Spectra Lines are formed when light  is focused through a spectrum analyzer after being passed through a slit.  Spectra and Absorption Lines for the elements:  hydrogen (H), iron (Fe), calcium (Ca), nickel (Ni), magnesium (Mg), and Chromium (Cr) can be seen in figure 1.1.7. No two elements produce the same spectral pattern, much like a fingerprint on a person is unique to that person.  Notice how the black absorption lines (when an atom gains energy) occur at the same wavelengths as the emission lines (when an atom loses energy).  According to the Rutherford Model (classical physics), an electron would lose energy and spiral into the nucleus as it continuosly radiates its energy.  
 

The signature of each atom is contained within its spectral graph or plot.  By placing an element within its gaseous state and using the spectrum analyzer one can easily determine what element is present. Since the Rutherford Model was unable to resolve the observations of spectral lines, and called for the annihilation of the electron as it spiraled into the nucleus, it had to be reassessed.

The Danish Physicist, Niels Bohr brought us closer to our current understanding of the atom. Bohr studied first with Thomson but the two didn't get along well. Later, he met with Rutherford, a man after whom he emulated his scientific life.   While studying the hydrogen atom, Bohr broke with classical physics when he stated that:

    There are stationary states of energy for atoms
    There is a frequency condition for energy transfers in atoms.
    The electron's path is circular about the nucleus  under the coulomb force taking the
          form:

  

       where: q₁ = +1.60 x 10^-19 Coulombs and
                      q₂ =  -1.60 x 10^-19 Coulombs.

These ideas, revolutionary at the time, set forth the path for our present understanding of quantum electrodynamics QED.  The first statement, there are stationary states of energy for atoms means that an electron orbiting about its nucleus (an atom of hydrogen), can exist without radiating any energy.  As stated before, according to the classical model this would be impossible because the hydrogen electron, as it revolves around the nucleus, would be constantly radiating energy which would degrade its orbital status with each successive rotation and radiation, hence colliding into the nucleus over time according  to the equations: