In
the simple, single-bonded hydrocarbons discussed previously, some
of the four tetrahedrally arranged atoms around each carbon are hydrogen
atoms and some are carbon atoms. One can imagine a three-dimensional
tetrahedral framework in which all of the atoms are carbons and no
hydrogens are present. The result would be the tetrahedral structure
shown on the right, with each carbon atom connected to four other
carbon atoms by single electron-pair bonds. This is the structure
of diamond. Diamond is very hard and rigid because any breaking off
or deforming of part of the diamond structure requires a breaking
or stretching of strong electron-pair bonds. In contrast, paraffin
wax has linear chains of carbon atoms, but only weak van der Waals
forces to hold the molecules together. If subjected to external stress,
the molecules slip past one another to new positions. Wax is soft
because the van der Waals forces are weak; diamond is hard because
the electronpair bonds in its three-dimensional network are strong.