Francium occurs as a result of α disintegration of actinium. Francium is found in uranium minerals, and can be made artificially by bombarding thorium with protons. It is the most unstable of the first 101 elements. The longest lived isotope,223Fr, a daughter of227Ac, has a half-life of 22 minutes. This is the only isotope of francium occurring in nature, but at most there is only 20-30 g of the element present in the earth's crust at any one time. No weighable quantity of the element has been prepared or isolated. There are about 20 known isotopes.
This sample of uraninite contains some francium because of a steady-state decay chain. An estimate suggests there is about 10-20 grammes of francium (about 1 atom!) at any one time. Image adapted with permission fromProf James Marshall's (U. North Texas, USA)Walking Tour of the elements CD.
Binary compounds with halogens (known as halides), oxygen (known as oxides), hydrogen (known as hydrides), and other compounds of francium where known.
Isolation: francium is vanishingly rare and is found only as very small traces in some uranium minerals. It has never been isolated as the pure element. As it is so radioactive, any amount formed would decompose to other elements.
Actinium decays by β decay most of the time but about 1% of the decay is by α decay. The "daughter" element of this reaction, which used to be called actinium-K, is now recognized as22387Fr - the longest-lived isotope of actinium with a half life of about 22 minutes.
Density of solid: 2900 (estimated) kg m-3
Molar volume: 77 (rough estimate based upon density estimate) cm3
Thermal conductivity: 15 (estimate) W m‑1 K‑1
Melting point: maybe about 300 [about 30 °C, estimate] K
Boiling point: 871 [598 °C (1108 °F)] K
Enthalpy of fusion: 20.5 kJ mol-1
Atomic radius (empirical): (no data) pm
Molecular single bond covalent radius: 223 (coordination number 1) ppm
van der Waals radius: 282 ppm
Pauling electronegativity: 0.7 (Pauling units)
Allred Rochow electronegativity: 0.86 (Pauling units)
Mulliken-Jaffe electronegativity: 0.68 (s orbital)
First ionisation energy: 392.96 kJ mol‑1
Second ionisation energy: 2160 kJ mol‑1
Third ionisation energy: 3230 kJ mol‑1
Universe: (no data) ppb by weight
Crustal rocks: (no data) ppb by weight
Human: (no data) ppb by weight
