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Q & A: electron orbits?

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Quantum Mechanics
Q:
I have read the the electron moves in a fixed orbital around the nucleus. What "tells" the electron to move in a particular orbit? (or why can’t it be in an intermediate orbital?) I guess it is the electorstatic attraction between electron and protons that keeps the elctron from escaping. My two questions are: 1) what kind of repulsive force does it experience to keep it into a fixed orbital? 2) If an electron is "forced" or pushed into the nucleus? will it eventually cause a nuclear reaction?
- Anonymous
A:
1) The quantum electron wave function stays spread out because of an effect that can't be pictured as a classical force. The Uncertainty Relation implies that if the wave is squeezed into a small region, it must have a big range of momenta, and thus a high kinetic energy. The size of the cloud formed minimizes the total energy, including this kinetic piece and the electrostatic potential piece, which becomes more negative for small clouds.

2) Electrons are forced into the nucleus by very high gravity-induced pressure in some heavy stars. Under these high pressures, the electrons combine with protons to form neutrons. So that's what makes neutron stars.

mike w

2') Some nuclei do interact with the orbiting electrons, which do in fact spend a fraction of their time within the nuclear radius. The process is called "inverse beta decay", in which a proton captures an electron, turns into a neutron, and emits an electron-flavored neutrino, all in one operation. This only happens when it is energetically favored, which is only in a small class of elements (or in neutron stars!).

Tom

(published on 10/22/2007)

Follow-Up #1: beta decay

Q:
We know that electrons can not prensent inside the nucleus. Then how come an electrons come out from the nuclueus in the beta decay process?
- Ramesh Naidu (age 33)
Pondicherry, India
A:
The world really isn't made of a collection of fixed little building-block particles. There is no fixed number of electrons, so you don't need one in the nucleus to start with for one to come out. 
There are some things that are fixed ("conserved"), though. These include energy, momentum, angular momentum, and electrical charge. So for a nucleus to change, there has to be just the right combination of outgoing particles to conserve all the conserved quantities- the ones listed plus some others. Emission of a combination of an electron, a neutrino, etc, can work to conserve these in beta decay processes.

A fundamental beta decay processes is  neutron --> proton + electron + neutrino, e.g. carbon 14 decays into nitrogen 14 plus an electron and a neutrino.   The half life of this process is about 5,700 years, useful for ’carbon dating’ of archeaological artifacts.

Why a particular type of nucleus decides to undergo beta decay and how long it typically lives is another question to be asked.

Mike W. and Lee  H.

(published on 02/24/07)

Follow-up on this answer.