Seaborgís Change

 

††††††††††† By the 1940s, the majority of the elements had been discovered.A small problem arose with Mendeleevís model when the Rare Earth Metals began to be discovered.Most people have never heard of most of the elements from the Lanthanide and Actinide series (sometimes referred to as the Lanthanoids and Actinoids), despite the majority of them being mined here in the United States and Canada.These strange elements have properties similar to the transition elements with other properties that are unique to the group and the periodic table had no gaps to be filled, despite the fact that the atomic numbers of the Lanthanide series fit between Lanthanum (element 57, group 3) and Hafnium (element 72, group 4).It was decided to stick the fourteen Lanthanides off by themself until something else could be figured out about their true place on the periodic table.Thorium, Protactinium and Uranium seemed to fit in fairly well after Actinium in what seemed to be the order they should be in.

A different pattern evolved when the elements Neptunium and Plutonium.These elements fit well with Thorium, Protactinium and Uranium, but not so well with the elements in their vertical groups.The American nuclear scientist, Glenn Seaborg, suggested moving the five elements to be with the Lanthanides, below the periodic table.Colleagues attempted to persuade Seaborg not to publish his idea since it would most likely meet with persecution and may end his young career.Despite the promptings, Seaborg went ahead with suggesting his organizational scheme.Though the addition of the Actinide series as another rogue group on the periodic table met with significant resistance, eventually it was determined that these groups were different due to their electron configuration (during the time of Seaborgís proposal, the yet undiscovered f-block).It was determined to leave the Lanthanides and Actinides below the periodic table as it would make the table large and awkward to use if they were placed between groups three and four where they belong.Because of this, you may also hear the Lanthanides and Actinides referred to as the inner transition elements.We have synthesized every element with an atomic number higher than 92, filling the Lanthanide series and continuing the rest of the 7th periodís transition elements.Further expansion of the periodic table will require additional adjustments to find new places to put the elements.