Copper, proton number 29, and argon, proton number 18, are elements which have d

Question

Copper, proton number 29, and argon, proton number 18, are elements which have different physical and chemical properties. In the solid state, each element has the same face-centred cubic crystal structure which is shown below. The particles present in such a crystal may be atoms, molecules, anions or cations. In the diagram above, the particles present are represented by Which typos of particle are present in the copper and argon crystals? In each case, give their formula. At room temperature, copper is a solid while argon is a gas. Explain these observations in terms of the forces present in each solid structure.

Explanation / Answer

a)

copper  :     copper atoms : Cu

argon :         argon atoms : Ar

b)

copper crystal:

copper crystal is an example of metallic solid.

Metallic solids are held together by a high density of shared, delocalized electrons, resulting in metallic bonding. Metallic solids have, by definition, no band gap at the Fermi level and hence are conducting.

argon crystal is an example of molecular solid:

molecular solids consists of small, non-polar covalent molecules, and are held together by London dispersion forces (van der Waals forces). These forces are weak, resulting in pairwise interatomic binding energies on the order of 1/100 those of covalent, ionic, and metallic bonds. Binding energies tend to increase with increasing molecular size and polarity.

Solids that are composed of small, weakly bound molecules are mechanically weak and have low melting points; an extreme case is solid molecular hydrogen, which melts at 14 K (259 °C). The non-directional nature of dispersion forces typically allows easy plastic deformation, as planes of molecules can slide over one another without seriously disrupting their attractive interactions. Molecular solids are typically insulators with large band gaps.

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