What is the difference between an emission spectrum and an absorption spectrum? What is the Bohr model of the atom, and how does it explain both emission and absorption spectra?  

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ncchemist | eNotes Employee

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The Bohr model of the atom posits that there is a positively charged nucleus at the center of the atom.  This nucleus contains most of the mass of the atom.  Surrounding the nucleus are electrons.  They travel in distinct paths or orbits around the nucleus depending on their energy levels.  These energy levels are fixed amounts, or quantized.  In other words, the energy levels between electron orbitals are fixed.  This is what allows the Bohr model to explain these energy spectra.  They both essentiall measure the same thing just from different points of view.  When at atom is exposed to the exact wavelength of radiation that corresponds to the energy difference between electron orbitals, the electron moves into a higher energy excited state, and the absorption of that energy is recorded in the absorption spectrum.  When the electron eventually moves back down to the lower energy state, the atom gives off a specific wavelength of energy corresponding to the transition.  This is recorded in the emission spectrum.  So both types of spectra measure the same energy gap, one from the point of view of gaining the energy and the other from the point of view of losing the energy.