Re: Electron Rotation (general questions)

The image of the atom that your have in mind is known 
as the Bohr atom model. This model was proposed by the 
Danish physicist Niels Bohr in 1913 for the hydrogen atom, 
which is composed of one proton and one electron. His idea 
was based on mixing certain concepts of classical physics 
with the new the ideas of the quantum that were being developed 
at the time. Basically Bohr proposed that the electron 
moves around the proton as if it were a planet orbiting 
the Sun in a circular orbit. This was not new since 
physicists knew that electrons had to be moving in the 
atom in order not to fall upon the nucleus, but classical 
physics could not explain why electrons did not radiate 
as a result of their accelerated motion in the atom. Such 
radiation is predicted by classical physics for any accelerated 
charge, and would make the electron loose its energy in a 
matter of seconds and make it fall upon the nucleus anyways. 

To solve this failure of classical physics to explain the atom, 
Bohr proposed two postulates that deviate from classical physics. 
First, an electron in the atom can move only in certain orbits that 
have DISCONTINUOS values of angular momentum. Angular momentum is 
a physical quantity that can have any value according to classical 
physics. Bohr proposed that angular momentum in the atom, only 
takes discrete values, and after a few algebra steps, this assumption 
means that the energy of the electron only takes discrete values too. 
We then say that energy and momentum are QUANTIZED. 
The second Bohr postulate says that an electron radiates or 
absorbs one quantum of light when it changes from one orbit to the other. 
Remember that energy must be conserved, therefore the mechanical 
energy that the electron looses in a jump from a higher to 
a lower orbit must be transformed into a quantum of light, 
which is called a photon. 

The Bohr model does not explain why this is so, it simply 
postulates or assumes those two things that are not 
implicit in classical physics. Why make these two rather 
unorthodox assumptions? Because they explain rather 
accurately the observed spectral lines of hydrogen. As you 
see, assumptions or postulates are made in physics to fit 
lab measurements. In the Bohr atom model an electron cannot 
fall upon the nucleus because that orbit is not allowed by 
Bohr's postulates. This differs significantly from classical 
physics that allows, for example, a comet to plunge right 
into the Sun. Such orbit would have a zero angular momentum 
from the point of view of classical physics. Thus far 
we have talked of one electron orbiting a proton as in the 
hydrogen atom. When the Bohr model is applied to atoms with more 
than one electron, it fails completely to explain the observed 
spectra. According to the Bohr model electrons in an atom 
DO NOT necessarily rotate in the same direction or plane, 
but as you already have noticed, this model still leaves many 
unexplained questions. Electrons in an atom repeal one another 
through their electric fields, therefore they don't collide, but 
their orbits in the atom would be far more complicated that 
simple circles as in the hydrogen atom. Besides we now know 
that electrons can have zero angular momentum, so 
why they don't fall upon the nucleus anyways as the comet 
onto the Sun? 

Because of the above problems physicists abandonded the 
Bohr model for the most part. The problem lies in thinking 
that electrons behave as particles, and in patching classical 
physics with quantum physics to explain certain facts. 
The Bohr model was important because it showed that physical 
quantities must be quantized at the atomic level, but it 
was substituted by a more modern approach called wave mechanics, 
or quantum mechanics. Still today the Bohr model is sometimes useful 
to understand certain things about the atom, but as you have already 
noticed, its usefulness is very limited. 

Unlike the Bohr model, wave mechanics uses a completely different 
approach that assumes that the behavior of electrons in an 
atom can be described as waves of density. As the Bohr model, 
wave mechanics uses a set of principles but they are more 
fundamental and don't assume concepts of classical physics. 
Thus it has been far more successful than the Bohr model. 
According to the modern picture of the atom, electrons 
can have stable states in the atom. When they change from 
one state to another, the energy difference is emitted or 
absorbed as a quantum of light. Electrons cannot really 
collide with one another, or with the nucleus because 
they are "waves", not particles, although their states are 
determined by the presence of the nucleus and other electrons. 
An electron can be expelled from the atom if it receives 
enought energy, an can recombine with the atom again by 
radiating away energy and remain in a stable state.

Light is basically a wave of electric and magnetic fields. 
Therefore light can interact with an electron, which is 
an electrically charged particle or wave, and this makes it 
radiate or absorb light and change its state in the atom. 
Besides quantum mechanics shows that, strange as it may seem, 
we cannot detect the electron when it changes state. 

Finally it is possible that an electron collides with 
the atom nucleus, but not all nuclei can do that and 
sometimes the electron must have enough energy. This 
usually happens in stellar interiors or nuclear explosions. 
For example, the nucleus of the Be 7 isotope can capture 
an electron and become a Li 7 isotope plus a particle 
called a neutrino. 

If you have some physics background, I recommend that you 
work out the Bohr model formulae following books like Orear's 
"Fundamental Physics" or some other textbook for first year 
physics that treats the Bohr model in order to see how 
classical and quantum physics were mixed rather arbitrarily 
and still produced the right answer. 
There are several books that explain wave or 
quantum mechanics for people with no math or physics 
background like "Mr. Tompkins in Wonderland" by George Gamow 
or "Alice in Quantumland : An Allegory of Quantum Physics" 
by Robert Gilmore or "Taking the Quantum Leap : The New
Physics for Nonscientists" by Fred Alan Wolf

Vladimir Escalante Ramírez