# Can we "fill" an atom with alpha particles?

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1. Aug 9, 2014

### Meson080

The following passage has been extracted from the book "Modern's abc of Chemistry":
Lets fill an isolated atom by subatomic "Rutherford projectiles"-alpha particles. I hope it is possible. This doesn't seem to be a limit of our technology. Isn't it?

If we are successful in filling the an atom with alpha particles, we are decreasing the space for the electron and confining them to a least distance, isn't it?

Doesn't this experiment, give us belief of measuring simultaneously both the position and momentum with little greater (or even complete) certainty than what predicted by Heisenberg's principle? This doesn't seem to allow us to fill an atom with alpha particles.

So, can we fill an atom with alpha particles?

The above question is also posted in Physics stack exchange, interested folks can visit: Can we "fill" an atom with alpha particles?

2. Aug 9, 2014

### Staff: Mentor

First, you can't fill an atom with alpha particles. Alpha particles are the nuclei of helium-4 atoms and are have a charge of +2. They would repel each other and you'd never be able to contain them in such a small area.

In addition, it is a common misunderstanding that you can't measure both the momentum and position of a particle. The uncertainty principle is about measuring multiple particles that are prepared identically. In other words, if I shoot a million electrons through a slit, there is no way to know in advance what the momentum and position of any single electron will be with perfect accuracy for both. I can set up the experiment so that I will know either the position or the momentum of each electron to any arbitrary value prior to measurement, but this requires making the other value more uncertain. However, upon measuring each electron, I will find out both the position and momentum to whatever accuracy my experiment can see.

3. Aug 9, 2014

### Meson080

Why can't we use the electric fields to confine the alpha particles into the atom?

I didn't find any such explanation in any of my books, can you suggest me a book or a link to read about the uncertainty principle?

Isn't this in contradiction with the statement of the Heisenberg's Uncertainty Principle, I provided in the first post?

4. Aug 9, 2014

### phinds

Yes, because as Drakkith explained, what you posted was not what the HUP actually says. It is better understood now than it was then. This has been a subject of serious contention on this forum. There are still those who believe in the theoretical limit of measuring a single quantum object but most understand it to be as Drakkith says, a statistical thing. You set up an experiment to produce a particle and measure the results, then you run EXACTLY the same experiment and measure the results of that run. You don't get the same results.

5. Aug 9, 2014

### Staff: Mentor

Imagine you have a hollow metal sphere with a positive charged particle inside. You want to keep it right in the middle so you charge the entire sphere with a positive charge. Unfortunately, this leads to the electric field disappearing inside since there's no potential difference anywhere inside the sphere. In other words, the charged particle will experience no force and will act as though the sphere is uncharged. You simply cannot contain charges inside a volume of space by using repulsive electric forces.

After searching for a reference, I've come to the conclusion that I may not know what I'm talking about. I'll let someone else with more knowledge handle this.

Edit: I see Phinds has the same idea I do about this. It's honestly something that I've seen explained here on PF several times, but I can't be certain it's correct. Hopefully someone can clear this up.

6. Aug 9, 2014

### mishima

Actually it is the limit of our technology. You are talking about nuclear fusion. It is thought this might be possible to achieve in the laboratory in a practical manner in the next 50 years or so (successes so far have been limited in their applications). I believe it is considered a cutting edge research topic in the field of plasma physics, at least.

As far as I know the only time that happens in nature is inside the core of red supergiant class stars (the biggest stars) near the end of that phase of stellar evolution. To fuse, you actually have to overcome the EM repulsion until the attractive strong force takes over (or there can be some tunneling effects as well). 100 million Kelvins is the order of magnitude temperature we are talking for that to occur. (edit: not to mention astronomical amounts of gravitational acceleration)

The particular reaction of alpha particles fusing together is called the triple-alpha process. The result is beryllium (from 2 alpha combo) and carbon (from beryllium and yet another alpha), and like 7 million electron volts per reaction. The energy comes from the fact that when nuclei form, the sum of the parts is actually less that the resulting nuclei. It would be like measuring the mass of all the parts of a bicycle, summing them together, and then comparing that with the mass of the assembled bicycle and getting a different answer. Weird, huh?

7. Aug 9, 2014

### Staff: Mentor

Mishima, I was under the impression the OP was talking about shoving a bunch of alpha particles inside an atom without fusing them together inside the nucleus.

What say you, Meson?

8. Aug 9, 2014

### mishima

Sorry if I misread, I have no idea what you are talking about if that's the case. How can a positively charged particle be bound to a nucleus without strong force interactions?

9. Aug 9, 2014

### Staff: Mentor

It can't.

10. Aug 9, 2014

### Meson080

I am going to the class now. Wait I will be back!

Last edited: Aug 9, 2014
11. Aug 10, 2014

### Staff: Mentor

Even if you *could* fuse enough nucleons to make a nucleus with a size comparable to the atom size, this would be a nucleus with a ridiculously high A, about 4e22 (nucleons are tiny!). Such an element would be highly unstable to the degree of not being possible to create.

Regarding the HUP: It is impossible to know the position and momentum of a particle at the same time. If you measure x first, them p, then x again, you will get different results in your x measurements. (You can also do the corresponding thing with any two non-commuting operators, such as the spin of a particle in two different directions.)

12. Aug 10, 2014

### Meson080

But, this analogy doesn't scale properly/match properly with our experiment of confining alpha particles into the atom.

13. Aug 10, 2014

### Meson080

Anyhow, I didn't want the charged atom to be stable. I just wanted it to be fused for a moment, in your words.

14. Aug 10, 2014

### Staff: Mentor

You missed the point, which is that the field will not point in. You cannot even keep a single charge in the middle of the sphere, let alone fill the sphere with charges. The analogy is an even simpler task than what you want, but even that simple task cannot be accomplished.

15. Aug 10, 2014

### Staff: Mentor

The real problem is that you will never be able to do that either. There is simply not going to be a bound state with that many nucleons.

16. Aug 10, 2014

### Staff: Mentor

This is not correct. The wavefunctions of the electrons are not excluded from the nucleus. Even if the nucleus took up a large volume, the space for the electrons is not decreased.

17. Aug 11, 2014

### Meson080

What about dipping the atoms into the sea of alpha particles?

18. Aug 11, 2014

### Meson080

Can you elaborate?

If the atom gets filled by alpha particles, for the layman like me, the space for the electron should decrease. Or else electron should have got different space, does the electron buys different land around nucleus?

19. Aug 11, 2014

### Staff: Mentor

Unfortunately, nature didn't ask for the opinions of laymen when she picked the rules, but don't feel too bad, she didn't ask for the opinions of experts either.

Look at these images of electron orbitals for hydrogen:
http://en.wikipedia.org/wiki/Hydrogen_atom#Visualizing_the_hydrogen_electron_orbitals

The bright spots are regions where the electron is likely to be, and the dark spots are regions where the electron is unlikely to be. Note that, for the s orbitals, the brightest spot is right in the middle, on top of the nucleus. In other words, the s electron is more likely to be in the nucleus than in any other spot of similar volume.

Weird, but that is how it is. Increasing the number of nucleons doesn't change this at all, although increasing the number of electrons changes the wavefunctions quantitatively, but not qualitatively.

20. Aug 11, 2014

### Staff: Mentor

There is no such thing. Alpha particles are highly charged particles that are emitted by radioactive decay and immediately steal electrons from whatever material they happen to come to a stop in. You will not find a "sea" of alpha particles nor could you even make one.