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Question on Rutherford scattering experiment

How did Rutherford conclude that most of an atoms mass is concentrated in its nucleus when he did the scattering experiment? In the textbook answer it says the evidence were the large angle scatterings and some coming back, it says 'this can only occur if the alpha particle collides with another particle much greater than its own mass'.

But surely you could have - a not very massive nucleus (when I say massive I mean relative mass) but was just very highly charged and the electrostatic repulsion was great enough to cause the alpha particle to bounce back or be scattered large angles depending on its proximity to the nucleus? just because some bounced back or scattered large angles does not mean the nucleus mass is concentrated in its centre?
Original post by DonWorryJockIsHere
How did Rutherford conclude that most of an atoms mass is concentrated in its nucleus when he did the scattering experiment? In the textbook answer it says the evidence were the large angle scatterings and some coming back, it says 'this can only occur if the alpha particle collides with another particle much greater than its own mass'.

But surely you could have - a not very massive nucleus (when I say massive I mean relative mass) but was just very highly charged and the electrostatic repulsion was great enough to cause the alpha particle to bounce back or be scattered large angles depending on its proximity to the nucleus? just because some bounced back or scattered large angles does not mean the nucleus mass is concentrated in its centre?


You're making the assumption that the nucleus the alpha collides with can't move. At these energies, of only a few MeV, the actual alpha-nucleus interaction is mediated by the Coulomb interaction anyway, and the charges simply dictate the height of the barrier. However, there will be momentum transfer during a collision. Think about colliding a relatively large alpha with a relatively small nucleus (ie colliding a cannon ball with a table tennis ball). Can you think of any way the cannon ball will be deflected substantially? No, because the cannon ball has suffered little change in momentum, but the table tennis ball has suffered a large change.

So if a 'heavy' alpha hits a 'light' target, the target nucleus gets dislodged and the alpha is relatively unaffected
Original post by Cora Lindsay
You're making the assumption that the nucleus the alpha collides with can't move. At these energies, of only a few MeV, the actual alpha-nucleus interaction is mediated by the Coulomb interaction anyway, and the charges simply dictate the height of the barrier. However, there will be momentum transfer during a collision. Think about colliding a relatively large alpha with a relatively small nucleus (ie colliding a cannon ball with a table tennis ball). Can you think of any way the cannon ball will be deflected substantially? No, because the cannon ball has suffered little change in momentum, but the table tennis ball has suffered a large change.

So if a 'heavy' alpha hits a 'light' target, the target nucleus gets dislodged and the alpha is relatively unaffected


I understand what you are saying with the change of momentum, but could you please elaborate on this point I don't know what you are saying.
Original post by DonWorryJockIsHere
How did Rutherford conclude that most of an atoms mass is concentrated in its nucleus when he did the scattering experiment? In the textbook answer it says the evidence were the large angle scatterings and some coming back, it says 'this can only occur if the alpha particle collides with another particle much greater than its own mass'.

But surely you could have - a not very massive nucleus (when I say massive I mean relative mass) but was just very highly charged and the electrostatic repulsion was great enough to cause the alpha particle to bounce back or be scattered large angles depending on its proximity to the nucleus? just because some bounced back or scattered large angles does not mean the nucleus mass is concentrated in its centre?


Most atoms would just be emitted and pass straight through, some reflected back because of the charges, this suggested that the positively charged nucleus was relatively dense, small and in the centre of the atom...

I think it's explained better in the chemistry "Atom evolution" section...

http://www.nobeliefs.com/atom.htm
Original post by DonWorryJockIsHere
I understand what you are saying with the change of momentum, but could you please elaborate on this point I don't know what you are saying.


Because both alpha particle and nucleus are positively charged, the Coulomb interaction between them presents an energetic barrier to collision and, if the particles are physically to come into contact, then the alpha has to get over this barrier. At alpha particle energies (about 5 MeV), that doesn't happen, so there won't be direct contact between them.

Think of it as being a bit like trying to push two magnets together, north to north. It gets harder and harder, the closer they are, and it's really very difficult to get them to actually touch.

Hope this helps

C

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