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
But let us get back to the bombardment of uranium with neutrons research that Fermi had begun. After he had reported his work, other physicists repeated it and also got a variety of beta particles and were also unable to decide what was going on.
Lise Meitner and Otto Hahn in their laboratory in the 1930s.
One way to tackle the problem was to add to the system some stable element that was chemically similar to the tiny traces of radioactive isotopes that might be produced through the bombardment of uranium. Afterwards the stable element could probably be separated out of the mixture and the trace of radioactivity would, it was hoped, be carried along with it. The stable element would be a “carrier”.
Among those working on the problem were Otto Hahn and his Austrian co-worker, the physicist Lise Meitner (1878-1968). Among the potential carriers they added to the system was the element, barium, which has an atomic number of 56. They found that a considerable quantity of the radioactivity did indeed accompany the barium when they separated that element out of the system.
A natural conclusion was that the isotopes producing the radioactivity belonged to an element that was chemically very similar to barium. Suspicion fell at once on radium (atomic number 88), which was very like barium indeed as far as chemical properties were concerned.