# Rutherford alpha scattering?

From Rutherfords alpha scattering, where alpha where fired at a thin peace of metal foil, he concluded that the nuclei were positively charged. He made this conclusion from the fact that alpha particles were scattered at different angles but would a negative charge not scatter the alpha the same way as a positive? As far as i can see it the scattering pattern caused by alpha been deflected of there path by an attractive force would be the same as a replusive. So why did he conclude the nucleus was positively charged??

The Geiger–Marsden experiment was performed in 1909, 12 years after the electron was identified as particle. It was already known that electron is negative and thousand times lighter than the lightest atom - hydrogen. Electrons could not scatter the alpha particles because the alpha particle is thousands of times heavier than the electron so the alpha particle would just go through. Something comparably heavy has to scatter the alpha particle and this something has to have charge of opposite sign than the electrons have for the atom to be neutral.

• Electrons certainly scatter off of alpha particles, and follow Rutherford cross sections as well. They just take very little energy, as you correctly state. As @Joseph states, either an attractive or repulsive force can cause the scattering (consider a comet and the sun with gravity a purely attractive force). They knew of the electron, knew it was part of the atom, and were looking for where the positive charge was. The observed scattering events were few in number, and large in energy transferred, pointing towards a small number of heavy scatterers, rather than the plum pudding model. May 2, 2014 at 19:05

From his experiments Rutherford concluded that the atom contained a heavy nucleus, more than about 1,000 times smaller in diameter than the atom. He also concluded that the nucleus carried a large charge, which in the case of gold he estimated to be about $$100e$$. However, he was not able to conclude from his results that the nuclear charge was positive, and pointed out that they could be equally well explained by a negative nucleus.

Although it was known that the atom was electrically neutral, it was not known that the nucleus and the electrons were the only constituents of atoms. Several models of the atom had been proposed, none of which was generally accepted. The facts that any stationary arrangement of positive and negative charges was inherently unstable (Earnshaw's Theorem), and that circulating charges would continuously radiate energy, leading to the collapse of the atom, meant that all such models were viewed with scepticism. Even Rutherford's model did not start to attract adherents until it had been revived by Bohr, who showed how it could account for atomic spectral lines.

So it could not be concluded at that time that the nucleus had to be positively charged.

The most convincing reason Rutherford gave for assuming the nucleus to be positively charged is that it would explain how alpha particles which are ejected from radioactive atoms can have such high energies.

The deductions from the theory so far considered are independent of the sign of the central charge, and it has not so far been found possible to obtain definite evidence to determine whether it be positive or negative.

It may be possible to settle the question of sign by consideration of the difference of the laws of absorption of the β particles to be expected on the two hypothesis, for the effect of radiation in reducing the velocity of the β particle should be far more marked with a positive than with a negative center.

If the central charge be positive, it is easily seen that a positively charged mass if released from the center of a heavy atom, would acquire a great velocity in moving through the electric field. It may be possible in this way to account for the high velocity of expulsion of α particles without supposing that they are initially in rapid motion within the atom.

The reason he concluded that it was a positive nucleus was because the alpha particles, a very small fraction of them got deflected directly back where they came from (or to be precise, angles greater than 90 degrees). This could not have been caused by electrons as they were big enough for the fraction to be significantly higher, plus he already proved that alpha particles where positively charged (+2), even though he didn't know it was actually itself a nucleus. So only a massive but small positive charge concentrated in a region could have caused this big deflection.