Potassium

In October 1807 Humphry Davy ran the current from a large voltaic battery into slightly moist caustic potash and watched small globules of a bright metal appear at the negative electrode, some of which caught fire on the spot — his assistant recorded that Davy danced about the room in ecstasy. It was the first element ever isolated by electrolysis, and it kept the old English name for potash while its symbol K came from the Latin kalium. About 0.012% of natural potassium is ⁴⁰K, a long-lived beta and gamma emitter, which makes potassium-rich food mildly radioactive and gives the banana equivalent dose its name; the same decay is the source of atmospheric argon. Potassium is the dominant cation inside your cells and the third number on every fertiliser bag.