Physics 2000 Science Trek Quantum Atom

Angular Momentum of an Electron

I think I can see how Bohr's idea of different energy levels sort of goes with Balmer's formula, but I don't understand how angular momentum fits in.

Bohr knew that a photon's energy was equal to Planck's constant times its frequency (this formula was discovered by Einstein during his work on the photoelectric effect). If the Bohr model was correct, he also knew that an emitted photon's energy was the same as the difference between the upper and lower energy levels involved. So he had a relationship between the energy levels and the frequencies of the photons...

But Balmer's formula specified the wavelength, not the frequency.

Ah, but don't forget that the two are related. The speed of a wave is equal to the product of its wavelength and its frequency, as I was telling Kyla earlier. A photon, or burst of electromagnetic radiation, travels at the speed of light, c.

Oh, so


Quite so--and since we know that


it follows that


from Balmer's formula. Now, we can write the energy levels in terms of the kinetic and potential energy of the electrons:


where m is the electron's mass, and v and r are its speed and orbital radius at the upper and lower levels.

I'm beginning to see where angular momentum could go into this equation. If the electron is in a circular orbit, then


which means that

right?

Absolutely. Thus you can now write everything in terms of r and L:


To find out what r is, we can apply Newton's second law, F=ma, to the electron. The force on the electron can be found using Coulomb's Law:


If the electron is in uniform circular motion, its acceleration is


so


Substituting the value for v you obtained in equation (6) and solving for r, we find that


With everything in terms of L, we get the rather nice equation


which means, from equation (3), that


The two sides of that equation look really similar. Inside the parentheses, both sides have 1 over something squared minus 1 over something else squared, and all that stuff outside the parentheses on the left is just a constant. So we should be able to pick some value of L that would make the two sides be exactly the same...

That's just what Bohr did. It seemed logical to assume that the squared terms on the right were related to his idea of energy levels. He associated each energy level with an integer--called, originally enough, n--with n=1 corresponding to the ground state (the lowest possible energy level). Then the 2 and the n in the Balmer series could represent electrons falling from the nth level into the second...

I get it--so the 656 nm line would be produced by an electron falling from the third energy level into the second, and so on. And then the Lyman series would come from electrons falling into the first energy level, and in the Paschen series they'd be falling into the third --this makes so much sense!

Doesn't it? Bohr realized that everything would work out beautifully if he just assumed that the electron's angular momentum in the nth level was equal to n times some constant. To find the constant, all he had to do was find the value that makes equation (13) true. It turns out that the one that works is


This implies that


If you plug in the values of all those fundamental constants--the speed of light, the electron's charge and mass, and so on--you will end up with just the value of the



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