Limits of classical views of the atomic structure

[mnb96]

Hello,
this might be a trivial question for many of you.

I would like to know what are the simplest phenomena/experiments, if any, (and perhaps the first historically) that contradict the assumption that subatomic particles like electrons, protons, neutrons..., can be thought of as "small concrete (spherical) objects moving across the space".

Thanks!

 

[ZapperZ]

Er.. the fact that electrons in a stable "orbit" in an atom does not radiate?

Zz.

 

[mnb96]

Ok.
Could you please mention what is the experiment that proves that fact?

Thanks.

 

[Rap]

Check out the Davisson-Germer experiment at http://en.wikipedia.org/wiki/Davisson–Germer_experiment. The first experiments illustrating the wave-particle duality were actually showing that a wave had particle properties, and this was done earlier, e.g. Einstein's interpretaton of the photoelectric effect, and Planck's resolution of the ultraviolet catastrophe showed that an electromagnetic wave had particle properties. De Broglie then postulated that particles exhibited wave properties, and this was confirmed by the Davisson-Germer experiment.

 

[Vanadium 50]

Could you please mention what is the experiment that proves that fact?

The fact that all the atoms on Earth don't vanish in a flash of light in less than a microsecond for starts.

 

[mnb96]

Thanks a lot for both your answers.

It seems that the Davisson-Germer experiment essentially confirmed the De Broglie hypothesis, which deals with the wave-particle duality of matter.
If I recall correctly, many (or probably all) the phenomena/experiments in which the wave-particle duality was observed, led to the development of the basis of quantum theory.

--However, ZapperZ's answer still leaves me thinking about few issues:

1) How could physicists in the past have accepted such a näive model for the atom which could not even explain why "...all the atoms on Earth don't vanish in a flash of light in less than a microsecond..."

2) Could we consider this fact alone (without even resorting to the experimental evidence of the photoelectric effect & co...) to immediately reject the hypothesis that electrons are not "electrically charged tiny orbiting planets" ?

3) What's the current explanation of the fact that "electrons in a stable "orbit" in an atom do not radiate"?

 

[Rap]

1) They did not accept it. It was a problem waiting to be solved and was what motivated further study.
2) Yes, we could immediately reject that hypothesis, but it does not show that electrons, protons, etc are not particles, only that applying the "tiny orbiting planets" model, obeying the macroscopic laws is not tenable. It either has to be modified, extended, or discarded.
3) The simple answer is: Quantum mechanics. The fundamental idea is that action is quantized in multiples of Planck's constant, and this leads to the conclusion that the energy of an orbiting electron can only take on discrete values. Rather than continuously radiate small amounts of energy, the electron must emit a quantum of energy, or not. When the electron is in its ground state, there is no lower energy state that it can drop to, and so it remains in that state. Two electrons cannot occupy the same state (Pauli exclusion for fermions), so the next electron occupies the next lowest energy state, and cannot drop lower, so it too stays in that state, etc. etc.

 

[jtbell]

1) How could physicists in the past have accepted such a näive model for the atom which could not even explain why "...all the atoms on Earth don't vanish in a flash of light in less than a microsecond..."

As far as I know, physicists knew from the time the naive classical planetary model of the atom was first proposed, that it had a serious flaw because of radiation from the centripetally-accelerating electrons. I don't think it was ever really "accepted," but instead taken as a possible starting point for a better model.

[Aha, Rap beat me to it while I left the thread open on my screen without refreshing it.]