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I have read everywhere that the gold foil experiment performed by Rutherford was done hoping that the alpha particles face only a little deflection/no deflection however this doesn't sit right with me, the old model of Thomson clearly shows that atom is a positive sphere with electrons embedded in it, what made him think that positively charged alpha particles moving through this atom wont face a repulsive force from the positively charged cloud?

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    $\begingroup$ rutherford predicted the range of electrostatic scattering angles based on the thompson model before he did his experiment and found the result did not match the prediction. you should probably read his original paper to understand his approach. $\endgroup$ Jul 9, 2023 at 17:56
  • $\begingroup$ @nielsnielsen would a 11th grader understand it? i skimmed through it its not really comprehensible. $\endgroup$
    – sanya
    Jul 9, 2023 at 17:58
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    $\begingroup$ Then I suggest you search on "rutherford scattering experiment" until you find one that is comprehensible. $\endgroup$ Jul 9, 2023 at 18:06
  • $\begingroup$ @nielsnielsen well thats why i asked this question on here $\endgroup$
    – sanya
    Jul 9, 2023 at 18:13
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    $\begingroup$ see the wiki article on the experiment, en.wikipedia.org/wiki/Geiger%E2%80%93Marsden_experiments "According to Thomson's model, if an alpha particle were to collide with an atom, it would just fly straight through, its path being deflected by at most a fraction of a degree" $\endgroup$
    – anna v
    Jul 9, 2023 at 18:16

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The Thomson or "plum pudding" model of the atom assumed a cloud of positive charge within which electrons orbited. In this model, the positive charge was so spread out that even if an alpha particle passed right through an atom, it would suffer only a very small deflection. The results of Rutherford's experiment showed that the positive charge and most of the mass of an atom is concentrated in a small volume (the atomic nucleus), leading to large deflection angles (which implies a large change in momentum) for the small proportion of the alpha particles that passed close to a nucleus. The proportion of alpha particles that were deflected through large angles allowed Rutherford to estimate the size of the nucleus in a model known as the Rutherford model of the atom.

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  • $\begingroup$ @sanya You have to work out the kinematics to know that small slow masses never deflect fast moving masses by much. It is not very difficult to do; it is the same argument as in basic physics of billiard ball collisions. $\endgroup$ Jul 10, 2023 at 1:36
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Atoms are neutral, so the mean density of negative charge (mean charge per unit volume) due to the electrons would be the same as the density of positive charge due to the rest of the atom (an atom-sized sphere of positively charged matter in Thomson's theory). Therefore the mean repulsive force on the alpha particle from the foil would be zero.

Alpha particles were expected to penetrate the foil, but suffer many very small deflections due to close encounters with electrons. These deflections would be very small owing to the alpha particle's mass being thousands of times greater than the electron's mass.

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  • $\begingroup$ i dont understand what you mean by mean repulsive charge though i tried to understand some parts and i have come up with something if you can verify or add to it. the positive charges were distributed in the atom so most likely it wasnt very dense, as a result the individual protons present here and there werent strong enough to repel the alpha particles? though im yet to understand why something with a lighter mass would have a negligible electric field, maybe because the alpha particles are so pumped with K.E that losing a bit in overcoming the repulsive force doesnt affect it $\endgroup$
    – sanya
    Jul 9, 2023 at 18:36
  • $\begingroup$ (a) I wrote "repulsive force". I was referring to the last line of your question. (b) There were no "individual protons" in Thomson's model (the model which Rutherford expected – mistakenly – to verify). In Thomson's model the atom was a homogeneous positively charged sphere with electrons embedded in it. $\endgroup$ Jul 9, 2023 at 18:43
  • $\begingroup$ "The mean density of negative charge (mean charge per unit volume) due to the electrons would be the same as the density of positive charge due to the rest of the atom, since atoms are neutral. Therefore the mean repulsive force on the alpha particle from the foil would be zero." Basically i dont understand this how having same mean density proves that the repulsive force would be zero. $\endgroup$
    – sanya
    Jul 9, 2023 at 18:47
  • $\begingroup$ The foil as a whole is neutral. The individual atoms are neutral. I said that the mean charge density of negative charge was the same as that for positive. $\endgroup$ Jul 9, 2023 at 19:05
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    $\begingroup$ No... For an atom with a small-sized positive nucleus of large mass and electrons orbiting it, when an alpha particle penetrates the electron's orbits and gets near the nucleus, the electrons, being roughly symmetrically disposed about the alpha particle, exert almost no resultant electrostatic force on the alpha. The only significant force on the alpha is repulsion by the nucleus. $\endgroup$ Jul 9, 2023 at 21:06
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This is an extended comment. The |Thomson model of the atom, called plumb pudding,

plumb puddingmodel

The plum pudding model of the atom, as envisioned by Thomson

The Thomson atom is a sphere of electric charge anchored in space by its mass. Thus the alpha particle will not bounce off the atom like a tennis ball hitting a basketball, but will pass right through if the atom's electric fields are weak enough to permit it. Thomson's model predicted that the electric fields in an atom are too weak to affect a passing alpha particle much, given how fast and heavy alpha particles are. Both the negative and positive charges within the Thomson atom are spread out over the atom's entire volume, and Rutherford had calculated that this volume was too large for strong deflection to happen.

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  • $\begingroup$ The Thomson atom is a sphere of electric charge anchored in space by its mass. Can you explain this? $\endgroup$
    – sanya
    Jul 9, 2023 at 18:49
  • $\begingroup$ "Both the negative and positive charges within the Thomson atom are spread out over the atom's entire volume" and the volume is that of a sphere with the dimensions of an atom as it was known then. $\endgroup$
    – anna v
    Jul 9, 2023 at 19:24

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