The Upper Limit of the Periodic Table
The Periodic Table has been acknowledged as one of the most influential keys to understanding modern chemistry. A wealth of information is organized into a readily interpretable array of essential atomic data. Since the days of Dmitri Mendeleev, who is credited with arranging our modern periodic table on the basis of physical similarities, atomic physicists have drastically extended the number of elements by the preparation of artificial elements. These are atoms not found naturally on Earth due to radioactive decay instability but have been created synthetically by atomic bombardment and collisions.
The very first synthetic element was the result of many years of searching for the elusive missing element to be inserted between molybdenum and ruthenium, an omission noted and a space left open by Mendeleev. Many efforts claiming to have identified element #43 were made but not substantiated. Conclusive evidence for the production of a new element was made by Emilio Segré and Carlo Perrier in 1937 after they collided molybdenum atoms with the heavy isotope of hydrogen known as deuterium. Later trace amounts of technetium were identified among the decay products of uranium fission. The name technetium was chosen from the Greek word for artificial.
The next synthetic element, #61, promethium, was produced by a similar method. Jakob Marinsky and Larry Glendenin at MIT bombarded neodymium atoms with neutrons obtained as byproducts of uranium decay. Their 1946 announcement named the new element after the mythological Prometheus, who, according to legend was responsible for bringing fire to mankind.
The decade of the 1940’s also marked the creation of the first trans-uranium element. Neptunium was the result of Berkeley scientists Edwin McMillan and Philip Abelson colliding uranium with neutrons as was the concurrent production of element 94, named plutonium in the sequence correlating with the modern group of solar system planets. One name suggested for element 94 was “extremium” offering the proposition that this artificially produced element was the upper limit or heaviest possible atom.
Since that time, the quest for producing super-heavy elements has continued with the question of where and when that upper limit, if it exists, will be reached. Currently, (2009) the as-yet unnamed Element 118, a member of the noble gas family, maintains its status as the heaviest element. Three atoms of element 118 were reportedly created by fusing californium atoms with calcium atoms in 2006 at Lawrence Livermore Laboratory. In the last year, claims suggesting the existence of Element 122 have also been reported but as yet, experimental replications have failed to reproduce this evidence.
Is there an upper limit to the periodic table? The intrinsic instability with respect to nuclear decay appears to limit the production of elements with atomic numbers greater than that of uranium. Most of the trans-uranium elements have extremely short half-lives and very limited production quantities. Attempting to load the tiny atomic nucleus with 100+ protons appears to provide a barrier that may have reached its synthetic limit.