n the case of the diamond lattice it can be seen that each carbon atom is surrounded by four others (tetrahedral arrangement). The structure consists of a tetrahedral extending in all dimensions, which suggests covalent bonding between adjacent atoms. This in turn explains the observed properties of diamond. The strength of the bonding and the fact that it exists throughout the crystal accounts for the extreme hardness and high sublimation temperature of diamond. (as well as the high melting and boiling points of these solids in general). This is quite understandable since boiling, melting and or sublimation, requires the rupturing of covalent bonds and very high temperatures are required to do this.
The structure of graphite consists of layers. The layers are parallel to one another and there is a relatively wide separation between adjacent layers.
At least three of the four valence electrons of each carbon atom are used in forming bonds with the three adjacent carbon atoms in the layer. The fourth valency electron of each carbon atom is not located by bond formation. The electrical conductivity of graphite can be explained on the basis of the mobility of these electrons. The separate layers are too widely spaced to be bonded together by covalent bonds and are probably bound together by weak Van der Waals forces which explains why graphite is so soft and flaky.
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