Bismuth-209 is radioactive, decaying like any other unstable atom over time and emitting radiation as it goes. But its rate of alpha decay is slower than any other.
Radioactive decay is the process by which an unstable atom sheds energy and material. The types of material that are spit out in this fashion determine the type (and danger) of that radiation. Alpha decay, for example, is a cluster of two protons and two neutrons; it’s heavy, slow, and doesn’t travel far, but if it gets stuck in your lungs you’re in trouble. Marie Curie may have been felled by inhaled alpha particles, although they are just one of a few potential culprits.
As radioactive material sheds particles it gets smaller. The standard measurement for this loss is the “half-life” – the average time it takes for radioactive material to halve in size. Radium-223, for example, has a half-life of eleven and a half days. Take a lump of radium-223 and wait a week and a half, and you’ll have half that amount of radium-223.
Many exotic and rare isotopes have an extremely small half-life. Hydrogen-7 is the fastest at 2.9 yoctoseconds. There are a septillion (1,000,000,000,000,000,000,000,000) yoctoseconds in a second, so hydrogen-7 really doesn’t stick around long. On the other end of the time scale is bismuth-209.
For a long time, nobody thought that bismuth-209 was radioactive at all. In 2003, researchers discovered that it did actually emit alpha particles, just very very slowly. The half-life of this isotope is 19 quintillion years. For scale, the universe itself has only been around for less than 14 billion years. We don’t even have a great idea of what things will look like in 19 quintillion years. Under some models, the universe itself will no longer exist.
(End-note: bismuth-209’s half-life is the slowest as the result of alpha decay, but there is at least one other isotope with a longer half-life: tellurium-128, estimated to be in the range of a septillion years. But it just doesn’t look as cool as bismuth.)