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Read " Spectroscopy "on pages 771 to 773 of your physics textbook.

At the beginning of the 20th century a model was proposed that finally began to answer some of the questions of atomic structure and spectra.  In 1913 Niels Bohr proposed a model of atomic structure using hydrogen as the example model.  Bohr's model not only described the structure of the atom, but it also explained atomic spectra and correctly predicted the existence of more atomic lines.  Bohr's model seemed to be everything that physicists were looking for.  But there was one problem - Bohr's model stepped outside the realm of classical physics and ventured into the newly emerging world of quantum physics.  This left many scientists skeptical of the model.  Nonetheless, Bohr's model was far superior to any previous model and was accepted as a semi-classical model of the atom.


Spectroscopy: the study of the light emitted and absorbed by different materials


Bohr started with a planetary model of the atom.  However, to avoid the problems that confounded Rutherford, Bohr made several assumptions, including the following:

  • Electrons orbit the nucleus.  They are held in orbit by an electrostatic force.
  • Electrons can only be in certain, permitted orbits and an electron does not emit radiation when it is in one of these orbits.  In these allowed orbits, the energy of the electron is constant.  These orbits are called stationary states because the electron's energy is constant.  In other words, the energy of the electron is quantized - it can only have certain values thus the allowed orbits can be referred to as energy states.
  • An electron only emits radiation when it "falls" from a higher energy state to a lower state.  The change in energy of the electron (from the higher state to the lower state) is equal to the energy of the emitted photon, thereby obeying the conservation of energy principle.  Similarly, an electron only absorbs radiation when it "jumps" to a higher energy level.  Again, the change in energy of the electron is now equal to the energy of the absorbed photon.
  • The radii of the allowed orbits are also quantized because each energy state has a specific radius.


Stationary state: a stable state with a fixed energy level



Energy level: a discrete and quantized amount of energy


Bohr's model was allowed to have stationary states because of de Broglie's work on the wavelength of matter. The electron must have a certain speed for the Finward and Felectric to be equal.  For this to occur, the electron has a specific wavelength which is equal to the circumference of the stationary state.  The circumference of the second stationary state is equal to twice the electron's wavelength and so on.  This agrees with quantum theory.  Refer to "Figure 15.24" on page 782 of the textbook.

By applying his assumptions, Bohr was able to develop expressions for the allowed energy levels and the allowed radii for the hydrogen atom.  Using these expressions, Bohr calculated all the allowed electron energy levels for hydrogen.

An energy level diagram, like the one shown here, often illustrates energy levels.  An energy level diagram displays several things, such as:

  • The energy levels of hydrogen are not evenly spaced.  As an electron moves to higher and higher levels, the difference in energy between the levels becomes smaller and smaller.
  • The energy of each level is reported as a negative number.  As an electron moves to a higher energy level, its energy increases (becomes less negative).
  • When an electron makes a transition, it moves from one energy level to another.  The spacing between energy levels represents the magnitude of the change in energy of the electron.  For example, an electron moving from n = 3 to n = 1 has a greater change in energy than an electron moving from n = 3 to n = 2