Rutherford experiment

  1. 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?
    Last edited: Dec 17, 2013
  2. jcsd
  3. Simon Bridge

    Simon Bridge 14,417
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    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.
  4. jtbell

    Staff: Mentor

    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.
  5. Simon Bridge

    Simon Bridge 14,417
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    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.
  6. Ah makes sense. Thanks for all the replies :) I can continue reading.
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