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Hydrogen and Flourine, as well as Chlorine, Bromine and Iodine, exist as diatomic molecules. Both diatomic Hydrogen and diatomic Florine exist as gasses at room temperature and exhibit extremely low melting and boiling points. If the temperature of the system is lowered below the respective melting points of Hydrogen or Fluorine, molecular motion is reduced and a crystalline lattice is formed, creating a solid.
Elemental Hydrogen and Elemental Fluorine, H2 and F2 respectively, exist as gases at room temperature. Though the melting point of H2 is 20.4 K (-252.75 °C), a temperature of 14.0 K (-259.15 °C) is required for the solidification of H2.3,4 As the solid structure is formed, the diatomic Hydrogen molecules adopt a hexagonal close packing (hcp) structure.4 Fluorine exhibits similar behaviour. The melting point of Fluorine is 53.54K (-219.61 °C), at which point the diatomic Fluorine adopts a cubic close packed crystalline arrangement (ccp), instead of a hexagonal close packed arrangement.3,4
When solid, both dihydrogen and difluorine are small enough to allow for rotation within the solid structure (the radius of Fluorine is about equivalent to that of Hydrogen). This rotation occurs about a central axis midway between the two distal atoms of the molecule, creating two equal radii from the middle of the H-H or F-F bond to the outer boundary of each H or F atom. This rotation occurs 360° in the x, y and z directions of the central axis, as well as all manner of combinations of these three variables. The rotation of the molecule in all directions creates what can be though of as a spherical shell, the boundary of which is created by the distal atoms.4
Some common uses for solid hydrogen include gamma-ray ablation cages for elemental analysis, and a solid state cage in which to look at hydrogen bonding within molecules. 1,2
1. Why can F2 and H2 be considered similar molecules and thus exhibit similar behaviour in rotation?
2. What is the different between a cubic close packing structure and hexagonal close packing?
3. Why do difluorine and dihydrogen exhibit different packing structures.
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