Did Ernest Rutherford know that the alpha particles he used to shoot at a gold leaf were helium nuclei?
During Rutherford's gold leaf experiment, he shot alpha particles (helium nuclei) at a piece of gold foil and observed that while some particles went straight through, others bounced back, suggesting that atoms consisted of some kind of positive matter. This would mean that Rutherford knew that the particles he was using were positive. If he knew they were helium nuclei, then he would have already known that atoms have a positive nucleus. So what did he think the alpha particles were?
Ernest Rutherford was a British chemist and one of the key people in the development in the modern understanding of the atom and nuclear physics. He did so much important work that he actually did his most famous work (the gold foil scattering experiment and development of the proton) after already winning the Nobel Prize. Prior to the gold foil experiment, Rutherford had already concluded that alpha particles were some type of charged helium atom. The purpose of the gold foil experiment was not really to determine the charge of alpha particles but to determine the distribution of the mass and charge of an atom. The "plum pudding" model of the atom at the time posited that the density of an atom was evenly distributed and the alpha particles should all pass through the foil. But the fact that they found that a small percentage of the particles were deflected at high angles showed that the mass of the atom was mostly concentrated in a very small area which we now know as the nucleus. Also, the nucleus contained the positive charge of the atom since it deflected the positively charged alpha particles. So this famous gold foil experiment led directly to the Rutherford model of the atom and the discovery of the proton.