One
oxygen atom can satisfy all of its electron-sharing needs by using
both unpaired electrons in a double bond to another oxygen atom, as
shown in the figure on the right. Oxygen therefore occurs as a diatomic
gas, 0=0 or O2,
similar to N2
gas but totally unlike the infinite solid structures of carbon in
diamond or graphite. Solid boron and solid carbon are held together
by covalent bonds in a three-dimensional network, and melt only at
very high temperatures: 2037C for boron and 3500C for graphite. Molecules
of solid N2 and
O2 are
held together only by van der Waals forces, and therefore melt at
a low -210C and -219C, respectively.