Rutherford's Experiment: Explaining Why Positive Charges Repel

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Discussion Overview

The discussion revolves around the interpretation of Rutherford's experiment and the implications of Thomson's model of the atom. Participants explore the behavior of positive alpha particles when interacting with gold foil, questioning why one would expect them to pass through rather than be repelled or deflected.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the expectation that alpha particles would pass through gold foil based on Thomson's model, arguing that positive charges should repel each other.
  • Another participant suggests that the presence of negative charges in Thomson's model would attract the alpha particles, potentially cancelling out the repulsion from positive charges.
  • A participant describes Thomson's model as a "plum-pudding model," where positive charges are embedded in a uniform negative charge, and explains how this might allow alpha particles to pass through without significant deflection.
  • One participant compares the interaction of alpha particles with the atomic model to a larger mass moving through a crowd, suggesting that while deflection occurs, complete reversal is unlikely.
  • Another participant raises questions about why alpha particles can penetrate the atom without being stuck or ripping electrons out, seeking clarification on the nature of atomic structure.
  • Concerns are expressed about the mechanics of scattering, with a participant drawing parallels to billiard balls and discussing the dynamics of collisions in the context of atomic interactions.
  • One participant notes the difference in mass between alpha particles and electrons, suggesting this may influence the outcomes of interactions in the atomic models.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of alpha particles in relation to Thomson's model and Rutherford's findings. There is no consensus on the expectations for particle behavior or the implications of the atomic models discussed.

Contextual Notes

Participants highlight various assumptions about atomic structure and interactions, including the roles of positive and negative charges, the nature of scattering, and the mass differences between alpha particles and electrons. These assumptions remain unresolved within the discussion.

t1mm3h
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Hello, I have just started reading my first chemistry book and have a question about Rutherford's experiment.

As stated in the book:
Thomson's model of the atom suggests that positive and negative charges were evenly distributed around the atom. Then Rutherford came with his experiment: he shot tiny alpha particles at a thin sheet of gold foil. Alpha particles have a positive charge.

So far so good. But I don't get the following part:

"If atoms look like Thomson's model, you'd expect the positive alpha particles to fly on through the gold foil, with maybe slight deflections when they get near the mixture of positive and negative charges in the gold atoms. "

Why would you expect this? I wouldn't. With Thomson's model (positive charges, protons, evenly distributed) I would expect the positive particles to deflect / bounce back when shot at the gold foil (full with atoms with evenly distributed charges). Because positive charges repel positive charges.

Can somebody explain me why you would expect what the book suggets to expect?
 
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Why would you expect this? I wouldn't. With Thomson's model (positive charges, protons, evenly distributed) I would expect the positive particles to deflect / bounce back when shot at the gold foil (full with atoms with evenly distributed charges). Because positive charges repel positive charges.
You have forgotten about the effect of the negative charges.

The negative charges would attract the alphas - pretty much cancelling the effect of the repulsion of the positive charges.
 
Also, in Thomson's model the positive charges are small "raisins" scattered around inside a more uniform "pudding" of negative charge. Hence the common name "plum-pudding model" after a certain English Christmas-time concoction.

Instead of raisins, think of small metal balls. Now fire another one of those metal balls into the pudding. It will often go right through the pudding without hitting any of the embedded balls. If it does hit one of the embedded balls, it will "scatter", but always in the forward direction (if the balls have the same mass), never backwards.

In Rutherford's model, all the embedded balls are glued together at the center of the pudding, with a large total mass. A ball fired at it can rebound backwards (think of a ping-pong ball bouncing off a bowling ball), which is what Rutherford first observed.
 
Yah - the alpha going through the Thomson model of the foil would be like a giant (4x the mass remember) shouldering his way through a crowd or regular sized people. He'll get deflected sure - but he won't get pushed right back.
 
Ah makes sense. Thanks for all the replies :) I can continue reading.
 
Hi. I am still having trouble with the fundamental concepts of the Rutherford experiment.

Simon Bridge said:
The negative charges would attract the alphas - pretty much cancelling the effect of the repulsion of the positive charges.

If the negatively charged "pudding" would attract the alpha particles, why do the alphas go through the foil? Why do they not get stuck or rip electrons out (for the Rutherford or Thomson model)? I guess what I'm asking is: Why is the electron part of the atom so soft and penetrable?

jtbell said:
Instead of raisins, think of small metal balls. Now fire another one of those metal balls into the pudding. It will often go right through the pudding without hitting any of the embedded balls. If it does hit one of the embedded balls, it will "scatter", but always in the forward direction (if the balls have the same mass), never backwards.

Why can it not scatter backward, like a "glancing blow"? This happens in pool all the time and is why I'm constantly pocketing the cue ball...

I know these questions are low level, but if anyone can help I'd appreciate it.
 
jtbell said:
Instead of raisins, think of small metal balls. Now fire another one of those metal balls into the pudding. It will often go right through the pudding without hitting any of the embedded balls. If it does hit one of the embedded balls, it will "scatter", but always in the forward direction (if the balls have the same mass), never backwards.

student#x said:
Why can it not scatter backward, like a "glancing blow"? This happens in pool all the time and is why I'm constantly pocketing the cue ball...

Let's make sure we're thinking about the same situation. In my admittedly limited experience, when I shoot the cue ball head-on at another (single) ball, the cue ball pretty much comes to a stop and the struck ball goes forward at practically the same speed as the incoming cue ball. Any deviations from "comes to a stop" and "same speed" are probably due to effects from the balls rolling (not sliding frictionlessly without rotation), and from friction between the ball and table if a ball slides briefly after impact instead of rolling immediately.

In a glancing collision, both balls go off in a generally forward direction, at various angles depending on the "glancingness" of the collision, but one of them never comes back in my general direction, except maybe in a very glancing collision that causes the balls to spin and introduce frictional effects. Without spin and friction, it wouldn't happen.

If you have multiple scattering (cue ball hits ball A which in turn hits ball B, etc.), then I suppose you could have the final ball coming out in a backwards direction. Then it comes down to how often you would expect multiple scattering in Thomson's model of the atom, and how much energy you'd expect a backwards-scattered particle to have. I suspect that the probability of a backwards-scattered particle with large energy might not be absolutely zero, but it would still be much smaller than with Rutherford's model.

Also consider that Rutherford was using alpha particles as his projectiles, and the "plums" in Thomson's model were electrons IIRC (embedded in a positively charged "pudding"). Alpha particles are much more massive than electrons.
 
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