The atoms of metallic elements have an outer layer of electrons that give metals many of their characteristic properties, including malleability, electrical conductivity and luster. Unlike the electrons in non-metallic substances, metallic solids and liquids share the outer electrons freely. When light shines on a lustrous metal, the loosely bound electrons reflect incoming light, giving the metal a shiny appearance.
Sea of Electrons
Although the atoms of most metals organize themselves into crystalline patterns as solids, their outer or valence electrons do not participate in the orderly structure; they move freely throughout the substance in what scientists call a “sea of electrons.” The valence electrons in non-metallic elements move about and participate in chemical reactions, but not to the extent that electrons in metals do. For example, in carbon, a non-metal, the valence electrons form strong bonds but do not have the freedom of those in a metal such as copper. As diamond, carbon can be transparent when light arrives at right angles to the surface, and reflective at other angles due to the smooth flatness of the crystal. The highly mobile electrons of metals reflect light back to the source and are not transparent to visible light.
Atoms, Electrons and Light
Objects have optical properties such as transparency, reflectivity and color due to the organization of atoms and electrons in the substance. Light consists of intertwined waves of electricity and magnetism; the way the electric waves interact with the atoms and electrons in a substance determines its optical behavior. If incoming light waves produce vibrations that each layer of atoms pass along to the next, the substance is at least partly transparent. In some instances, the electrons respond only slightly; transparency results from this effect also. If the electrons absorb light energy, the object is opaque. The free electrons in metals vibrate easily in response to light; the vibrating electrons, in turn, produce light of their own, resulting in a reflection or lustrous shimmer.
Some pure metals such as lead have high luster when initially cut or polished, but over time they take on a dull appearance. The electrons in these metals have attracted oxygen, carbon and other substances in the air, forming a layer of tarnishing compounds. The electrons in the compounds are not as free to move as in the pure metal. You can restore the luster of metal objects by removing the outermost layers through polishing or chemical treatments. Gold, platinum and other so-called “noble metals” have very low reactivity with air; they remain untarnished for very long periods of time.
Other Electromagnetic Frequencies
Luster is an effect of visible light -- a very narrow part of the electromagnetic spectrum, which in its entirety spans a range from low-frequency radio waves to high-energy gamma rays. Metals exhibit most of the properties at the frequencies for radio waves as they do for light; the same applies to infrared and ultraviolet light; unlike many minerals and compounds, however, most pure metals do not fluoresce under UV. Much higher frequencies produce X-rays, which the valence electrons in metals scatter or absorb. Gamma rays, having greater energy than X-rays, penetrate through metal depending on its density; it takes an inch or more of lead to absorb significant amounts of gamma rays; lighter metals such as aluminum and iron will also block gamma rays, although proportionally greater thicknesses are required to equal the shielding effect that lead provides.
- University of California, Davis: Metallic Bonding
- Purdue University: Valence Electrons
- Georgia State University: Physical Connections to Electric Permittivity and Magnetic Permeability
- Environmental Protection Agency: Radiation Shielding
- Georgia State University: The Interaction of Radiation with Matter
- National Park Service: The Use Of Ultraviolet Fluorescence In The Examination Of Museum Objects
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