Bless you, Bob Lee
--
oozing on the muggy shore of the gulf coast
l...@fbtc.net
--- Begin Message ---Morning wonderful CSers, Color in Complex Ions,"White" light consists of a continuum of energies, each associated with a particular wavelengh of electromagnetic radiation through the equation E = hc/lambda, where E is the energy of the radiation, h is Planck`s constant, c is the velocity of light (3 x 10+10 cm sec-1), and lambda is the wavelength. The visible region in the spectrum constitutes only a small fraction of the range of electromagnetic radiation. Many aqueous solutions of complex ions appear colored because of absorption by the ions of visible light in certain specific wavelength regions. The color observed is a combination of those not absorbed, that is in general, the complementary color of the absorbed color. The question remains as to why some complexes are colored and others are not. The key lies with an analysis of the electronic energy states of the species. Absorption of visible light requires an electronic transition between two energy states separated by an energy equal to that of the light involved. If available energy states are too widely separated, the higher-energy ultraviolet light will be absorbed. If they are too closly spaced, infrared light (lower energy) will be absorbed. Only in the case of energy-level spacings corresponding to the rather narrow spectral region of visible light will color be seen. Apparently the energy differences involved in electronic energy states of many transition metal complexes are appropriate for the absorption of visible light. The electronic environment of the central ion in a complex may be such that relatively small energy differences can exist between possible states for certain electrons (e.g., alternative ligand-ion orbitals or alternative energy levels for loosely held electrons of the metal ion). The energy associated with a specific wavelength of light may, then, induce an electronic transition from a lower to a higher energy condition. On spontaneous return to the lower energy state, an equivalent amount of energy will be lost, some as light of the original wavelength ( but scattered in all directions so that only a small percentage continues in the direction of the incident light beam) and some as other forms of energy, such as vibrational energy of the complex. Since complex ions in solution exist as families of structures of continually varing distortions from the idealized complex geometry, the electronic environments around central ions will also have minor variations. Thus light energy appropriate for electronic transitions will be a more or less narrow band of wavelengths rather than a single wavelength. Vibrational and rotational excitations, in addition to the electronic transitions, also contribute to the breadth of the absorption band. Different wavelength components of the absorption band are absorbed in different amounts (intensities), so that the wavelength at which maximum absorption occurs may be reported as a characteristic of the substance. Absorbed Observed ------------------------------ infrared (none) 7500A ------------------------------ red blue-green 6500 ------------------------------ orange blue 5900 ------------------------------ yellow indigo 5600 ------------------------------ yellow-green violet 5400 ------------------------------ green purple 5100 ----------------------------- blue-green red 4900 ------------------------------ blue orange 4600 ----------------------------- indigo yellow 4200 ----------------------------- violet yellow-green 4000 ------------------------------ ultraviolet (none) Why Absorption is Noticed If light energy causes electrons to be promoted from a lower to a higher energy state and these electrons spontaneously return to their more stable condition, why isn`t the light reemitted so that no net absorption occurs? Remember that covalent bonds in molecules are always undergoing motions described as bending, stretching ,and rotation. Some of the reemitted energy may appear as energy causing changes in these molecular motions; that is, some of the light energy may be converted to kinetic energy. Indeed, covalent bonds may actually be broken in rare cases. However, only some of the light energy is lost by such processes in most cases, and some other explanation must be invoked to account for additional absorption. The other reason for observed absorption is the scattering of the reemitted radiation. The original light beam is directional (toward the observer). When absorption occurs, followed by reemission, the light given off by the sample is scattered in all directions, so that only a small fraction continues on in the direction of the observer. Bless you Bob Lee -- oozing on the muggy shore of the gulf coast l...@fbtc.net -- The silver-list is a moderated forum for discussion of colloidal silver. 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