Longest half-life by alpha decay

Half-life is the measure of how long it takes an unstable element to decay. A half-life of one day means that it would take one day for half the atomic nuclei in a sample to decay into a more stable element. In 2003, a team of French scientists discovered that bismuth-209, previously believed to be radioactively stable, in fact gradually decayed with a half-life of around 20 billion billion years – more than a billion times the age of the universe.

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