Characteristics
Francium is the most unstable of the naturally occurring elements: its most stable isotope, francium-223, has a maximum half-life of only 22 minutes. In contrast, astatine, the second-least stable naturally occurring element, has a maximum half-life of 8.5 hours. All isotopes of francium decay into either astatine, radium, or radon. Francium is also less stable than all synthetic elements up to element 105.
Francium is an alkali metal whose chemical properties mostly resemble those of caesium. A heavy element with a single valence electron, it has the highest equivalent weight of any element. Liquid francium—if such a substance were to be created—should have a surface tension of 0.05092 N/m at its melting point. Francium's melting point was claimed to have been calculated to be around 27 °C (80 °F, 300 K). However, the melting point is uncertain because of the element's extreme rarity and radioactivity. Thus, the estimated boiling point value of 677 °C (1250 °F, 950 K) is also uncertain.
Linus Pauling estimated the electronegativity of francium at 0.7 on the Pauling scale, the same as caesium; the value for caesium has since been refined to 0.79, although there are no experimental data to allow a refinement of the value for francium. Francium has a slightly higher ionization energy than caesium, 392.811(4) kJ/mol as opposed to 375.7041(2) kJ/mol for caesium, as would be expected from relativistic effects, and this would imply that caesium is the less electronegative of the two.
Francium coprecipitates with several caesium salts, such as caesium perchlorate, which results in small amounts of francium perchlorate. This coprecipitation can be used to isolate francium, by adapting the radiocaesium coprecipitation method of Glendenin and Nelson. It will additionally coprecipitate with many other caesium salts, including the iodate, the picrate, the tartrate (also rubidium tartrate), the chloroplatinate, and the silicotungstate. It also coprecipitates with silicotungstic acid, and with perchloric acid, without another alkali metal as a carrier, which provides other methods of separation. Nearly all francium salts are water-soluble.
Read more about this topic: Francium