How long did it take to discover francium?
Francium is a natural chemical element but the total quantity of
francium in the earth crust is only approx. 30 g. Francium has ca.
40 isotopes and isomers but only two are natural: 221Fr (in the
neptunium decay chain series) and 223Fr (in the actinium decay
chain series). Artificially preparation of francium isotopes is
also extremely difficult and expensive; and the chemistry and
physics of isotopes was developed essentially after 1940.
Supplementary, the most stable isotope of francium has a half life
of only 21,8 minutes and is strongly radioactive; the half lives of
the artificially prepared isotopes are more smaller.
This is sufficient reason to explain why francium was later
discovered and why even today is only slightly studied.