A transuranic element
is an element
with a greater atomic number
, atomic number 92, as it has the highest atomic number of any naturally occurring element.
The first of these elements was neptunium, produced at the University of California at Berkeley in 1940 by E. M. McMillam and P.H. Abelson. An isotope of element 93, neutrons bombarding uranium-238 produced uranium-239 and after a few days of beta decay neptunium-239 was formed.
238U + 1 --> 239U
239U --> 239Np + 0
Plutonium, atomic number 94, was next. A cyclotron accelerated deuterons and were slammed into uranium to produce neptunium-238 which later decayed to plutonium-238. This isn't what most people think of as plutonium today which is used in weapons and reactors, which is plutonium-239
238U + 2H --> 238Np + 21n
238Np --> 238Pu + 0e
Americium (atomic number 95) and curium (atomic number 96) helped scientists understand how elements after actinium should fit into the periodic table. Originally it was thought that these elements belonged under the d-transition elements after actinium and uranium ended up in Group VIB under tungsten. Glenn T. Seaborg thought perhaps a second series of elements belonged under the periodic table and they were named the actinides. This helped scientists separate these later elements due to their expected chemical properties which could be determined by their plane in the table.
Other than the best known uses, weapons and reactors, these elements are often used commercially. Since plutonium-238 only produces weak alpha radiation it is used as a power source for things like satellites and pacemakers. Gamma rays from Americium-241 are used to measure the thickness of metal sheets and their alpha particles are present in home smoke detectors to determine is there is smoke in the air.
Ebbing, Darrell D. and Gammon, Steven D. General Chemistry - Seventh Edition. Boston: Houghton Mifflin Company, 2002.