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21: Spectra and Structure of Atoms and Molecules

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    In the following sections we are going to study the way in which matter can both absorb energy and emit it in the form of electromagnetic radiation such as light. The pattern in which matter absorbs or emits radiation is called its spectrum. In the past, and still to this day, studies of the spectrum of a substance have furnished important clues to the structure of matter. At the same time, the spectrum of a substance is often a very useful way of characterizing and hence identifying and analyzing that substance.

    • 21.1: Prelude to Spectroscopy
      Absorption of an appropriate quantity of energy can raise the hydrogen atom from a lower to a higher energy level, while emission of electromagnetic radiation corresponds to a change from a higher to a lower energy level. Although Bohr’s theory is quantitatively accurate only for hydrogen, his idea of energy levels is useful for all other atoms and even for molecules.
    • 21.2: The Nature of Electromagnetic Radiation
    • 21.3: Atomic Spectra and the Bohr Theory
    • 21.4: Bohr Theory of the Atom
      In a classic paper published in 1913, the young Niels Bohr, then working with Rutherford in Manchester, England, proceeded to show how Rydberg’s formula could be explained in terms of a very simple model of the hydrogen atom. The model was based on the nuclear view of atomic structure which had just been proposed by Rutherford.
    • 21.5: The Spectra of Molecules- Infrared
      When we turn from the spectra of atoms to those of molecules, we find that the region of most interest to chemists is no longer the visible and ultraviolet but rather the infrared. As its name implies, the infrared extends beyond the red end of the visible spectrum, from the limit of visibility at roughly 0.8 μm (800 nm) up to about 100 μm where the microwave region begins.
    • 21.6: The Visible and Ultraviolet Spectra of Molecules- Molecular Orbitals
      When molecules absorb or emit radiation in the ultraviolet and visible regions of the spectrum, this almost always corresponds to the transition of an electron from a low-energy to a high-energy orbital, or vice versa.
    • 21.7: Molecular Orbitals
      In order to explain both the ground state and the excited state involved in an absorption band in the ultraviolet and visible spectra of molecules, it is necessary to look at the electronic structure of molecules in somewhat different terms. We need to look upon electrons in a molecule as occupying orbitals which belong to the molecule as a whole. Such orbitals are called molecular orbitals, and this way of looking at molecules is referred to as molecular-orbital (abbreviated MO) theory.
    • 21.8: Delocalized Electrons
    • 21.9: Conjugated Systems
      In some molecules the delocalization of electron pairs can be very much more extensive than in ozone and benzene. This is particularly true of carbon compounds containing conjugated chains.

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