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Once cooled below their respective melting points, the Group 18 elements form an ordered solid. Group 18 elemental solids are useful in providing a place for electrons to become trapped or reactions to take place. Some examples include the use of solid argon to study highly reactive molecules, and solid neon to allow reaction and formation of xenon hydrides.
In their solid states, Group 18 elements (or 'noble gases') form cage structures in the Cubic Close Packing (CCP) formation, also known as Face-Centered Cubic (FCC).1,2 These cages have found great use in research laboratories, serving as a stable, nonreactive substrate to perform spectroscopy or study highly reactive molecules.
Packing of Solid State Group 18 Elements
The solid states of all Group 18 Elements take on an arrangement of Cubic Close Packing (CCP structures), also known as Face-Centered Cubic (FCC) packing.4 This packing shape is based on a structure where each atom is surrounded by 12 other atoms.3 This type of packing formation is represented by stacking spheres together in a plane. Next, a second layer of spheres is applied which covers each of the holes left between the spheres, as viewed from above. Finally, a third layer of spheres is applied to the holes created by the second layer of spheres. In CCP structures, these spheres form an ABA pattern, in which the spheres of the third layer are in the same arrangement as the spheres of the first layer.3,4
Conditions of Crystallization
The elements found in Group 18 are gases at Standard Temperature and Pressure (STP, 273 K and 1 atm). With very little interactive forces to hold the atoms together, these elements must be cooled from STP to form a solid structure. At 1 atmosphere of pressure, the melting point of Neon is 24.5 K, Argon is 84 K, Krypton is 116 K, Xenon is 161 K, and Radon is 202 K.4 It is important to note that while the other Group 18 elements can be solidified under 1 atmosphere of pressure, Helium cannot be and must be under greater pressure than 1 atmosphere in order to solidify.4 At these temperatures, the only attractive force keeping the solids together are Van der Waals forces, due to the lack of bonds present in the elemental forms.4
Cages for Reactions
The solid states of the Group 18 Elements have found uses in research laboratories worldwide. In the formation of xenon hydrides, a solid neon lattice serves as an ideal setting for formation of these compounds, as they are nonreactive even under harsh conditions.1,2 In addition, solid argon has been used on multiple occasions in reactions to serve as a stage for more harsh reaction conditions, such as the formation of difluoronitroxide radical.5 Solid argon has also been used in spectroscopy of highly reactive compounds to study their structures without fear of reaction.6
1) What type of packing do the Group 18 Elements take on in the solid state?
2) Why must these elements be so cold to study their packing states?
3) Why are Group 18 Elements useful in their solid states?
4) Draw or model a diagram of CCP (FCC) Packing, both in the unit cube and as a series of spheres.
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