Francium

Marguerite Perey found francium at the Curie Institute in 1939 while purifying actinium, noticing a decay activity that did not belong to anything known; it was the last element discovered in nature rather than made in a laboratory. Francium-223 has a 22-minute half-life, so nothing accumulates, and the largest quantity ever assembled is a magneto-optical trap holding about 300000 atoms — enough to measure a spectrum, not enough to see. That trap work is the point: francium's huge nucleus amplifies parity-violating weak-force effects on atomic electrons, making it an unusually sensitive laboratory test of the Standard Model. Its properties as a bulk metal, including whether it is more reactive than caesium, are extrapolations that will probably never be checked.