Glenn T Seaborg – making new elements with sub-atomic particles

Glenn T Seaborg, with his co-workers identified 11 elements. These all have atomic number greater than 92 (uranium) and are man-made rather than occurring naturally. The discovery that uranium atoms could be bombarded with neutrons to create new elements led to an extension of the Periodic Table beyond uranium.

The elements found by chemists before Seaborg were discovered – they existed on Earth already, combined with other elements to form compounds in most cases. Chemists had to extract them and show that they really were new elements. The elements found by Seaborg and his colleagues were actually made – they are elements that do not exist naturally on Earth. The heaviest element that does exist on Earth is uranium which has 92 protons.

Protons are positively charged and tend to repel one another (they are held in the nucleus against this repulsion by a force called the strong nuclear force). Atoms whose nuclei have more than 92 protons tend to break apart because of this repulsion. This is why they are not found on Earth. Scientists found that when they fired neutrons at uranium atoms, one would occasionally stick to a uranium nucleus. This increased the relative atomic mass of the atom by one but kept the atomic number the same. Sometimes this neutron then ‘spat out’ an electron and turned into a proton. This meant that the nucleus now had 93 protons and was a new element, of atomic number 93, which was christened neptunium, Np. This was actually done by Edwin McMillan and Philip Abelson. Seaborg then took over working as the leader of a group of scientists. Bombardment with other sub-atomic particles allowed them to make elements numbers 94 - 103 in the same sort of way.