atomic spectra

(noun)

emission or absorption lines formed when an electron makes a transition from one energy level of an atom to another

Related Terms

  • α-particle
  • nucleus

Examples of atomic spectra in the following topics:

  • Hydrogen Spectra

    • For decades, many questions had been asked about atomic characteristics.
    • From their sizes to their spectra, much was known about atoms, but little had been explained in terms of the laws of physics.
    • (It was a running joke that any theory of atomic and molecular spectra could be destroyed by throwing a book of data at it, so complex were the spectra.)
    • In some cases, it had been possible to devise formulas that described the emission spectra.
    • As you might expect, the simplest atom—hydrogen, with its single electron—has a relatively simple spectrum.
  • The Bohr Model of the Atom

    • In 1913, after returning to Copenhagen, he began publishing his theory of the simplest atom, hydrogen, based on the planetary model of the atom.
    • From their sizes to their spectra, much was known about atoms, but little had been explained in terms of the laws of physics.
    • This atom model is disastrous, because it predicts that all atoms are unstable.
    • Therefore, his atomic model is called a semiclassical model.
    • Niels Bohr, Danish physicist, used the planetary model of the atom to explain the atomic spectrum and size of the hydrogen atom.
  • High Resolution Spectra

    • In assigning mass values to atoms and molecules, we have assumed integral values for isotopic masses.
    • Because the strong nuclear forces that bind the components of an atomic nucleus together vary, the actual mass of a given isotope deviates from its nominal integer by a small but characteristic amount (remember E = mc2).
  • Spin-Spin Splitting

    • This is a common feature in the spectra of compounds having different sets of hydrogen atoms bonded to adjacent carbon atoms.
    • The signal splitting in proton spectra is usually small, ranging from fractions of a Hz to as much as 18 Hz, and is designated as J (referred to as the coupling constant).
    • The splitting patterns found in various spectra are easily recognized, provided the chemical shifts of the different sets of hydrogen that generate the signals differ by two or more ppm.
    • Longer-range coupling may be observed in molecules having rigid configurations of atoms.
    • Test your ability to interpret 1H nmr spectra by analyzing the seven examples presented below.
  • Emission Spectrum of the Hydrogen Atom

    • The emission spectrum of atomic hydrogen is divided into a number of spectral series.
    • Similarly, the emission spectra of molecules can be used in chemical analysis of substances.
    • All observed spectral lines are due to electrons moving between energy levels in the atom.
    • You need to understand convergence, production of UV, vis, IR, excitation, concentric energy levels and be able to draw the line spectra.
    • The various series are named for the atomic energy level they end on (n1).
  • X-Ray Spectra: Origins, Diffraction by Crystals, and Importance

    • X-ray shows its wave nature when radiated upon atomic/molecular structures and can be used to study them.
    • However, since atoms and atomic structures have a typical size on the order of 0.1 nm, x-ray shows its wave nature with them.
    • When x-ray are incident on an atom, they make the electronic cloud move as an electromagnetic wave.
    • This is called Rayleigh Scattering, which you should remember from a previous atom.
    • Not only do x-rays confirm the size and shape of atoms, they also give information on the atomic arrangements in materials.
  • Carbon NMR Spectroscopy

    • The following diagram depicts three pairs of isomers (A & B) which display similar proton nmr spectra.
    • These difficulties would be largely resolved if the carbon atoms of a molecule could be probed by nmr in the same fashion as the hydrogen atoms.
    • Unlike proton nmr spectroscopy, the relative strength of carbon nmr signals are not normally proportional to the number of atoms generating each one.
    • The isomeric pairs previously cited as giving very similar proton nmr spectra are now seen to be distinguished by carbon nmr.
    • The C8H10 isomers in the center (red) box have pairs of homotopic carbons and hydrogens, so symmetry should simplify their nmr spectra.
  • Characteristics of Mass Spectra

    • The following diagram displays the mass spectra of three simple gaseous compounds, carbon dioxide, propane and cyclopropane.
    • The molecular ion is the strongest ion in the spectra of CO2 and C3H6, and it is moderately strong in propane.
    • The unit mass resolution is readily apparent in these spectra (note the separation of ions having m/z=39, 40, 41 and 42 in the cyclopropane spectrum).
    • Even though these compounds are very similar in size, it is a simple matter to identify them from their individual mass spectra.
    • Since a molecule of carbon dioxide is composed of only three atoms, its mass spectrum is very simple.
  • Introduction

    • Photon energies associated with this part of the infrared (from 1 to 15 kcal/mole) are not large enough to excite electrons, but may induce vibrational excitation of covalently bonded atoms and groups.
    • We must now recognize that, in addition to the facile rotation of groups about single bonds, molecules experience a wide variety of vibrational motions, characteristic of their component atoms.
    • The complexity of this spectrum is typical of most infrared spectra, and illustrates their use in identifying substances.
    • The inverted display of absorption, compared with UV-Visible spectra, is characteristic.
    • Infrared spectra may be obtained from samples in all phases (liquid, solid and gaseous).
  • Isotopes

    • Since molecules of bromine have only two atoms, the spectrum on the left will come as a surprise if a single atomic mass of 80 amu is assumed for Br.
    • Thus, the bromine molecule may be composed of two 79Br atoms (mass 158 amu), two 81Br atoms (mass 162 amu) or the more probable combination of 79Br-81Br (mass 160 amu).
    • The center and right hand spectra show that chlorine is also composed of two isotopes, the more abundant having a mass of 35 amu, and the minor isotope a mass 37 amu.
    • Loss of a chlorine atom gives two isotopic fragment ions at m/z=49 & 51amu, clearly incorporating a single chlorine atom.
    • It should be noted that the presence of halogen atoms in a molecule or fragment ion does not change the odd-even mass rules given above.
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