The diamond form has a close-packed crystal structure that gives it its
property of extreme hardness. In it each carbon is bonded to four other carbons in
a tetrahedron arrangement like this: These covalent solids form crystals that can
be viewed as a single giant molecule made up of an almost endless number of
covalent bonds. Because all of the bonds in this structure are equally strong,
covalent solids are often very hard, and they are notoriously difficult to melt.
Diamond is the hardest natural substance. At atmospheric pressure, it melts at
3,550°C.
TIP
Diamond uses the sp^3 hybrid orbitals to explain its tetrahedron structure.It has been possible to make synthetic diamonds in machines that subject
carbon to extremely high pressures and temperatures. Most of these diamonds are
used for industrial purposes, such as dies.
The graphite form is made up of planes of hexagonal structures that are
weakly bonded to the planes above and below. This explains graphite’s slippery
feeling and makes it useful as a dry lubricant. Graphite is also mixed with clay to
make “lead” for lead pencils. Its structure can be seen below. Graphite also has
the property of being an electrical conductor.