Rutherford's Experiment: Explaining Why Positive Charges Repel

[t1mm3h]

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?

 

[Simon Bridge]

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.

 

[jtbell]

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.

 

[Simon Bridge]

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.

 

[t1mm3h]

Ah makes sense. Thanks for all the replies :) I can continue reading.

 

[student#x]

Hi. I am still having trouble with the fundamental concepts of the Rutherford experiment.

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?

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]

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...

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.

 

[Simon Bridge]

Probably best to get a better picture of what Thompson had in mind... have you tried looking for other sources?
I.e. http://en.m.wikipedia.org/wiki/Plum_pudding_model