Element Francium, Fr, Alkali Metal

In 1939 by Marguerite Perey at Curie's Institute in Paris was purifying actinium-227 when she detected β-emission produced by unknown isotope with a half-life of 21 minutes. More thorough research showed that the beta decay is detected not only for the main series Actinium- RadiumActinium - ActiniumX, but also by the lateral Actinium- Actinium K-Actinium X series. The element was first known as Actinium K. Chemical analyses showed that its properties were very close to those of eka-caesium. After the World War II which interrupted the research, Perey's results were confirmed. In 1946 it was named Francium, by Perey for her native country.

Alkali metal Francium does not have stable isotopes. 27 known radioactive isotopes have mass numbers from 202 to 229. Most long-lived radionuclides are ²¹²Fr [half-life T_1/2 20 min, α-decay (44%), electron capture (56%)] and ²²³Fr [half-life T_1/2 21.8 min, β-decay (99.9%)]; the latter is a constituent part of natural actino-uranium. The Francium/Uranium ratio in Earth's crust is 4x10^-16. Electron configuration: 5s²5p⁶5d¹⁰6s²6p⁶7s¹, oxidation number: +1; electronegativity: 0.7 (Pauling scale); atomic radius: 0.29 nm, Fr⁺ ion radius: 0.178 nm.