The
connection between delocalized electrons and metallic luster can
be illustrated in a simple way by looking at a series of linear
conjugated molecules of the type encountered in Chapter 9 and illustrated
opposite.
The maximum wavelength absorbed and the color seen in this series
of molecules are given in the table for molecules with chain lengths
from n = 1 to 8. As the length of the delocalized chain increases,
the absorption of radiation shifts from the ultraviolet into the
visible, the complement of the absorbed color is seen, and at n
= 7 or 8 a metallic luster begins to appear. Compounds of longer
molecules in this series are a shiny black, like graphite, because
the region of absorption is so broad that it encompasses virtually
the entire visible spectrum. Graphite, with its endless hexagonal-mesh
sheets, can be considered as the infinite extension of aromatic
molecules like benzene and naphthalene.