The conventional picture of metallic bonding is of a crystalline array of positive metal ions immersed in a sea of delocalized valence electrons, which act as a kind of glue to hold the metal together. High electrical conductivity is also well explained by a band theory model. When an electron absorbs energy from an electric field, it is excited into an unfilled molecular orbital, and this totally delocalized orbital provides a kind of electronic highway through the solid that allows charge to be transported. Finally, the characteristic shininess of metals can also be understood in band theory terms. Figure 2 illustrates the kind of electronic transitions that can occur in a metal having a band only partially filled with electrons.
Absorption can take place over the entire visible spectrum.
The vertical arrows in Figure 2 represent transitions that take place when photons 1 of energy, h n , collide with the metal, transferring energy to one of the electrons, which is then promoted to an orbital of higher energy. The transitions shown in this figure range from those of low energy, characteristic of red and IR photons, to those of much higher energy, characteristic of blue and UV photons. 1 See pages 291 - 294 of the text for more information on the properties of photons.