Francium is the most expensive naturally occurring element in the world. The extremely rare, highly radioactive metal is found in trace quantities in uranium and thorium ores where it is continually forming and decaying into astatine, radium and radon.
The periodic table is a plan of the 118 chemical elements arranged in rows so that elements with similar chemical properties appear in vertical columns.
The atomic number, the number of protons in the nucleus, ranges from 1 (Hydrogen) to 118 (Oganesson)
A hole in the periodic table was filled with discovery of the element with atomic number 87 (between radon, 86 and radium, 88). Claimed by many (with names russium, alkalinium, viginium, moldavium, eka-caesium, actinium K), the first confirmed
discovery was by Marguerite Perey in 1939, who named it after her country, France.
The Chemical Detective 