Neils Bohr and the Beginnings of Quantum Theory

Although Max Planck's work with the ultraviolet catastrophe is arguable the beginning of Quantum Physics, Neils Bohr is generally considered the father of that field.

This is because it was Bohr who was able to apply the “mathematical quirk” that Planck had found to the basic structure of matter and, in doing so, explain a number of things that were not understood. 

At the time that Bohr first proposed his theory there were several “unsolved mysteries” that we need to consider:

1. When things have a lot of energy, they give off light. This could be the glowing of embers in a fire, or the fire itself. It also could be the light coming out of a “normal” incandescent bulb or out of a fluorescent tube. It could even be the sparks given off when a piece of flint is hit with a piece of steel.

2. The light given off by an excited element is rarely, if ever, a single color of light, nor is it a smooth rainbow of colors. Instead, an excited element will give off a few (or occasionally a few dozen) very distinct, separate colors of light.

3. The work of Balmer, Paschen and Lymen found that the wavelengths of the light (visible, IR and UV) emitted by hydrogen could be found with a set of mathematical equations.

Bohr was able to synthesize an explanation that addressed all of these in a single, relatively simple idea. That idea was that:

The (Potential) Energy of Electrons is Quantized

Short? Yes. Simple? Ehhh…

Let’s pull apart what Bohr is saying:

1. The potential energy of electrons…  Because electrons are negative and the nucleus is positive they attract each other. When they are not in the same place, they have potential energy. The amount of potential energy depends on the amount of attraction and the distance. So, an electron that is further from the nucleus has more potential energy than one that is closer.

2. Energy is quantized… this means that all amounts of energy must be a multiple of some small basic amount. This is Bohr’s use of Planck’s idea.

Putting those ideas together creates a very distinct picture of the atom, that addresses and explains the three observations listed above.