|Name, Symbol, Number||francium, Fr, 87|
|Chemical series||alkali metals|
|Group, Period, Block||1, 7, s|
|Atomic mass||(223) g/mol|
|Electron configuration||[Rn] 7s1|
|Electrons per shell||2, 8, 18, 32, 18, 8, 1|
|Density (near r.t.)||? 1.87 g/cm³|
|Melting point||300 K
(27 °C, 80 °F)
|Boiling point|| ? 950 K
(? 677 °C, ? 1250 °F)
|Heat of fusion||ca. 2 kJ/mol|
|Heat of vaporization||ca. 65 kJ/mol|
|Crystal structure||cubic body centered|
(strongly basic oxide)
|Electronegativity||0.7 (Pauling scale)|
|Ionization energies||1st: 380 kJ/mol|
|Electrical resistivity||? 3 µΩ·m|
|Thermal conductivity||(300 K) ? 15 W/(m·K)|
|CAS registry number||7440-73-5|
Francium (chemical symbol Fr, atomic number 87) is a radioactive metal found in minute amounts in uranium and thorium ores. Although many isotopes of this element have been produced, they are all highly unstable. Consequently, knowledge of the properties of this element is very limited. Nonetheless, this metal is notable for having the lowest electronegativity and electron affinity of all the elements. In addition, it is the heaviest alkali metal.
This element, which was named for France, was the last element discovered in nature. Marguerite Perey of the Curie Institute in Paris discovered it in 1939, when examining the products of radioactive decay of actinium. Its existence, however, was predicted by Dmitri Mendeleev in the 1870s, based on his examination of the periodic table. He called it eka-caesium, as he recognized that its properties would closely track those of cesium.
Francium lies in group 1 (former group 1A)—the alkali metal group—of the periodic table. It is the heaviest alkali metal, situated just below cesium. In addition, it is placed in period 7, just before radium.
Although francium occurs naturally in uranium minerals, it has been estimated that there might be only 340 to 550 grams of francium in the Earth's crust at any one time, making it the second rarest element in the crust, next to astatine. It is also the most unstable element among the first 101 and has the highest equivalent weight of any element. Francium is the least electronegative of all the known elements, with cesium being the second least.
Many radioactive isotopes of francium have been produced, with atomic mass numbers between 199 and 232. Of these, the longest-lived isotope is 223Fr, with a 22-minute half-life. It is a daughter isotope of actinium-227 and is one of two isotopes of francium that occur naturally. The second naturally occurring isotope of francium is 224Fr, a member of the thorium radioactive series. All known isotopes of francium are highly unstable, therefore knowledge of the properties of this element comes only from radiochemical procedures.
A small number of images of francium have been obtained, but of at most 350,000 atoms at a time. The images were made by trapping the atoms and using a special fluorescent imaging camera. The atoms were produced by a nuclear transformation with a particle accelerator at the State University of New York at Stony Brook. The nuclei of 18O were accelerated to an energy of 100 million electron volts (MeV), to give them sufficient energy to fuse with gold nuclei and produce francium nuclei. The francium nuclei typically last for three minutes and must be trapped and observed before they decay.
All links retrieved April 25, 2017.
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