 |
 |
4 of 18 |  |
 |
 |
M-4
It is important to remember that molecules and ions in solution do not absorb all wavelengths (or
energies) of light (photons) equally: only specific wavelengths/energies are absorbed. The
energy of photons corresponds to the intervals between the various allowed energy states for the
molecule or ion. A plot of absorption of the radiation versus wavelength is referred to as an
absorption spectrum.
Conversely, molecules in an excited state may lose a photon and revert to ground state
molecules. A plot of the emitted wavelength of the electromagnetic radiation (photon) versus
varying wavelengths is referred to as an emission spectrum.
The absorption of a photon increases the total energy of a molecule. This may include changes
in the electronic energy, vibrational energy and rotational energy of the molecule. For example,
absorption of radiation from the visible region of the electromagnetic spectrum gives rise to
electronic transitions in the molecule while absorption within the infrared region results in
vibrational and rotational transitions in the molecule. The overall absorption of energy by a
molecule in the visible region is manifested as rather broad bands, instead of sharply defined
absorptions, because both vibrational and rotational transitions are superimposed upon these
electronic transitions.
D.
Beer’s Law
The amount of light absorbed by a sample will depend on the number of ions or molecules in the
pathway of the light (photons). It follows that more light will be absorbed as the concentration
of the absorbing species increases. Similarly, the longer the light path followed by the photon
beam through the solution, the more photons will be absorbed. In addition, for a given
wavelength of light, each molecule or ion in solution has a certain probability of absorbing a
photon and attaining a short-lived excited state in the process.
These statements have been expressed in a mathematical form known as the Beer-Lambert Law
or simply Beer’s Law:
A =
bc
