Refers to chemical elements with an atomic number higher than that of uranium, i.e. Z > 92. Because these heavy elements tend to be highly radioactive, they generally do not occur in nature because they are very unstable and decay with a short half-life into lighter elements. Some examples of these elements include plutonium, americium, mendelevium, and rutherfordium.
These elements are often created by bombarding some stable element with sub-atomic particles, most often neutrons or perhaps the atomic nuclei of lighter elements. Neutrons are the most effective at producing transuranic elements, because they are neutral and are subject to the strong nuclear force. Most weapons-grade plutonium is produced by nuclear reactors that bombard Uranium-238 with neutrons. Neutron capture changes the U-238 into U-239, which decays in about 24 minutes to Neptunium-239, which then beta decays into Plutonium-239 2.3 days after. Atomic nuclei of light elements such as alpha particles have also been used to create elements with high atomic number.