Covalent and molecular solids

A covalent solid consists of atoms that are held together by covalent bonds, creating a network that extends throughout the crystal, while a molecular solid is formed when molecules are held together by van der Waals forces or hydrogen bonds or a mixture of the two types of intermolecular forces.

Diamond is an example of a covalent solid with a structure that is similar to that of zinc blende. However, carbon atoms (orange atoms in diagram above) instead of zinc cations occupy four tetrahedral holes within a face-centred cubic structure that is again formed by carbon atoms (blue atoms) and not anions. Just like zinc blende, the unit cell of diamond is face-centred cubic. Other elements like silicon and germanium in group 14 also have the same structure.

An example of a molecular solid is ice. Water molecules, when cooled below 273.15 K at standard atmospheric pressure, are held together in a chair configuration (see diagram I above) by hydrogen bonds. When viewed from the top, we see hexagonal rings extend throughout the crystal, with the unit cell being hexagonal (demarcated by red lines in diagram II).

The structure of graphite, on the other hand, has features of both a covalent solid and a molecular solid. The diagram above shows carbon atoms covalently bonded to form giant planar sheets of hexagonal rings that are stacked in an XYXY… manner. Each sheet is called graphene. Every carbon in graphite is sp2 hybridised, leaving an electron in the orbital that is free to form a π-bond with one of three neighbouring orbitals. One may expect alternating single and double bonds throughout each sheet. However, x-ray diffraction data shows that all covalent bonds in the sheets have a similar length of 0.141 nm, implying that the orbitals overlap to form a conjugated -electron system where the electrons are delocalised. This is the reason why graphite conducts electricity. Experiments also shows that graphite is only electrically conductive in the direction parallel but not perpendicular to the sheets, suggesting that the sheets are not held together by the overlap of orbitals in the perpendicular direction. Layers of graphene are instead loosely stabilised by van der Waals forces and can slide against each other, making graphite a good lubricant. The thermodynamically stable form of graphite, as shown in the diagram above, has a hexagonal unit cell containing four carbon atoms.



Why does the unit cell of graphite contain four carbon atoms?


The edges of the hexagonal unit cell of graphite cuts through three layers of graphene (see above diagram). The top sheet has four corner atoms, each being shared by six unit cells, and one other atom shared by two unit cells \left ( \frac{4}{6}+\frac{1}{2} \right ). The same occurs for the bottom layer. For the middle graphene, four peripheral atoms are again shared by six unit cells but one atom remains entirely within the unit cell \left ( \frac{4}{6}+1\right ). Therefore, in total, there are four atoms in a single hexagonal unit cell of graphite.


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