The Bohr Model

While the Rutherford model focused on describing the nucleus, Niels Bohr turned his attention to describing the electron.  Prior to the Bohr Model, the accepted model was one which depicted the electron as an orbiting planet.  The flaw with the planet-like model is that an electron particle moving in a circular path would be accelerating (see circular motion).  An accelerating electron creates a changing magnetic field. This changing magnetic field would carry energy away from the electron, eventually slowing it down and allowing it to be "captured" by the nucleus.

absorption spectrum (4k)
Absorption Spectrum

Emission spectrum  (3k)
Emission Spectrum

Bohr built upon spectroscopic observations of atoms. Spectroscopists noticed that an atom can only absorb certain energies (colors) of light (the absorption spectrum) and once excited can only release certain energies (the emission spectrum) and these energies happen to be the same.  Bohr used these observations to argue that the energy of a bound electron is "quantized."  Quantized is a fancy word meaning only certain quantities of energy are allowed.  This explanation addresses the true origin of light.   Since only certain energy levels are allowed it is actually possible to diagram the atom in terms of its energy levels.  In the animation below you will see a model of a Hydrogen atom and to the right of it, a Bohr energy level diagram.

Bohr atom & energy levels (25k)
The Hydrogen Atom

In the animation you will notice that if the energy of the photon of light is just right, it will cause the electron to jump to a higher level.  When the electron jumps back down, a photon is created for each jump down.  A photon without the right amount of energy (the pink one) passes through the atom with no effect.  Photons with too much energy will cause the electron to be ejected which ionizes the atom.  An ionized electron is said to be in the n=infinity energy level.  Keep in mind that these rings are not actually orbits, but are levels that represent the location of an electron wave.   The number n corresponds to the number of complete waves in the electron.

Ephoton = Einitial - Efinal

This formula can be used to determine the energy of the photon emitted (+) or absorbed(-).

Ephoton = hf

where h = 6.6 x 10-34 Js
or 4.14 x 10-15 eVs

This formula can be used to determine the energy of a photon if you know the frequency of it.  Planck's constant, h, can be used in terms of Joule(s) or eV(s).
(note: the Regents reference table only gives it in terms of  Js)




1999 Science Joy Wagon