Enduring Understanding 1.D: Experimental Data and Atomic Structure

  • The current model of the atom is based on quantum mechanics (QM) and Coulomb's Law.
  • QM predicts that electrons exist in regions of space called orbitals, and no more than two electrons can be in a single orbital. If two electrons are in an orbital, they must have opposite spin.
  • An early model of the atom (Dalton's model) predicted that all atoms of the same element must be identical.
  • However, experimental evidence obtained by Mass Spectrometry (MS) showed that this is not correct.
  • In MS, samples of atoms or molecules are vaporized and ionized in a magnetic field. The gaseous ion curves through the magnetic field, and the degree of the curvature gives information about the charge and the mass of the ion.
  • Example: Mass spectrum of Bromine, Br2:
  • Isotopes have the same number of protons but different number of neutrons. Every element has a characteristic relative abundance of its isotopes.
  • The graphic above shows the mass spectrum of bromine gas, Br2. Natural bromine consists of two isotopes of bromine, at a nearly equal abundance, with atomic masses of 79 and 81. Molecular bromine (Br2) can therefore be composed (25% probability) of two atoms of 79Br and have a mass of 158, one atom of 79Br and one of 81Br (50% probability) with a mass of 160, or two atoms of 81Br (25% probability) with a mass of 162. The MS above shows the signals for the three peaks corresponding to the three isotopic compositions of Br2, and also the peaks from fragmentation to a bromine cation at 79 and 81. The average atomic mass of bromine is 79.9, which is the weighted average of the masses of the two isotopes.
  • The structure of atoms and molecules can be probed by examining light energy (photons) that is absorbed or emitted by the atom or molecule. This is called spectroscopy.
  • Photons of light have different energies based on their frequency, according to Planck's equation: E=hv.
  • Absorption and emission of different wavelengths results from different kinds of molecular motion:
  • Infrared photons represent changes in molecular vibrations. This can be useful for the detection of organic function groups, like alcohols (-OH) and ketones (C = O)
  • Visible and ultraviolet photons represent transitions of valence electrons between energy levels.
  • X-rays can result in ejection of core electrons (see photoelectron spectroscopy)
  • Molecules absorb light to a degree proportional to their concentration. This means that the concentration of a molecule can be determined using Beer's Law: A = εbc, where A Is the absorbance, ε is the molar absorptivity of the molecule, b is the path length, and c is the concentration.
  • UV/V is spectroscopy is especially useful for measuring the concentration of colored species in solution.

  • Example. Gas A absorbs light at 440 nm and is orange in color. Gas B does not absorb at 440 nm and is colorless. Which of the following can we conclude about A and B? A has more vibrational modes than B, A has a lower first ionization energy than B, or A has lower energy electron transitions than B?
  • We can conclude that A has lower energy electron transitions than B. Visible light spectroscopy involves electron energy level transitions, not vibrations (infrared spectroscopy) or ionizations (photoelectron spectroscopy).

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