities were not due to transuranium elements but were due to fission products. According to the Periodic Table of that time, without the Actinide series, element #94 is below Osmium (#76). According to the present Table, Eka-Osmium would be #108.

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Glenn T. Seaborg (age 79) and astronomer Clyde W. Tombaugh (age 85), discoverer of the planet Pluto, in June 1991 at a press conference at the Sandia National Laboratory in Albuquerque, New Mexico.
Courtesy of the Sandia National Laboratories, Albuquerque, New Mexico. Photo by Randy Montoya..

Esperium, Hesperium

In 1934, Enrico Fermi (1901-1954) and his co-workers Amaldi, D'Agostino, Segrè, and Rasetti, after having bombarded Uranium with neutrons, believed to have synthesized the first transuranium elements. These were named Ausonium (#93) and Hesperium/Esperium (#94). See further at Neptunium.

Further reading:

• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 842-846.
• Earl K. Hyde & Glenn T. Seaborg, Transurane : Teil A 1, I: Die Elemente. Gmelins Handbuch der anorganische Chemie, Ergänzungswerk zur 8. Aufl.; Band 7a. Weinheim/Bergstrasse: Chemie, 1973.
• Glenn T. Seaborg, Early History of LBNL, A transcript of the lecture on the 65th Anniversary of Lawrence Berkeley National Laboratory, August 26, 1996 (on-line).
• Enrico Fermi, Artificial radioactivity produced by neutron bombardment. Nobel Lecture, 12 December 1938 (on-line PDF-file).