Why was it expected for alpha radiation to pass through the Plum Pudding model?

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username1204031
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I'm watching a lecture right now about the discovery of the nucleus, and apparently it was expected for a-particles to pass through the plum pudding atom in Rutherford's experiments. I know what the backscatter proves, etc, but the one thing I don't understand is why alpha radiation is expected to pass through?

a-particles are positively charged, and the pudding part of the atom is positively charged. So why did Rutherford expect all radiation to pass through the gold foil? Shouldn't it be expected that the radiation is deflected by the positive charge?
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Arkasia
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Rutherford's experiments were to test the plum pudding model, namely, the theory that an atom is a sphere is positive charge with negatively charged electrons dotted around the outside. He used Alpha particles because he knew they were positively charged, and so he wanted to test what happened when he bombarded the gold foil with them. He didn't know they were going to get through, that was the whole point of his experiment. Since some of the atoms got through, but only a tiny minority deflected, Rutherford concluded that only a tiny part of an atom is positively charged, with the rest being mostly empty space, and so they threw out the idea of the 'plum pudding' model (in favour of the nuclear model), which would have yielded different results.
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Stevenebrey
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I'm with JordanL we learn that Rutherford expected the alpha particles to pass through, that was the working hypothesis. Why though? I'd have thought if atoms are plum puddings ie no empty space then the alpha particles would have been deflected. So why was his hypothesis not that most of the alpha particles would be deflected
(Original post by Arkasia)
Rutherford's experiments were to test the plum pudding model, namely, the theory that an atom is a sphere is positive charge with negatively charged electrons dotted around the outside. He used Alpha particles because he knew they were positively charged, and so he wanted to test what happened when he bombarded the gold foil with them. He didn't know they were going to get through, that was the whole point of his experiment. Since some of the atoms got through, but only a tiny minority deflected, Rutherford concluded that only a tiny part of an atom is positively charged, with the rest being mostly empty space, and so they threw out the idea of the 'plum pudding' model (in favour of the nuclear model), which would have yielded different results.
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Joinedup
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(Original post by Stevenebrey)
I'm with JordanL we learn that Rutherford expected the alpha particles to pass through, that was the working hypothesis. Why though? I'd have thought if atoms are plum puddings ie no empty space then the alpha particles would have been deflected. So why was his hypothesis not that most of the alpha particles would be deflected
The plum pudding model was that a neutral atom was like a cloud of positive charge with electrons stuck in it... at the scale of an alpha particle each atom is a region of neutral charged space.

The pudding killer was that a tiny number of alphas were deflected at high angles which wasn't possible with the low density cloud of positive charge with electrons in it, but is possible with a compact and dense positive nucleus.
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Stevenebrey
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Thanks for responding JoinedUp - great explanation!
(Original post by Joinedup)
The plum pudding model was that a neutral atom was like a cloud of positive charge with electrons stuck in it... at the scale of an alpha particle each atom is a region of neutral charged space.

The pudding killer was that a tiny number of alphas were deflected at high angles which wasn't possible with the low density cloud of positive charge with electrons in it, but is possible with a compact and dense positive nucleus.
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