Most unstable naturally occurring element

Francium (discovered by Maguerite Perey, Curie Institute, Paris, France in 1939, given the symbol Fr and atomic number 87) is the most unstable naturally occuring element in the Periodic Table. Francium's most stable isotope, francium-223, possesses a half-life of only 22 minutes compared to the next least stable naturally occuring element, astatine, whose half-life is 8.5 hours. Given this extreme rate of decay, a significant mass of solid francium has never been observed due to the extreme heat of the decay instantly vapourising the sample. It is one of the two least electronegative elements (the other being caesium), and would produce a very impressive explosion of energy if introduced to water. Francium is extremely rare in nature, with possibly as little as 20–30 g (1 oz) existing in the Earth's crust at any one time.