Predicting the color of a compound can be extremely complicated. An example with vanadium(III) VCl 3 has a distinctive reddish hue, whilst V 2O 3 appears black. It is important to note, however, that elemental colors will vary depending on what they are complexed with, often as well as their chemical state. Charge-transfer complexes tend to have very intense colors for different reasons.Įxamples Colors of metallic ions Lycopene is a classic example of a compound with extensive conjugation (11 conjugated double bonds), giving rise to an intense red color (lycopene is responsible for the color of tomatoes).
Similarly, color is due to the energy absorbed by the compound, when an electron transitions from the HOMO to the LUMO. Organic compounds tend to be colored when there is extensive conjugation, causing the energy gap between the HOMO and LUMO to decrease, bringing the absorption band from the UV to the visible region. (see Transition metal#Colored compounds). Transition metal compounds are often colored because of transitions of electrons between d-orbitals of different energy. sodium chloride) and organic compounds (e.g.
The vast majority of simple inorganic (e.g. This can only be used as a very rough guide, for instance if a narrow range of wavelengths within the band 647-700 is absorbed, then the blue and green receptors will be fully stimulated, making cyan, and the red receptor will be partially stimulated, diluting the cyan to a greyish hue. This utilizes the scientific CMY and RGB color wheels rather than the traditional RYB color wheel. īelow is a rough table of wavelengths, colors and complementary colors. For example, beta-carotene has maximum absorption at 454 nm (blue light), consequently what visible light remains appears orange. If photons of a particular wavelength are absorbed by matter, then when we observe light reflected from or transmitted through that matter, what we see is the complementary color, made up of the other visible wavelengths remaining. The relationships between the energies of the various quantum states are treated by atomic orbital, molecular orbital, Ligand Field Theory and Crystal Field Theory. Where E is the energy of the quantum ( photon), f is the frequency of the light wave, h is Planck's constant, λ is the wavelength and c is the speed of light. The relationship between energy and wavelength is determined by the Planck-Einstein relation E = h f = h c λ E=hf=\,\! The perception of light is governed by three types of color receptors in the eye, which are sensitive to different ranges of wavelength within this band. However the release of energy visible to the human eye, commonly referred to as visible light, spans the wavelengths approximately 380 nm to 760 nm, depending on the individual, and photons in this range usually accompany a change in atomic or molecular orbital quantum state. There are various types of quantum state, including, for example, the rotational and vibrational states of a molecule. The amount of energy absorbed or released is the difference between the energies of the two quantum states. Theory The UV-vis spectrum for a compound that appears orange in DimethylformamideĪll atoms and molecules are capable of absorbing and releasing energy in the form of photons, accompanied by a change of quantum state. The study of chemical structure by means of energy absorption and release is generally referred to as spectroscopy. This spectral perspective was first noted in atomic spectroscopy. What is seen by the eye is not the color absorbed, but the complementary color from the removal of the absorbed wavelengths. The color of chemicals is a physical property of chemicals that in most cases comes from the excitation of electrons due to an absorption of energy performed by the chemical.
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