What Surprised Geiger in the Gold Foil Experiment on Atom Structure?

[pvnrt]

Hi, I'm confused a little about the gold foil experiment.
I tried to put myself in Geiger's shoes, the author of the experiment.
I'm firing alpha particles at a gold film, given that the plum pudding model is the pinnacle of human understanding of the atom, meaning I believe matter is made of indivisible particles - amorphous, negatively charged pieces of mass, containing electrons, which neighbour one another, why would I expect all the alpha particles to be able to pass through the gold relatively undisturbed rather than the vast majority of them bouncing off due to the multilayer obstacle of gold plum pudding atoms?

 

[Charles Link]

At that time, they didn't think that matter contained any atomic-sized, basically point, positive charges (that also contained a relatively sizable mass) that would be able to repel or scatter the alpha particles. If the positive charge were more uniformly distributed, (as in the plum pudding model), the scattering would not occur. They also would not have guessed that these point positive charges would contain not only one positive charge, but in the case of gold, 79 positive charges, localized almost to a point. (It may also be worth pointing out that the scattering that occurs is due to electrostatic forces which are long range, (with inverse square law), rather than some kind of interaction due to the mass of the particles).

 

[sophiecentaur]

why would I expect all the alpha particles to be able to pass through the gold relatively undisturbed rather than the vast majority of them bouncing off due to the multilayer obstacle of gold plum pudding atoms?

It isn't clear what you mean here. You would (well, I would) expect some interaction and scattering for all incident particles if you assumed that the mass of the atom is spread about within the volume it appears to occupy, The actual experimental result was a surprise because it was only a tiny proportion of the particles that were affected to any degree. The model of all the mass in the centre was revolutionary but it was needed in order to explain the results.