Question about Rutherford experiment: Which of these could not be concluded?

[songoku]

Homework Statement

In the early 20th century, scattering experiments were carried out by directing high speed alpha particles at thin metal foils. Which of the following could not be concluded from these experiments?
A. Most of the atom is empty space.
B. The nucleus of the atom is positively charged.
C. There is a concentration of charge in the atom.
D. There is a concentration of mass in the atom.

Relevant Equations

None

I don't think there is correct answer for this question. This is my reasoning:

A) This is correct since most of alpha particles move in straight line

B) This is correct since the deflection of some alpha particles is away from center point

C) This is correct because a small number of alpha particles were deflected at large angles, which indicated that there is a concentration of positive charge in the atom

D) This is correct since very few alpha particles deflected straight back so it means that the mass is concentrated in a tiny volume in the atom

Is there mistake in my reasoning?

Thanks

 

[Steve4Physics]

It's hard to reply without giving the answer away! But here's a question: what would be the difference in experimental results if the nucleus were negative?

 

[songoku]

It's hard to reply without giving the answer away! But here's a question: what would be the difference in experimental results if the nucleus were negative?

I think everything will be the same.

But when doing this experiment, didn't Rutherford already know alpha is positively charged?

 

[Steve4Physics]

But when doing this experiment, didn't Rutherford already know alpha is positively charged?

Yes, but that’s not enough information.

There was additional information available apart from his team's (Geiger and Marsden) experimental results. There was the belief that atoms (neutral) were a combination of electrons (low mass, negative charges) and positive charges.

Whether the positive charge was spread-out (Thomson’s ‘plum pudding model’) or distributed in some other way was unknown.

I don't think that the experimental results alone would distinguish between between deflection by a small massive positive charge and deflection by a small massive negative charge. For example, back-scattering by a small massive negative charge could occur if an $\alpha$ particle performed a part-orbit (like a comet going around the sun). I’m sure someone will correct me if I’m wrong!

However, the original question is not entirely clear: is it about using only Rutherford's results or about using the results plus the additional information available at the time?

Just my opinion/thoughts though!

 

[songoku]

From that picture, I thought it was clear from "some deflection" that the nucleus is positively charged since the deflection is always going away, never going towards

So maybe it is enough to conclude the force is repulsive and since alpha is positively charged then there should be another positive charge repelling alpha?

Thanks

 

[Orodruin]

You do not know which atom deflected the alpha particle so you don’t know if it was deflected towards or away from the nucleus.

 

[Steve4Physics]

View attachment 353150
From that picture, I thought it was clear from "some deflection" that the nucleus is positively charged since the deflection is always going away, never going towards

So maybe it is enough to conclude the force is repulsive and since alpha is positively charged then there should be another positive charge repelling alpha?

The $\alpha$ particles could be attracted to - and therefore deflected when passing close to -fixed negative charges. Like this:

(No accurately drawn.)

Without some additional information, you can't tell solely from the experimental results (angular distribution of scattered $\alpha$ particles) whether the deflections are caused by positive or negative charges.

But, with the premise that an atom consists of negative electrons and positive charge, it is reasonable to propose that the fixed charges are in fact positive.

 

[songoku]

You do not know which atom deflected the alpha particle so you don’t know if it was deflected towards or away from the nucleus.

The $\alpha$ particles could be attracted to - and therefore deflected when passing close to -fixed negative charges. Like this:

View attachment 353157
(No accurately drawn.)

Without some additional information, you can't tell solely from the experimental results (angular distribution of scattered $\alpha$ particles) whether the deflections are caused by positive or negative charges.

Oh I see. The correct answer is (B)

But, with the premise that an atom consists of negative electrons and positive charge, it is reasonable to propose that the fixed charges are in fact positive.

Why not negative?


Who finally proposed the nucleus is positively charged and what further experimental evidence to support it?

Thanks

 

[Steve4Physics]

Oh I see. The correct answer is (B)

I'd choose B. But I'm assuming the intended question is:
"Which of the following could not be concluded from these experiments alone?"

But if the intended question is:
"Which of the following could not be concluded from these experiments and existing knowledge of electrons and atoms?"
then I'd say: none.

It shows the importance of asking unambiguous questions! Do you have the official answer?

Why not negative?

Because the evidence was that atoms were neutral and that electrons (negative) were part of atoms. The remainder of the atoms would therefore have to be ________ (fill in the blank!).

Who finally proposed the nucleus is positively charged and what further experimental evidence to support it?

No one 'finally proposed' it. There's a process of gradual acceptance by the scientific community. But, using the model, Bohr was able to accurately explain the spectrum of hydrogen,which was a huge step forwards. Read about about the history of atomic theory for yourself - there's plenty information out there. For example, do you know why we believe atoms exist? It's worth doing some reading as the history of physics is fascinating.

 

[songoku]

It shows the importance of asking unambiguous questions! Do you have the official answer?

Just got it. The answer is (B).

Because the evidence was that atoms were neutral and that electrons (negative) were part of atoms. The remainder of the atoms would therefore have to be ________ (fill in the blank!).

Positive. But why not considering something like "protons (positive) were part of atoms. The remainder of the atoms would therefore have to be negative"? Is it because from plum pudding model the electrons are scattered around so the "clump" one should be positive?

No one 'finally proposed' it. There's a process of gradual acceptance by the scientific community. But, using the model, Bohr was able to accurately explain the spectrum of hydrogen,which was a huge step forwards. Read about about the history of atomic theory for yourself - there's plenty information out there. For example, do you know why we believe atoms exist? It's worth doing some reading as the history of physics is fascinating.

I actually did search about Rutherford experiment and found that many sources from google search stated Rutherford was the one proposing the nucleus to be positive; that's why I said there was no correct answer for the question.

One of the source is this: https://www.aps.org/apsnews/2006/05/rutherford-discovery-atomic-nucleus

and part of the text in it is "After thinking about the problem for over a year, Rutherford came up with an answer. The only explanation, Rutherford suggested in 1911, was that the alpha particles were being scattered by a large amount of positive charge concentrated in a very small space at the center of the gold atom. The electrons in the atom must be orbiting around this central core, like planets around the sun, Rutherford proposed"

So since it maybe Rutherford who proposed the nucleus to be positively charged and the conclusion also came from alpha particle scattering experiment, we can say option (B) is wrong?

Thanks

 

[Steve4Physics]

But why not considering something like "protons (positive) were part of atoms. The remainder of the atoms would therefore have to be negative"? Is it because from plum pudding model the electrons are scattered around so the "clump" one should be positive?

I don't understand your question. It was already known that neutral atoms contained electrons (negative particles of small mass), so part of each atom must be positive. It was logical to ask how the electrons and positive charge(s) are arranged in an atom.

Since the mass of each electron is very small and electrons are (relatively) easily removed from atoms, the electrons could not be responsible for scattering of the (much larger mass) $\alpha$ particles - so it was reasonable to consider the possibility that the positive charge was responsible for the scattering.

In the plum pudding model, the positive charge is spread over a relatively large volume. This could not produce a strong enough electric field to cause large deflections of $\alpha$ particles.

So since it maybe Rutherford who proposed the nucleus to be positively charged and the conclusion also came from alpha particle scattering experiment, we can say option (B) is wrong?

IMO the original question isn't written clearly enough for a definite answer - as already explained in Post #9.

 

[songoku]

I understand. Thank you very much for all the help and explanation Steve4Physics and Orodruin

 

[Astronuc]

One of the source is this: https://www.aps.org/apsnews/2006/05/rutherford-discovery-atomic-nucleus

and part of the text in it is "After thinking about the problem for over a year, Rutherford came up with an answer. The only explanation, Rutherford suggested in 1911, was that the alpha particles were being scattered by a large amount of positive charge concentrated in a very small space at the center of the gold atom. The electrons in the atom must be orbiting around this central core, like planets around the sun, Rutherford proposed"

So since it maybe Rutherford who proposed the nucleus to be positively charged and the conclusion also came from alpha particle scattering experiment, we can say option (B) is wrong?

Consider what was understood about the electron and when (1897).

https://www.aps.org/apsnews/2000/10/discovery-of-the-electron

and the Millikan oil drop experiment.

The oil drop experiment was performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the elementary electric charge (the charge of the electron).
https://en.wikipedia.org/wiki/Oil_drop_experiment

https://physics.aps.org/articles/v5/9

https://pubs.aip.org/aapt/pte/artic...Robert-A-Millikan-and-the-Oil-Drop-Experiment