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Unformatted text preview: 7-1Experiment 7 SPECTROSCOPY I. Learning Objectives… ♦To construct a simple but accurate spectroscope containing a built-in quantitative calibration system. ♦To use the spectroscope to compare various continuous emission sources. ♦To obtain the atomic line spectra of light emitted from discharge tubes. ♦To calculate the photon wavelengths, frequencies, and energies from the line spectra data II. Background Information Atoms, ions, and molecules contain electrons that occupy discrete energy levels. The actual energy of each state (level) is dependent upon several factors: the nuclear charge, the distance of the electron from the nucleus, and the number of electrons between the nucleus and the electron in question. The transition of an electron from one level to another must be accompanied by the emissionor absorptionof a discrete amount of energy. The magnitude of this energy depends on the energy of each of the levels between which the transition occurs. Figure 7.1. Energy Level Transitions In the instance illustrated above in Figure 3, the energy involved in the electron transition (ΔE) will equal the energy difference between E2and E1: ΔE = E2- E17-2For energy emission to occur, electrons must first be given energy from some external process – e.g., an electrical discharge, a combustion reaction, or a heated wire. The electrons are then said to move from the ground state to an excited state. The excited electrons then "relax" back to their original levels, and energy is emitted. The number and type of these transitions depend both on the particular structure of the energy levels in a given chemical species and on various quantum selection rules. These properties are unique to each individual species and give rise to an emission of energies that characterizes the species. If the value of ΔE lies within the visible region of the total electromagnetic spectrum, the frequency corresponds to visible light, and the emission can be seen by the eye. Since each electron can undergo many transitions and since many species have many electrons, emission spectra usually consist of a very large number of discrete frequencies. The wave theoryof radiation is particularly useful in providing models for interpreting the behavior of light emitted from atoms. Electromagnetic radiation is a form of energy consisting of oscillating electric and magnetic fields that move the direction of propagation at the speed of light. The wave motion, which is illustrated in Figure 2, is described in terms of some fundamental properties such as amplitude A, wavelength λ, and frequency ν. Figure 7.2. A Wave The frequency is defined as the number of waves (of wavelength λ)passing a point per second. The relationship that links λand νis given below where c is the speed of light (3.00 x 108m/s)....
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This note was uploaded on 04/05/2012 for the course CHEM 101L taught by Professor Austell during the Spring '08 term at UNC.
- Spring '08