Editing Metallic bonding (section)

==Optical properties==
The presence of an ocean of mobile charge carriers has profound effects on the [[optical properties]] of metals, which can only be understood by considering the electrons as a ''collective'', rather than considering the states of individual electrons involved in more conventional covalent bonds.

[[Light]] consists of a combination of an electrical and a magnetic field. The electrical field is usually able to excite an elastic response from the electrons involved in the metallic bonding. The result is that photons cannot penetrate very far into the metal and are typically reflected, although some may also be absorbed. This holds equally for all photons in the visible spectrum, which is why metals are often silvery white or grayish with the characteristic specular reflection of metallic [[lustre (mineralogy)|lustre]]. The balance between reflection and absorption determines how white or how gray a metal is, although surface tarnish can obscure the lustre. Silver, a metal with high conductivity, is one of the whitest.

Notable exceptions are reddish copper and yellowish gold. The reason for their color is that there is an upper limit to the frequency of the light that metallic electrons can readily respond to: the [[plasmon frequency]]. At the plasmon frequency, the frequency-dependent dielectric function of the [[Free electron model#Dielectric function of the electron gas|free electron gas]] goes from negative (reflecting) to positive (transmitting); higher frequency photons are not reflected at the surface, and do not contribute to the color of the metal. There are some materials, such as [[indium tin oxide]] (ITO), that are metallic conductors (actually [[degenerate semiconductor]]s) for which this threshold is in the [[infrared]],<ref>{{cite journal|doi=10.1021/jp026600x|title=Indium Tin Oxide Plasma Frequency Dependence on Sheet Resistance and Surface Adlayers Determined by Reflectance FTIR Spectroscopy|year=2002|last1=Brewer|first1=Scott H.|last2=Franzen|first2=Stefan|journal=The Journal of Physical Chemistry B|volume=106|issue=50|pages=12986–12992}}</ref> which is why they are transparent in the visible, but good reflectors in the infrared.

For [[silver]] the limiting frequency is in the far ultraviolet, but for copper and gold it is closer to the visible. This explains the colors of these two metals. At the surface of a metal, resonance effects known as [[Surface plasmon resonance|surface plasmons]] can result. They are collective oscillations of the conduction electrons, like a ripple in the electronic ocean. However, even if photons have enough energy, they usually do not have enough [[momentum]] to set the ripple in motion. Therefore, plasmons are hard to excite on a bulk metal. This is why gold and copper look like lustrous metals albeit with a dash of color. However, in [[colloidal gold]] the metallic bonding is confined to a tiny metallic particle, which prevents the oscillation wave of the plasmon from 'running away'. The momentum selection rule is therefore broken, and the plasmon resonance causes an extremely intense absorption in the green, with a resulting purple-red color. Such colors are orders of magnitude more intense than ordinary absorptions seen in dyes and the like, which involve individual electrons and their energy states.