Glenn T. Seaborg
Philip H. Abelson
What happened was exactly what was expected. Uranium-238 (92 protons, 146 neutrons) added a neutron to become uranium-239 (92 protons, 147 neutrons), which emitted a beta particle to become neptunium-239 (93 protons, 146 neutrons).
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.