Physics 2000 Science Trek Quantum Atom

Bohr's Atom

To explain the spectral line puzzle, Bohr came up with a radical model of the atom which had electrons orbiting around a nucleus.

That doesn't sound so radical. We've already seen how electrons can orbit around a positively charged nucleus.

Yes, but in order to explain the "signature colors," Bohr came up with an extraordinary rule the electrons had to follow: Electrons can only be in "special" orbits. All other orbits just were not possible. They could "jump" between these special orbits, however, and when they jumped they would wiggle a little bit...

And that would cause radiation!

To see this happening, try clicking on different orbits in the model of an atom below.

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Hey, when I click on a smaller orbit, a little colored squiggle goes shooting out, but when I click on a bigger orbit, a squiggle comes in and kind of "bumps" the electron up.

Those squiggles are little bursts of light (electromagnetic energy). We call them photons.

But when we played with the orbits earlier, we saw that just about any orbit and any speed is possible. It doesn't make sense that only some orbits would be "allowed."

Now you can see why the Bohr model was considered so radical! It said that energy could only change in little jumps. These are called quanta and that's why this kind of physics is called Quantum Mechanics.

Is that where the term "quantum leap" comes from?

Yup. Ironically, everyday use of the term has come to mean a big jump, but physicists use it to mean a jump between allowable orbits, which is usually very, very tiny. The important part is that these jumps cannot be broken down into smaller steps. For an electron on the move it's all or nothing.



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