In fact, neptunium-239 also emitted a beta particle so it ought to become an isotope of an element even higher in the atomic number scale. This one, element 94, was named “plutonium” after Pluto, the planet beyond Neptune. The isotope, plutonium-239, formed from neptunium-239, was only feebly radioactive, however, and it was not clearly identified until 1941. The actual discovery of the element plutonium came the year before, however, when neptunium-238 was formed. It emitted a beta particle and became plutonium-238, an isotope that was radioactive enough to be easily detected and identified by Glenn Theodore Seaborg (1912- ), and his co-workers, who completed McMillan’s experiments when he was called away to other defense research. Neptunium and plutonium were the first “transuranium elements” to be produced in the laboratory, but they weren’t the last. Over the next 30 years, isotopes were formed that contained more and more protons in the nucleus and therefore had higher and higher atomic numbers. At the moment of writing, isotopes of every element up to and including element 105 have been formed. A number of these new elements have been named for some of the scientists important in the history of nuclear research. Element 96 is “curium”, named for Pierre and Marie Curie; element 99 is “einsteinium” for Albert Einstein; and element 100 is “fermium” for Enrico Fermi. Element 101 is “mendelevium” for the Russian chemist Dmitri Mendeléev, who early in 1869 was the first to arrange the elements in a reasonable and useful order. Element 103 is “lawrencium” for Ernest O. Lawrence. “Rutherfordium” for Ernest Rutherford has been proposed for element 104. And “hahnium” for Otto Hahn (1879-1968), a German physical chemist whose contribution we will come to shortly, has been proposed for element 105. Neptunium, however, was not the first new element to be created in the laboratory. In the early 1930s, there were still 2 elements with fairly low atomic numbers that had never been discovered. These were the elements with atomic numbers 43 and 61. In 1937, though, molybdenum (atomic number 42) had been bombarded with neutrons in Lawrence’s laboratory in the United States. It might contain small quantities of element 43 as a result. The Italian physicist Emilio Segrè (1905- ), who had worked with Fermi, obtained a sample of the bombarded molybdenum and indeed obtained indications of the presence of element 43. It was the first new element to be manufactured by artificial means and was called “technetium” from the Greek word for “artificial”. The technetium isotope that was formed was radioactive. Indeed, all the technetium isotopes are radioactive. Element 61, discovered in 1945 and named “promethium”, also has no stable isotopes. Technetium and promethium are the only elements with atomic numbers less than 84 that do not have even a single stable isotope. The Discovery of Fission