Covalent Solids

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Covalent Solids

Post by Algerien1970 on Sat 23 May - 2:33



Extended Covalent Bonding: "Framework Structures"

In covalent compounds both atoms have similar and fairly high electronegativities. The nature of the bonding therefore relies upon electron-sharing through orbital overlap. In contrast to ionic and metallic solids, the bonding in extended covalent solids is highly directional, this leads to low coordination numbers (e.g. 4).

Diamond, analogy to methane

In methane the bonding network of the carbon atoms is terminated by the H atoms. Because the H atoms can only form one covalent bond, which is confined within the molecule, there is no possibility for extended covalent bonding between different molecules. The inter-molecular bonds must therefore rely on weaker dispersion forces for their cohesive forces.

In diamond, which can be constructed by replacing the H atoms in methane by other C atoms, the bonding is not terminated and the s sp3 bonds between the C atoms are extended throughout the solid. These are strong bonds, as a result diamond has a very high melting (>3550°C) and boiling (4827°C) point. (for a more detailed description of the bonding go to Conduction properties)


Crystal Structure of Diamond


Methane


The crystal strucutre of diamond is comprised of tetrahedrally coordinated C atoms. The unit cell is fcc, with 2 carbon atoms per lattice point, and 8 atoms per cell.
The diamond structure is also adopted by several well known semi-conductors such as Si, Ge (and also gray Sn).
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