# atomic model debate and Rutherford scattering experiment

Discussion in 'Physics' started by logicman112, Feb 14, 2011.

1. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
My question is about Rutherford experiment when a gold foil is bombed by alpha particles. We know that at that time only two types of force were discovered, electric forces and gravitational. And also there was another phenomenon and force that could not be explained:
When two objects hit each other, each one exerts a force on the other. These two objects are neutral electrically so the repulsion that happens can not electrical. Besides gravitational force is always a form of attraction. So repulsion of the objects could not be explained those times.
Now lets get back to the Rutherford experiment. He monitored alpha particles and saw some of them were reflected as much as 180 degrees so he concluded that there must be a lump of positive charges!!
My question is that why that lump must be positive? It may be a lump of mass electrically neutral and positive and negative charges have been distributed uniformly inside the atom like the model J.J.Thomson suggested.

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I vaguely remember reading something similar in The Radioactive Boy Scout (do a search on it, it is a strange story).

I'm thinking the nucleus, protons and neutrons.

3. ### steveb Senior Member

Jul 3, 2008
2,433
469
The Thomson model did not fit the observations. The expected result for the Thomson model is for all particles to pass through the foil with only minor deflections. In the actual results, the occasional particle which is reflected by 180 degrees can only be explained by a very strong and localized positive charge, which we now understand as a nucleus of protons and neutrons.

If the intuitive explanation does not convince you, you can work out the physics formulas and do the math. Keep in mind that when teachers explain these old experiments, they leave out a lot of details. You can bet the scientists of that time thought about and debated very deeply on this.

Jul 7, 2009
1,585
141
The reason it was concluded to be a "lump" of positive charge is because the alpha particles were known to be positively charged. The key experiments were done in 1909 by Geiger (of Geiger tube fame) and Marsden, both students of Rutherford. Rutherford was investigating radioactivity and had shown that there were two types of radiation from the disintegration of uranium, which he called alpha and beta. He showed that the charge to mass ratio for the alpha particle was half that of the proton. Rutherford suspected the alpha particles were doubly-ionized helium. So what his group did was to let a radioactive substance decay in a vacuum chamber. Then they detected helium gas by spectroscopy. Clever!

I remember when I was a student reading that someone described the surprise of seeing alpha particles deflected from the thin metal foils at angles as large as 90 degrees to 180 degrees. They said it was as surprising as if you fired a 12 inch naval shell at a piece of tissue paper and it bounced off the tissue paper and came back and hit you. Quite descriptive. (The wikipedia page says Rutherford said it was a 15 inch shell; I don't know why 12 inches is stuck in my brain.) The "tissue paper" came from the fact that the gold foil they were using in the experiments was only about 400 nm thick.

The book "Intro to Modern Physics" by Richtmeyer and Kennard (1947) commented that all students interested in modern physics should read Rutherford's paper: Phil. Mag. v. 21, p 669 (1911). Rutherford also showed that the observed deflections of the alpha particles do not result from lots of little deflections.

The interesting thing Rutherford did was to construct a model that predicted what the scattering rate should be as a function of angle if one assumed a point-type nucleus. They observed the scattered particles by the scintillations produced when the alpha particle struck a phosphorescent screen. Because of the experimental results of Geiger and Marsden (their data agreed with the model's predictions over measured angles of 0 to 150 degrees and fit the predicted curve $sin^{-4} \frac{\theta}{2}$ nearly perfectly over nearly 6 orders of magnitude of number of alpha rays per unit solid angle), the group was able to calculate that the radius of the gold nucleus is less than 3e-14 m (cf. "Foundations of Modern Physics" by Tipler, Worth, 1969). Remember that the size of atoms was on the order of 0.1 nm, so the radius of the nucleus was four orders of magnitude smaller, a surprise to everyone.

This group's work was truly a watershed event in our understanding of the atom.

5. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Probably because that was about the limit of naval gun calibers in 1909. There were some 13.5" guns afloat in 1910, but 15" did not come into existence until after his experiments.

Last edited: Feb 14, 2011
6. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The history of cannon and naval gunnery is a fascinating story in it own right

7. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
As is the analog computers that let the ranges for successful engagement go beyond 3 miles or so. Try Friedman's "Naval Firepower".

Jul 7, 2009
1,585
141
I've always been interested in those big guns. I have a book on the Missouri that I've enjoyed reading over the years. Around 25 years ago I was in the Seattle area for something, so I made a point to go to Bremerton to look at the Missouri. I enjoyed the heck out of it and was a bit awed by the size of those 16" projectiles and the sheer size of those guns. On top of that those Iowa class boats were beautiful ships. There's a picture in the book showing that they could turn inside destroyers.

If I could find someone to host the pictures, I'd scan some of my photos in so you guys could see them. She was also next to the Hornet, another famous ship.

9. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The USS Alabama in Mobile is a battleship you can walk around in. A veritable floating city of 5000 crew. Amazing.

10. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
Thank you very much for the replies, however I do not understand the subject yet. The large deflection can show a large lump of neutral material and this aggregation of material can return Alpha particles as much as 180 degrees. It is like the collision of two objects when one object is fixed some where and another object is thrown to it. Or like a ball which hits a wall and returns straight to us.
I want to say that why the material in the centre must be positive? It can be neutral naturally or it can be a mixture of positive and negative charges stuck together hardly, at any case it can return alpha particles. I know that its mass must be high but why it should be charged and positively charged?

11. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
You are going way back in physics on this one, all the way to static electricity. We "knew" there were charges, and they were opposite. If you take an atom and remove its electrons then it becomes positive, ergo there is something in there with a charge.

It wasn't until the early 20th century that electrons were understood to be the negative particle, Ben Franklin basically flipped a coin and took a guess as to which was which. As it happens he was wrong, but he did some really amazing science for the time, and we have people argueing conventional theory (invented by Ben Franklin) today. The evidence is pretty overwhelming that electron flow theory is correct, but when you use something for over a 150 years that works it builds an inertia of its own.

12. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
You will also find that atoms are electrically neutral. They are known to be surrounded by electrons, carrying negative charge. The only way to make the atom electrically neutral is for the nucleus to have an equal positive charge.

13. ### steveb Senior Member

Jul 3, 2008
2,433
469
It's not clear what is troubling you. Are you saying that things can bounce off of thick objects like walls? If so, keep in mind that the experiment you are talking about uses an ultra-thin sheet of gold. It's so thin that a model like the Thompson model predicts that the alpha particles should pass straight through with only minor deflection possible. Remember that the Thomson model has no neutral parts to it. It is a smeared out positive charge with localized negative charges spread around. A thin sheet of material made of such "bread pudding" atoms can't return alpha particles back at 180 degrees. In other words, the model fails completely when compared to the experimental data.

Could one conceive of other models that reflect at 180 degrees? Possibly, but they would not agree mathematically with the data to such a high degree as the currently accepted model. Again, remember my point that there is much detail left out of basic descriptions that we typically get. Real science is not so neat and clean and much debating happens before a model can be accepted. When it's all over, we tend to simplify the story and make it seem so obvious in hindsight.

14. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
Thank you Steveb, my question is that a smart intelligent person like Rutherford why did not ask himself that the material which blocks electrons and forces them to return may not have any charge(like Neutron). Or he may have considered this possibility and we do not know about it.

15. ### steveb Senior Member

Jul 3, 2008
2,433
469
Ah ok. That is a good question because it is possible for neutrons to scatter off each other, and I believe that even 180 degrees would be possible. However, the processes of neutron scattering are not as simple as electrostatic repulsion/attraction and neither these processes, nor the neutron as a particle were known then. Electrodynamics and the concept and evidence of charged particles were very well understood at the time, so it's logical that the model was based on existing physics. It turns out that the full quantum mechanical nature of atoms still needed to be discovered, but the Rutherford model was a very good start along that path.

He very well may have thought of some ideas along these lines, but, if he did, he could not come up with a model that fit known physics and the data. Even today, I can't think of a model based on neutron interactions, that could fit the data.

As I mentioned above, we get a simplified version of the story today. We can't be sure of all the ideas Rutherford and other scientists of the day considered and how hard they really debated. There is some historical record that one can dig up in publications and letters, but the full details are probably untraceable.

16. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
(Based on our discussion here, our little knowledge of history and the exact details of Rutherford experiment)
The bad thing is that our atomic model today is based on Rutherford experiment and his preconditions. He supposed atoms have only negative and positive charges. We accepted the idea and then Neutron was discovered. We started from the fact that there is not uncharged particles, we did other experiments and then we concluded that our initial condition is wrong!! While that initial condition made us conclude this!
Our discussion suggest me that atomic model is just a model, one guess and state from many possible states. It is very frightful and shows that we may be very far from reality.

17. ### steveb Senior Member

Jul 3, 2008
2,433
469
I agree with your assessment that atomic models are just models. But, to me this is not frightful, and, in fact, we are getting closer and closer to simulating and predicting reality over time.

In other words, we are clearly homing in on better and better models over time. I suppose you can argue that a model never equals reality, and is therefore very far from it, but this is missing the entire point of science. Science seeks to describe the world. Descriptions and models are basically the same thing, or at least do similar things, and neither purports itself to be the thing it describes.

18. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
You also have to be very careful when encountering statements like that, it is precisely the kind of statement a con man will make when trying to confuse your reality to substitute his own. The question to answer is usually the same, does the new model (because it is still a model) explain the facts better or worse? It it does less well then it is probably further off base.

Another mistake many people make, if they can't understand a concept then no one can.

19. ### russ_hensel Distinguished Member

Jan 11, 2009
820
47
You said:

My question is about Rutherford experiment when a gold foil is bombed by alpha particles. We know that at that time only two types of force were discovered, electric forces and gravitational. And also there was another phenomenon and force that could not be explained:
When two objects hit each other, each one exerts a force on the other. These two objects are neutral electrically so the repulsion that happens can not electrical. Besides gravitational force is always a form of attraction. So repulsion of the objects could not be explained those times.

But:

2 objects that collide do repel each other via electrical forces. At a distance the fact that they are neutral provents any force, but an object has the outer shells of the electrons around it ( in a simple model ) and when they get close the shells collide and repell, the farther away positive parts are not so important ( but I am assuming some of the rutheford model by putting the electrons outside ). My point is you do not need a new force to explain why the objects repel given that you have the right structure for the object, the + and - do not always cancel.

20. ### logicman112 Thread Starter Active Member

Dec 27, 2008
69
2
we did not know about Rutherford experiment in details!!