Rutherford's Alpha particles experiment

  • #1
According to JJ Thomson's atomic model, Rutherford expected deflections of alpha particles through small angles. I'm unable to understand why he had expected "small angles". Can someone please explain.
 

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  • #2
jfizzix
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Before the Rutherford experiments, it was very popular to believe that an atom consisted of dot-like electrons suspended in a spread out positively charged cloud like a plum pudding (this being known as the plum pudding model of the atom).

If that were the case, then the alpha particles would mostly pass through or get deflected by small angles, because the only hard objects to scatter off of would be the tiny (and relatively light) electrons, which were much lighter than the alpha particles.

Since some of the alpha particles were actually deflected at large angles, the model of the atom was revised to be a small positively charged nucleus surrounded by orbiting negatively charged electrons.

Interesting side question:
At the time that those experiments were done, did scientists understand alpha particles to be charged nuclei of helium, or were they just thought of as different particles all their own?
Also:
Where'd the plum pudding model come from in the first place?
 
  • #3
SteamKing
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Before the Rutherford experiments, it was very popular to believe that an atom consisted of dot-like electrons suspended in a spread out positively charged cloud like a plum pudding (this being known as the plum pudding model of the atom).

If that were the case, then the alpha particles would mostly pass through or get deflected by small angles, because the only hard objects to scatter off of would be the tiny (and relatively light) electrons, which were much lighter than the alpha particles.

Since some of the alpha particles were actually deflected at large angles, the model of the atom was revised to be a small positively charged nucleus surrounded by orbiting negatively charged electrons.

Interesting side question:
At the time that those experiments were done, did scientists understand alpha particles to be charged nuclei of helium, or were they just thought of as different particles all their own?
Yes, it was understood that alpha particles were in fact helium nuclei. Rutherford himself had done experiments which proved such:

http://en.wikipedia.org/wiki/Alpha_particle

Rutherford was the scientist who classified radiation into the three types known today: alpha, beta, and gamma.

Also:
Where'd the plum pudding model come from in the first place?
The plum pudding model was proposed by JJ Thompson in 1904. This model was later shown to be incorrect once experiments showed that the atom has a nucleus.
Again, Rutherford provided the key analysis of the experimental data which showed the the plum pudding model was not an accurate representation of the atomic structure:

http://en.wikipedia.org/wiki/Plum_pudding_model
 
  • #4
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According to JJ Thomson's atomic model, Rutherford expected deflections of alpha particles through small angles. I'm unable to understand why he had expected "small angles". Can someone please explain.
You would expect small angles unless the mass is distributed very unevenly throughout the atom, and before Rutherford there was no particular reason to expect such an uneven distribution. It's easy to overlook just how extraordinary the mass distribution in an atom is: for example the nucleus of a hydrogen atom occupies about one one-billionth of the volume of the atom, yet represents 99.9% of the mass.

One of Rutherford's contemporaries (I do not remember who) remarked that the large angles were like firing a naval gun at a sheet of paper and having the shell sometimes bounce back instead of going through; that's not something that you'd expect. (My favorite image is to consider how surprised we'd be to find that a one-ton elephant is actually an elephant-sized cloud of elephant-colored weightless vapor surrounding a one-ton mosquito).
 
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  • #5
SteamKing
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One of Rutherford's contemporaries (I do not remember who) remarked that the large angles were like firing a naval gun at a sheet of paper and having the shell sometimes bounce back instead of going through; that's not something that you'd expect. (My favorite image is to consider how surprised we'd be to find that a one-ton elephant is actually an elephant-sized cloud of elephant-colored weightless vapor surrounding a one-ton mosquito).
It was Rutherford himself who made this quote, in a lecture delivered at Cambridge U:
It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.
http://en.wikipedia.org/wiki/Geiger–Marsden_experiment
 
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  • #6
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It was Rutherford himself who made this quote, in a lecture delivered at Cambridge U:
Yes, that's the quote I was thinking of... Thanks.
 
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According to JJ Thomson's atomic model, Rutherford expected deflections of alpha particles through small angles. I'm unable to understand why he had expected "small angles". Can someone please explain.
To fix the ideas, if during a match a player kicks some penalties, the ball (alpha particle) may have more collisions with a lot grass (electrons), deviating a little, but it will continue to move in that direction; if the player can kick the ball off a goal post (nuclei), the same ball would bounce off.

Since nuclei was unknown and one was inducted to think that it was always possible to cross the atom, the fist time the opposite phenomenon was observed, someone said that it was like to shoot at a paper (atom) and then to see the bullet (alpha particle) bouncing back...
 
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