transuranium elements
/ trans-yoo-RAY-nee-um /
Uranium, element 92, is the heaviest element that occurs on Earth in any real quantity. Everything beyond it — neptunium, plutonium, americium, curium, and on toward the end of the actinide row — is a transuranium element, and almost all of them are human-made, conjured into existence inside reactors and accelerators because they decay away too fast to survive from the Earth's formation. They are the elements we manufactured rather than found.
How are they made? The first and most important route is neutron capture. Put uranium-238 in the intense neutron flux of a nuclear reactor and a nucleus occasionally swallows a neutron; the resulting heavier nucleus is unstable and undergoes beta decay, in which a neutron turns into a proton, nudging the element one place up the periodic table. Do this once and uranium becomes neptunium; again and the neptunium becomes plutonium. Repeated capture and decay climbs the ladder to americium and curium, which is exactly how these elements are produced in kilogram (plutonium) to gram or milligram (the heavier ones) quantities. Chemically the transuranium elements are actinides: the lighter ones (neptunium, plutonium) inherit the early actinides' flexible, multi-valent chemistry, while from americium onward they drift toward the lanthanide-like +3 state as the 5f electrons sink in.
Why they matter runs from the practical to the everyday to the cautionary. Plutonium-239 is a fission fuel and a weapons material; plutonium-238 is a long-lived heat source that has powered deep-space probes for decades; americium-241 is the tiny radioactive source inside many household smoke detectors. The honest caveats are real: these elements are intensely radioactive and often acutely toxic, must be handled remotely behind shielding, and the heaviest of them exist only as a handful of atoms with lifetimes of seconds — so their 'chemistry' is sometimes inferred from a few events rather than measured in a flask. Discovering and naming them, element by element, is how the actinide concept and the modern bottom of the periodic table were built.
A common smoke detector contains a speck of americium-241 — about 0.3 micrograms — a transuranium element produced from uranium by neutron capture in reactors. Its alpha particles ionize the air in a small chamber; smoke disturbs that ionized air and triggers the alarm.
Americium-241, a transuranium element, sits inside most smoke detectors.
Neptunium and plutonium do occur naturally in vanishingly tiny traces, formed by neutron capture in uranium ores, so 'entirely synthetic' is a slight overstatement; for all practical purposes, though, the transuranium elements are made by people.