A (Very) Brief History of the Atom

It is important to understand a little of the history of how we got to where we are. We could start with Democritus who postulated that all matter was made up of small particles called atoms. We've already discussed J.J. Thomson (go back and look if you don't remember). Thomson was in favor of a model of the atom called the plum pudding model. That is to say, the atom was a homogenous mixture of positive charges with electrons
scattered around through it. This was a fairly well accepted model until1911 when Ernest Rutherford was playing around with some alpha particles and some gold foil. Rutherford's Scattering Experiment consisted of a radium alpha source (a particle) and a very thin foil of gold, with a viewing screen around the apparatus that would detect alphas striking on it. He predicted that the
a particle, with a strong positive charge, would not be deflected significantly. From the plum pudding model, the electrons were not big enough to have an effect on the a particles, and the homogenous mixture of positive charge shouldn't cause any abnormal deflection. What he found was, while most of the alphas came through as predicted, some were scattered back to the back edge of the screen as if they had hit something and rebounded. For Rutherford, this was like shooting a cannonball at a sheet of tissue paper and having it rebound back. From these experiments, Rutherford proposed another model of the atom where the atom was composed of a dense, positively charged nucleus ion the order of 10-15 meters in diameter, surrounded by electrons in orbit. The size of the atom was known to be about 10-10 meters in diameter, so this led to the conclusion that the atom was mostly empty space. This was called the planetary or nuclear model of the atom.

Niels Bohr had been studying in Rutherford's lab and added to the model by combining the work of Rutherford with that of Planck and Einstein. Bohr proposed that the electrons circling the nucleus existed in specific orbits, and would jump from one energy level to another. Classical mechanics would predict:

  • Electrons could exist in any orbit.
  • Electrons would would radiate energy as they circled around the nucleus due to acceleration on the electrons.
  • Eventually the electrons would spiral into the nucleus.
  • Nothing in the model explained the emission spectra.

None of this was happening, so he proposed that the electron did not behave in accordance with classical models. What he ended up with was an electron that existed in orbits called stationary states". When an electron is in one of these stationary states, it does not emit any electromagnetic radiation. When an electron moved from one orbit to another, it either emits or absorbed light. It emits a photon when it changes from a higher state to a lower state, and it absorbs a photon when it goes from a lower state to a higher energy level. Further, the electron doesn't exist between levels. These were called quantum states or quantum conditions. The lowest energy state that the electron is in is called its ground state. The energy required to move an electron from the ground state is called the binding energy or ionization energy.

As an electron moves from a high energy state (Eh) to a lower energy state (Ef), it gives off a photon. The photon has energy E = hf, so we can say that the change in energy of the electron is related to the frequency of the photon given off, or

Eh - Ef = hf

We see this photon in emission spectra - frequency lines given off when electrons move to excited states. Bohr's atom also explains absorption spectra - the dark bands caused when a photon of exactly the right frequency has the energy required to move an electron from a lower energy state to a higher energy state.

There are some limitations to Bohr's model, the most important being that the model is only good for atoms with 1 electron (H, He+, Li2+) in the outer shell. This leads us to the Electron Cloud Model which we'll discuss at a later point.


Try these additional links:

Rutherford Scattering http://hyperphysics.phy-astr.gsu.edu/hbase/rutsca.html


For more one Bohr's Atom, try http://www.aip.org/history/electron/jjhome.htm
Finally, for a historical overview of Quantum Mechanics, try:

For Practice Problems, Try: Giancoli Multiple Choice Practice Questions (Don't worry if you can't solve all of them just yet!)