Lithium

At 0.534 g/cm³ lithium is the least dense solid element and would float on water if it did not react with it vigorously enough to fizz across the surface. Johan August Arfvedson found it in 1817 in petalite from a Swedish mine, and for a century it was a curiosity. Two things changed that. In 1949 the Australian psychiatrist John Cade reported that lithium salts calmed manic patients — still one of the most effective treatments in psychiatry, and one whose mechanism is not fully understood. Then lithium-ion cells, commercialised by Sony in 1991 and honoured with the 2019 Nobel Prize, made lithium the strategic metal of the energy transition. It is also one of only three elements made in the Big Bang, and stars destroy it, so cosmic lithium abundance is a long-standing puzzle.