 Polonium-210

The only isotope found on Earth because it occurs as part of the uranium 238 decay chain, i.e., it is not a primordial nuclide.

Interestingly it is a bit less stable than other isotopesL such as Polonium-208 (3 y) and Polonium-209 (124 y), but those aren't in any Earthly radioactive chain so they don't show up on Earth.

Polonium-210 is a radioactive isotope of polonium, a chemical element with the symbol Po and atomic number 84. It was discovered in 1940 by the scientists Marie Curie and her husband Pierre Curie, who named it after Poland. Polonium-210 is one of the most well-known isotopes of polonium, primarily due to its intense radioactivity and its use in various applications.