edited by
chern ed cornpact/ Remember AE = h Is Correct Only Half of the Time Howard R. LBO Onondaga Community College Syracuse. NY 13215
The change in energy for an atom or molecule resulting from photon emission is frequently related to the energy of the photon produced via AE = hv. While this relation is convenient, i t should he kept in mind that i t is not algebraically correct. When emission of a photon occurs by an atom or molecule the energy of the final state is, of course, lower than the initial state. By convention, the delta of a process is the difference between the final and initial conditions. Consequently, A E for the atom or molecule is negative. Since Planck's constant and freauencv are uositive auantities, AE and hu are opposite in sigd. ~ n a i ~ sbfi sthe emission process iustifies the incoruoration of a negative sign in the equation ;elating the quantities, bringing i t in line a i t h convention.
AE is the energy change occurring within the atom or molecule, whereas the photon produced by the transition appears in the surroundings. The appearance of the photon in the surroundings increases the energy of the surroundings, AEsE.,, = hu. For any process the change in energy of the surroundings is equal but opposite in sign to that of the the relasystem under consideration, AE,,, = -AE,,.'Thus, tion becomes hu = -AE,,, or simply hu = -AE. For an absorption process the opposite occurs. The energy of the surroundings is decreased while the energy of the system is increased by the photon, USE,,, = hu. Thus, the equation conforms with convention without the incorporation of a negative sign.
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A single atom or molecule cannot be treated thermodynamically. The terms system and surroundings are appropriate, however, since the atom or molecule is the focus of the analysis and it is separated fromthe rest of the world by imaginary boundaries.
Volume 65 Number 1 January 1988
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