Electron Cloud

Most recent answer: 10/22/2007

Q:
Electron accelaration inside the electron cloud: If electron changes its position so randomly that it seems as an electron cloud, then it must experience large accelerations. Hence, similar to Bohr’s earlier (wrong) planetary model, such accelerations should cause the electron to give off energy (photons) and collapse to the neuclus. Since this does not happen, hence, this model must also be wrong! Comment please?
- Mehran (age 53)
Lisle, Illinois
A:

Mehran- OK, another of your pointed, thoughtful questions.

The thing is that, so far as we now understand things, electrons simply don’t have exact positions. In, for example, the ground state of a hydrogen atom the electron state really is spread out and nothing is changing, not position or momentum or anything else. Now when something else (maybe say a high-energy photon) comes along things do change, and some sort of ’measurement’ process may occur. The outcome can be an electron whose position or momentum (but not both) are at least for a while more sharply defined than in the initial ground state.

If the electron starts out in some higher-energy state, it will ultimately radiate away the extra energy into electromagnetic waves, so something will change. An electron in the ground state cannot radiate energy because there is no lower-lying state it can fall into; that’s what’s meant by the ground state. It’s a non-classical feature of quantum mechanics that ground states exist at all. If you try to use a picture of electrons moving about in some sort of trajectory to calculate how fast an excited atom radiates energy, you won’t get the right answer except in some special cases.

In other words, we have no reason to think that the electron ’seems’ to be a cloud. It’s far easier to make a self-consistent description in which the electron is a cloud, not a point darting about.

Mike W.

p.s. Here’s another little fact that may help bring out these mysteries. The photons emitted are electromagnetic waves, so you might be tempted to think that they have particular values of electric and magnetic fields, just as you are tempted to think that electrons have position and momentum. However, for a single photon (or for any exact number of photons) the expected value of the fields is zero. On a small scale, things just don’t have the properties we might naively expect them to have using what we know from classical physics.


(published on 10/22/2007)