francium

Francium, the fabled element at the bottom of group 1 and the most reactive of the alkali metals (if you are not familiar with the reactivity of group 1 metals, have a look at the video elsewhere on this site).

However, the truth is a little more boring. Francium is vanishingly rare and is found only as very small traces in some uranium minerals. It is the most unstable of the 92 naturally occurring elements and as it is so radioactive that any amount formed would quickly decompose into other elements. Although there are about 20 known isotopes of the metal only Fr-223, with a half life of 22 minutes, occurs in nature. There is estimated to be only ever about 20-30 g of the element present in the earth's crust at any one time and no weighable quantity of the element has ever been prepared or isolated.

Bad news for those of us hoping to see what happens when you put it into a basin of water.

Francium was discovered by Marguerite Perey, who worked as an assistant to Marie Curie, in France (from which the element takes its name) in 1939.