Chemistry/Physics question

[Bruce]

It's a very simple question, but I never good, straight-forward answer in college:

The electron cloud is negative; the nucleus is positive. What is preventing the electron cloud from collapsing into the nucleus?

Every answer I got in college sounded an awful lot like perpetual motion to me. Professors said that atoms had vibrational/rotational energies that kept the electron cloud in orbit. So where does thermodynamics come into play?:confused:

[garys_2k]

Good question. I'd assumed that there was some minimal quantum energy level an orbital electron could assume, but I'm sure there's something more fundamental going on.

As for electron orbits being the same as perpetual motion, that isn't the same thing. Electrons in orbit can only assume specific energy levels. Some are stable, so once they reach that spin state they'll stay there until disturbed. They don't run down from the spin like a wheel on an axle.

[Bruce]

But that spin state is energy driven. We know that energy is involved because it can be released by splitting the atom. Why does that energy not slowly dissapate into heat like all other forms of energy?

Splitting atoms releases a tremendous amount of energy. The atom is fragmented into smaller particles. The energy is stored within the atom. As far as we know, the energy remains there forever as long as the atom is not disturbed (assuming it's not already unstable).

Fundamental questions are always the hardest to answer. I hope we can arrive at something. More importantly, I hope I'm smart enough to understand the answer!:D

[Skeptoid]

I'll take a stab at it. As garys_2k was getting at, the electron in say, the hydrogen atom, can only occupy certain energy levels. Once it reaches the ground state no energy is required for it to maintain its stability. A loose analogy would be that a planet in a stable orbit around a star needs no external energy to maintain that orbit for billions of years.

The splitting of an atom is an entirely different process. The electrons do not take part in the interaction. Say you have a nucleus of Uranium-235, it has a certain mass that we'll call 1. Suppose the nucleus splits (fission) into two parts, if we measure the mass of each fragment we'll find that the sum of the masses is less than 1. Where is the missing mass? It is the nuclear binding energy released when the atom split. Remember E=mc²?

Does this help?

[Stimpson J. Cat]

Bruce,

In the lowest energy state there is a nonvanishing probability of the electron being found arbitrarily close to the nucleus. But what does this mean?

We all have an intuitive idea of particles being little solid balls of matter, but this is not correct. The electron does not have any "size" as such. It is not meaningful to talk about it actually "striking" the nucleus. In fact, you could find it arbitrarily close to the nucleus, but then a short time later, back far away from it.

Your question seems to imply that you think that the electron should somehow be captured by the nucleus. The thing is that in order for this to happen, some sort of reaction between the electron and the nucleus must be possible. The simplest such reaction is for the electron and proton to combine to form a neutron, and omit a neutrino. The thing is that for stable nuclei, this reaction will not occur, because the total energy after the reaction is considerably more than before the reaction. It is possible for unstable nuclei, though. This is known as K capture.

Remember that in QM, it is not sufficient, or even necessary, for two particles to "touch" for a reaction to occur. In fact, it is not even really meaningful to talk about them touching. What is necessary is that a reaction be possible, and then the probability of such a reaction depends on the relative energy before and after the reaction, and the proximity of the wave-functions of the particles. This is why in QM, scientists refer to the "cross-section" of an interaction, rather than talking about "probabilities of hitting". The cross-section is in effect a direct measure of the likelihood of the reaction occurring.

Dr. Stupid

[MRC_Hans]

The analogy generally used to describe these small parts, "particle" is perhaps unfortunate. It makes us think of tiny balls racing around. But it is only an analogy. The real reason they are called particles is that they are the smallest parts with those particular (sic) properties.

Hans

[Agammamon]

The reason that electrons don't spiral into the nucleus is that the inner electron shell/orbital is a minimun energy state. To get the electron closer you have to add energy. Why the inner shell is at x distance and not somewhere closer or farther away is still one of the fundemental mysteries of the universe, along with all of the other constants.

[Bruce]

Originally posted by Agammamon
The reason that electrons don't spiral into the nucleus is that the inner electron shell/orbital is a minimun energy state. To get the electron closer you have to add energy. Why the inner shell is at x distance and not somewhere closer or farther away is still one of the fundemental mysteries of the universe, along with all of the other constants.

I thought I would fail QM miserably in college, but it actually helped me to understand chemical interactions better. It helped me get away from thinking of atoms in terms of hard spheres and +'s and -'s, and start thinking about probability fields and energies. I still have trouble with it though because I'm a Euclidian thinker. If I can't visualize a theory in terms of shapes and object, my head starts to throb.

I did a project in grad school dealing with binding energies between ligands and transition metals. I remember learning that there is an attractive force between the nucleus of one atom to the electron field of an adjacent atom that it is "bound" to, and a repulsive force between the electron fields of two atoms.

This led me right back to the +/- problem. If these forces exist (at least in theory), should there not be an attractive force between the electron field of the atom with it's own nucleus. If so, there must also be a repulsive force that holds the electron field at a fixed distance, thus minimizing the total energy. What is the nature of that repulsive force? I think that is the mystery that Agammamon alludes to.

I hate mysteries. I like to know how everything works. That is my driving force as a researcher. I think this mystery is better suited for a physicist to solve than a chemist. Thanks to everyone for your thoughts. Point me out to any links you know on this topic.

[BillyJoe]

Stimpy,

Originally posted by Stimpson J. Cat
The thing is that in order for this to happen, some sort of reaction between the electron and the nucleus must be possible. The simplest such reaction is for the electron and proton to combine to form a neutron, and omit a neutrino. The thing is that for stable nuclei, this reaction will not occur, because the total energy after the reaction is considerably more than before the reaction.I think that bruce thinks you are ducking the question.

He wants to know how this answers the question about why the opposite electric charges of the electron and the proton do not result in the mutual attraction of the electron and the proton.

[Tez]

Originally posted by Agammamon
The reason that electrons don't spiral into the nucleus is that the inner electron shell/orbital is a minimun energy state. To get the electron closer you have to add energy. Why the inner shell is at x distance and not somewhere closer or farther away is still one of the fundemental mysteries of the universe, along with all of the other constants.

This is not completely correct..

As stimpy pointed out, many orbitals have significant probability density right at the nucleus. This is the problem with using a Bohr model of the atom too literally...

[Captain_Snort]

sorrym drunken reply here, right, energy levels and stuff, I always remeber that associated electrons are in energy levels and can usually be relied on to be such, but it is also probability levels, and that say the atom of carbon a molecule of the alcohol I am drinking could happily be associated with an electron somewhere around jupiter. It got me thinking and decided that Quantum ***** should be left to those who are warped enough to deal with it.

I prefer to think of electrons as little manic buzzing bees that zip about all over the place never resting anywhere for somewhere is always better, and convince them of that and we get them to run in certain ways, we can use their energy.

I hope this had made sense, cos it hasn't to me