# How did Rutherford's gold foil disprove the plum pudding model?

What stops one of the two following scenarios from happening, consistent with the plum pudding model?

1. The $\alpha$ particle, attracted by the electrons on the outer shell of the pudding, orbits nearly parabolically around the atom, causing the near-180 degree deflection angle seen.

2. The $\alpha$ particle hits a plum pudding atom directly, and because the atom consists largely of positive charge, it is deflected by nearly 180 degrees.

Followup: did they know anything about the spacing of these plum-pudding atoms? Did they expect them to be lined up in a grid-like way; difficult to penetrate?

I am a physics undergraduate, I've only taken an intro QM class, it would be nice if that were kept in mind while answering.

• Have a look at this and its questions hyperphysics.phy-astr.gsu.edu/hbase/nuclear/rutsca2.html Dec 1 '14 at 8:15
• Hi user3347826, and welcome to Physics Stack Exchange! I removed your last sentence (while I was fixing up some other things about your question) because "you should know..." is physicists' way of warning you that you won't understand what they're about to write unless you already know some other fact, without offending people who already do know that fact. It's not meant to be patronizing. (I'm not claiming it's the best way to say that, either.) Of course, anything that is outright rude is not acceptable here, and please do report it if you see any such thing. Dec 1 '14 at 8:22
• The nucleus in the plum-pudding model doesn't consist largely of positive charge. It's a neutrally charged jumble of protons and electrons. Dec 1 '14 at 8:54

## 3 Answers

The $\alpha$ particle, attracted by the electrons on the outer shell of the pudding, orbits nearly parabolically around the atom, causing the near-180 degree deflection angle seen.

This wouldn't happen because of momentum conservation. It was reasonably established in 1909 (when the gold foil experiment was done) that electrons were light, so if an alpha particle were to be reflected by interaction with an electron, the electron would be kicked out of the gold atom with even higher velocity in the opposite direction. $v_e \approx \frac{m_\alpha}{m_e}\Delta v_\alpha$

Besides, in the plum pudding model, the electrons are distributed throughout the atom, not all on the surface. The same reasoning applies, though, regardless of how the electrons are distributed.

A gold nucleus is much more massive than an alpha particle, so it can reflect the alpha particle without recoiling very strongly itself.

The $\alpha$ particle hits a plum pudding nucleus directly, and because the nucleus consists largely of positive charge, it is deflected by nearly 180 degrees.

The defining feature of the plum pudding model is that there was no nucleus of positive charge. What you're describing here is exactly what did happen, it just wasn't a prediction of the plum pudding model.

• 1st part: it seems like to finish off the logic here we need the fact: "Rutherford searched for and did not detect any electrons scattered from the foil", but after looking through the paper here it doesn't seem like he did that: chemteam.info/Chem-History/Rutherford-1911/Rutherford-1911.html As to why he expected small angle deflections: he referenced an experimental paper on $\alpha$ deflections in matter by Geiger and then said he calculated the probabilities of large deflections to be "exceedingly small" by "the theory of probability". Dec 3 '14 at 3:21

This is the plum pudding model of the atom

Left: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. Right: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge.

There are no electrons and no outer shells, quantum mechanics was yet to come when the experiment was done and the plum pudding proposed. They knew of atoms, and they though since they were neutral that the charges and masses were distributed evenly within the the atom.

. According to the then-dominant plum-pudding model of the atom, where the small negative electrons were spread through a diffuse positive region, backscattering of the high-energy positive alpha particles should have been nonexistent.

Parabolic as a solution could not come to 180 degrees because the masses were considered similar in the plum pudding.

There are no nuclei in the plum pudding.

Based on thomson's model, all of the alpha particles should go through or reflect back but in Rutherford's experiment, it was more of in between, some reflecting back and some going through, disproving the theory.

• I'm looking more for specifics; I want thorough reasoning that justifies the conclusions claimed to be made by the experiment. Dec 1 '14 at 7:52
• @user3347826 - the Wikipedia entry for the Geiger-Marsden experiments contains a link PDF's of the original papers from 1909 - 1913. Dec 1 '14 at 22:04