Transuranium Element

A transuranium (beyond uranium) element is any of the chemical elements with atomic numbers higher than 92, which is the atomic number of uranium.

Ever since the eighteenth century when chemists began to recognize certain substances as chemical elements, uranium had been the element with the highest atomic weight; it had the heaviest atoms of all the elements that could be found on Earth. The general assumption was that no heavier elements could exist on this planet. The reasoning went like this: Heavy atoms are heavy because of their heavy nuclei, and heavy nuclei are unstable, or radioactive; they spontaneously transform themselves into other elements. Uranium and several even lighter elements—all those with atomic numbers higher than 83 (bismuth)—were already radioactive. Therefore, still heavier ones would probably be so unstable that they could not have lasted for the billions of years that Earth has existed, even if they were present when Earth was formed. In fact, uranium itself has a half-life that is just about equal to the age of Earth (4.5 billion years), so only one-half of all the uranium that was present when Earth was formed is still here.

If we could create atoms of elements beyond uranium, however, perhaps they would be stable enough to hang around long enough for us to study them. A few years, or even hours, would do. But in order to make an atom of an element with an atomic number higher than uranium which has 92 protons in its nucleus, we would have to add protons to its nucleus; one added proton would make an atom of element number 93, two added protons would make element 94, and so on. There was no way to add protons to nuclei, though, until the invention of the cyclotron in the early 1930s by Ernest Lawrence at the University of California at Berkeley. The cyclotron could speed up protons or ions (charged atoms) of other elements to high energies and fire them at atoms of uranium (or any other element) like machine-gun bullets at a target. In the resulting nuclear smashup, maybe some protons from the bullet nuclei would stick in some of the "hit" target nuclei, thereby transforming them into nuclei of higher atomic numbers. And that is exactly what happened. Shooting light atoms at heavy atoms has turned out to be the main method for producing even heavier atoms far beyond uranium.

Such processes are called nuclear reactions. Using nuclear reactions in cyclotrons and other "atom smashing machines," nuclear chemists and physicists over the years have learned a great deal about the atomic nucleus and the fundamental particles that make up the universe. Making new transuranium elements has been only a small part of it.