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Q & A: Bohr model revisited

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Most recent answer: 10/22/2007
Q:
What are the justification why the atomic model presented by Neils Bohr are accepted?
- Evalyn (age 21)
National Teachers College, Quezon City,Philippines
A:
Evalyn- You raise an interesting question. Although the Bohr model is often used to give a cartoon illustration of atoms, it has not actually been accepted as a good model of atoms since about 1925. Here are some of the things it gets wrong.
1. The Bohr model implies that each electron has a single position and momentum at any time. That's not so in quantum mechanics.
2. The Bohr model implies that the smallest value of the orbital angular momentum of an electron (a measure of how much the electron is rotating around the nucleus) is equal to something called Planck's constant. Actually, it's zero.
3. The Bohr model claims that each electron must at any time have a specific value of energy, not a combination of different values. That's false.

Historically, the Bohr model was the first model which described the emission and absorption spectrum of light from hydrogen. If an electrical discharge is made through hydrogen gas (don't do it yourself -- it can explode if there is any oxygen nearby), it will glow. If you use a prism or better yet, a diffraction grating to see what colors of light are present, only a few narrow bands will be seen. These were categorized by Balmer, Paschen, Lyman, Brackett, and Pfund from the 1880's through the 1920's.

This discretization of the energy levels was first explained (although not fully, and not correctly, either, as explained above) by Bohr's model. The Bohr model also predicted the approximate size of the hydrogen atom, and it predicted correctly the amount of energy needed to tear the electron away from the proton in a hydrogen atom.

The Bohr model appears in many textbooks because it can be shown in pictures, unlike the actual situation.

We have several other discussions of this issue, which you can find by searchuing for "electron" or "atom" on our site.

Mike W.

(published on 10/22/2007)

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