# Could a negatively charged nucleus have caused the results in the Rutherford scattering experiment? [duplicate]

Struggling to understand the answer to this question.

When alpha particles are directed at a thin gold foil it is found that most of the alpha particles go straight through undeflected. However a very small number are scattered through angles greater than 90°.

Which of the following is not a valid conclusion?

1. The atom is mainly empty space.
2. The nucleus must be positively charged.
3. The nucleus must contain most of the mass of the atom.
4. There is a large charge concentration in the centre of the atom.

The answer is 2. I assumed that a negative nucleus would not scatter alpha particles in this way. Could it?

• I am not sure that (3) is a valid conclusion either - unless you make other assumptions about things that you know about the atom. Commented Dec 12, 2017 at 19:10
• @sammygerbil No he's not. He's asking if the differential scattering cross-section (the thing the Geiger-Marsden experiment measured) between like charged particles is different from that between unlike charged particles. Commented Dec 12, 2017 at 19:33
• @dmckee Yes, thanks. I read the final question too literally, and out of context. I have edited the question. Commented Dec 12, 2017 at 19:49
• (3) is indeed a valid conclusion. To backscatter requires that the mass of the atom is concentrated, and if not in the nucleus where? Commented Dec 12, 2017 at 22:19
• Thank you, I can see now that its a duplicate question!. So how does an attractive force cause scattering of greater than 90 degrees? Surely thats repulsion? Commented Dec 13, 2017 at 9:34

The Coulomb scattering angular dependence is independent of the sign of the charge. It goes as $(Z_1 Z_2)^2$. See https://en.wikipedia.org/wiki/Rutherford_scattering for instance.
• In principle the full $\alpha + \mathrm{Au}$ interaction includes the nuclear force (that is it is not just the Coulomb force) in the attractive case and will be slightly different for repulsive and attractive forces for small impact parameters. In practice, I doubt the Geiger-Marsden apparatus had the sensitivity to detect the difference. Commented Dec 12, 2017 at 19:08