In 1909, Ernest Rutherford and his assistants, Hans Geiger and Ernest Marsden, conducted a series of experiments at the University of Manchester, where they fired a beam of alpha particles at a thin gold foil. They surrounded the foil with a fluorescent zinc sulphide screen to detect the deflection of the alpha particles. As each alpha particle struck the screen, it produced a burst of light called a scintillation, which was visible through a viewing microscope. To their surprise, they observed that most of the alpha particles passed through the foil, but some were deflected at large angles, and a very few even bounced back toward the source. This was contrary to the plum pudding model, which predicted that the alpha particles would only experience slight deflections due to the diffuse positive charge of the atom. Rutherford later said that this result was "as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you" He realized that this phenomenon could only be explained if the atom had a small, dense, positively charged core called a nucleus, in which nearly all the mass was concentrated, and that the rest of the atom was mostly empty space with electrons orbiting around the nucleus. He calculated that the nucleus was about 1/100,000ᵗʰ the size of the atom, and that it contained most of the positive charge and mass of the atom. He also proposed that the nucleus was composed of smaller particles called protons, which had a positive charge equal to that of an electron but opposite in sign. Rutherford’s experiment was a major breakthrough in understanding the structure and nature of atoms. It also paved the way for further discoveries such as neutrons, isotopes, nuclear fission, and nuclear fusion. It is considered one of the most important experiments in the history of physics and Rutherford is considered as 'the father of nuclear physics'. [Lab image courtesy Science and Society Picture Library]