Electrons orbit atomic nuclei of protons and neutrons in orbitals of very specific energies. If an orbital of lower energy, closer to the nucleus, becomes available, an outer, more energetic electron can lose energy as a burst of light, a photon, as it moves to the lower energy orbital. The movement of electrons to different orbitals within the same atom explains black-light (ultraviolet) illumination of fluorescent paint.
Electrons Can Move to Empty Orbitals: Absorbance and Fluorescence
Electrons can only move to empty orbitals, moving from higher (outer) to lower (inner) energy, unless extra energy is provided. Thus, if a photon of light increases the energy of an electron in an inner orbital it would have too much energy to remain in that location and may move to an empty outer orbital of the appropriate higher energy, i.e. light absorption. This would cause a cascade of electrons from higher energy orbitals releasing energy and moving to vacated lower energy orbitals. The released energy would be in the form of lower energy photons. This is a description of fluorescence in which ultraviolet light (short wavelength, high energy) is used to excite an appropriate atom or molecule, and emitted light from the downward cascading electrons is in the visible range (longer wavelength, lower energy.)
Absorb Ultraviolet and Emit Green or Red
A rainbow displays white light separated into its colored photons of different energies. The blues are the highest energy (short wavelength) and the reds are the lowest energy (long wavelength) photons of visible light. Photo pigments in the cones of the eye can absorb photons of restricted energies and permit detection of light of particular colors. Small molecules or atoms have large, high energy gaps between orbitals and can only absorb very energetic photons, i.e. ultraviolet light. Molecules with smaller energy gaps between their outer orbitals can absorb lower energy, redder, photons. Thus, an outdoor sign with letters of different colors will reflect the photons of the colors that are seen, but absorb the other photons. Blue letters absorb low energy red photons and red letters absorb high energy blue photons. Since electrons absorbing high amounts of energy are also more chemically reactive, the red pigments tend to fade most readily in sunlight.
Fluorescent Light Tubes Convert Ultraviolet Mercury Emissions into White Light
Heat and electrical conduction through gases can be used in light bulbs and fluorescent tubes to excite the electrons of atoms and emit photons of light as the electrons return to lower, emptied orbitals. The heated atoms literally crash together and convert their kinetic energy into electrons of higher energy and then into released photons of light. In the fluorescent tube, excited mercury atoms quickly release their energy as ultraviolet photons that are absorbed by the fluorescent pigment lining the inside of the glass tubes. The electrons of the excited pigment move to outer orbitals, but they have so much energy that they skip several orbitals and then cascade down giving off photons of several different energies in the visible range. The complexity of the photons of many different energies produces white light.
Looking for Uranium and Finding Scorpions
Larger atoms and molecules have orbitals that are tightly stacked and lower energies separates them. Thus, large atoms, such as copper can produce colorful blue salts, because their valence electrons can absorb the low energy photons of red light. Some of the large molecules or atoms can also fluoresce. Thus, uranium prospectors use ultraviolet lights to search for fluorescent uranium ore in Nevada. Those prospectors are frequently surprised to find vividly fluorescing scorpions. It turns out that in the bright Nevada sun, it is useful to absorb and reemit ultraviolet radiation using fluorescent organic molecules.
It Is All about Orbital Energies
The energy of electrons determines the properties of atoms and molecules. For example, oxygen with even its outermost, valence electrons in low energy orbitals, relatively close to a high proton charge in the nucleus, are very low in energy compared to other biological atoms such as carbon and hydrogen. If the outer electrons of carbon and hydrogen are crashed into the lower energy orbitals left open on oxygen, they will be stripped away. This is oxidation and the energetic crashing together of the atoms and release of energy is what happens in a fire. In cellular metabolism, electrons move from higher to lower energy orbitals and the energy releases is stored as chemical energy in the phophodiester bonds of ATP.
Thus, fluorescence and absorbance reveal chemical characteristics that are useful in everyday life, and the controlled movement of electrons between orbitals is the essence of life itself.
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