All elements with atomic number 84 (Polonium) and above are radioactive — they have no stable isotopes. But radioactivity varies enormously: some isotopes last billions of years, while others vanish in milliseconds.
Radioactivity is measured by specific activity — the number of decays per second per gram of material. The higher the specific activity, the more intensely radioactive an element is.
Naturally Occurring Radioactive Elements
These radioactive elements occur in nature and have had the most historical significance:
Polonium-210 has a specific activity of approximately 166 TBq/g (4,490 Ci/g), making it extraordinarily radioactive. Just 1 microgram is lethal. It was used to assassinate Alexander Litvinenko in 2006.
Radium-226 has a half-life of 1,600 years. Once used in luminous paint for watch dials ('Radium Girls'), it was the first radioactive element commercially exploited.
Radon-222 is a colorless, odorless radioactive gas with a 3.8-day half-life. It seeps from soil and rock into buildings, and is the second leading cause of lung cancer after smoking.
Actinium-227 is approximately 150 times more radioactive than radium. Its intense radioactivity makes it glow blue in the dark due to Cherenkov radiation in the surrounding air.
Astatine is the rarest naturally occurring element. At any given moment, less than 30 grams exist in the entire Earth's crust.
Uranium-238 has a half-life of 4.47 billion years — about the age of Earth. Its slow decay means it's only mildly radioactive, but it's the starting material for nuclear reactors.
Synthetic Superheavy Elements
The most unstable elements are all synthetic — created in particle accelerators and existing for fractions of a second:
Oganesson has a half-life of about 0.7 milliseconds. Only 5 atoms have ever been detected.
Tennessine has a half-life of about 51 milliseconds for its longest-lived isotope.
Moscovium's most stable isotope has a half-life of about 220 milliseconds.
Nihonium has a half-life of about 10 seconds — practically eternal by superheavy element standards.
Useful Radioactive Elements
Despite the dangers, several radioactive elements serve critical roles in medicine, energy, and everyday life:
Technetium-99m is the most widely used medical radioisotope, employed in over 40 million diagnostic procedures worldwide each year.
Americium-241 is in nearly every household smoke detector. Its alpha particles ionize air, allowing the detector to sense smoke.
Plutonium-238 powers deep-space missions including the Voyager probes, Curiosity rover, and New Horizons spacecraft.