 The arrangement for the experiment looked something like this. There was a gold foil, there was a source of alpha particles, and there was a zinc sulfide scintillator telescope. When an alpha particle hits the zinc sulfide screen, it lets out a little burst of light, and they could see the burst of light using a telescope. It was also possible to move the zinc sulfide telescope through a range of angles. The experiment was extremely time consuming. Geiger and Marsden had to get their eyes adjusted to the dark and then would sit in the dark for hours at a time, counting the little burst of light as the alpha particles bounced off the gold foil and into the scintillator. At the time of the experiment, scientists knew that atoms were normally neutral, and the common picture for what an atom looked like was something like this. This was known as the plum pudding model. There was a sea of positive charge and an equal amount of negative charge in the form of electrons occupying the volume of the atom. Remember that Rutherford had already identified that alpha particles were positively charged helium atoms. Because the plum pudding model has a distribution of charge, if an alpha particle was to come close, it would go on a straight line and would not be deflected very much at all. This meant it was a very big surprise when it was discovered that sometimes the alpha particles bounced straight backwards. When they observed that the alpha particles could bounce backward, Rutherford was amazed. He later gave a lecture at Cambridge University and he said the following. It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realised that this scattering backward must be the result of a single collision. And when I made calculations, I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre carrying a charge. If all the positive charge is in a tiny nucleus at the centre of the atom, we know the atom is neutral so the negative charge must be situated further away and we now know that the electrons move in orbits around the nucleus. During the 1920s, Rutherford and other scientists began to use alpha particles to convert one element into another, the first nuclear reactions. In many of these reactions, they observed hydrogen being emitted and they soon realised that a hydrogen nucleus is another fundamental particle that they called the proton. They then observed that some elements can have nuclei with different masses and this eventually led to the discovery of the neutron by Chadwick in 1932. By this stage, all the ingredients for our understanding of the atom were now known. The nucleus at the centre of the atom contains protons and neutrons and orbiting the nucleus are electrons. In a neutral atom, the number of protons is the number of electrons. As an example, here is a schematic drawing of lithium. Lithium has three protons and usually has four neutrons and these three protons and four neutrons sit in the middle in the nucleus and surrounding it are three electrons moving in orbits. The next step we need to undertake is to try and understand how and why the protons and neutrons stay together and combine to make this tiny nucleus at the middle of an atom. See you in the next lecture.