Phases of fluorine

Fluorine forms diatomic molecules (F
2) that are gaseous at room temperature with a density about 1.3 times that of air.[1][note 1] Though sometimes cited as yellow-green, pure fluorine gas is actually a very pale yellow. The color can only be observed in concentrated fluorine gas when looking down the axis of long tubes, as it appears transparent when observed from the side in normal tubes or if allowed to escape into the atmosphere.[3] The element has a "pungent" characteristic odor that is noticeable in concentrations as low as 20 ppb.

1892 observation of the color of fluorine gas by Henri Moissan, who first isolated the element, using end-view on 5 meter long tubes. Air (1) is on the left, fluorine (2) is in the middle, chlorine (3) is on the right.
Liquid fluorine

Fluorine condenses to a bright yellow liquid at −188 °C (−307 °F),[4] which is near the condensation temperatures of oxygen and nitrogen.

The solid state of fluorine relies on Van der Waals forces to hold molecules together, which, because of the small size of the fluorine molecules, are relatively weak. Consequently, the solid state of fluorine is more similar to that of oxygen or the noble gases than to those of the heavier halogens.

Animation showing the crystal structure of beta-fluorine.

Fluorine solidifies at −220 °C (−363 °F)[4] into a cubic structure, called beta-fluorine. This phase is transparent and soft, with significant disorder of the molecules; its density is 1.70 g/cm3. At −228 °C (−378 °F) fluorine undergoes a solid–solid phase transition into a monoclinic structure called alpha-fluorine. This phase is opaque and hard, with close-packed layers of molecules, and is denser at 1.97 g/cm3.[5] The solid state phase change requires more energy than the melting point transition and can be violent, shattering samples and blowing out sample holder windows.[6][7]

Solid fluorine received significant study in the 1920s and 30s, but relatively less until the 1960s. The crystal structure of alpha-fluorine given, which still has some uncertainty, dates to a 1970 paper by Linus Pauling.

angled lines showing linear pressure temp relations of the lower phase boundaries A parallelogram-shaped outline with space-filling diatomic molecules (joined circles) arranged in two layers
Low-temperature fluorine phases Alpha-fluorine crystal structure

Notes
  1. Density of air at 100 kilopascal and 0 °C is 1.2724 g/L.[2]

Citations
  1. Jaccaud et al. 2005, p. 2.
  2. Shelquist, Richard (2010). "An introduction to air density and density altitude calculations". Shelquist Engineering. Retrieved 29 April 2011.
  3. Burdon, J.; Emson, B.; Edwards, A. J. (1987). "Is Fluorine Gas Really Yellow?". Journal of Fluorine Chemistry. 34 (3–4): 471. doi:10.1016/S0022-1139(00)85188-X.
  4. Dean 1999, p. 523.
  5. http://jcp.aip.org/resource/1/jcpsa6/v49/i4/p1902_s1
  6. Young, David A. (1975). Phase Diagrams of the Elements (Report). Springer. p. 10. Retrieved 2016-02-25.
  7. Barrett, C. S.; Meyer, L.; Wasserman, J. (1967). "Argon—Fluorine Phase Diagram". The Journal of Chemical Physics. 47 (2): 740–743. Bibcode:1967JChPh..47..740B. doi:10.1063/1.1711946.

Indexed references
  • Dean, John A. (1999). Lange's handbook of chemistry (15th ed.). McGraw-Hill, Inc. ISBN 0-07-016190-9.
  • Ullmann, Franz, ed. (2005). Encyclopedia of Industrial Chemistry. Wiley-VCH. ISBN 978-3-527-30673-2.
    • Jaccaud, Michael; Faron, Robert; Devilliers, Didier; Romano, René (2000). "Fluorine". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a11_293. ISBN 3527306730.

Further reading
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