Ernest Rutherford’s 1911 gold foil experiment showed that the positive charge and most of the mass of an atom was found in the center of the atom. He called this center of the atom the nucleus. Most of the alpha particles passed through the empty space in the gold foil so they were not deflected. If an alpha particle came close to a nucleus of a gold atom, it was repelled by the positive charge of the nucleus and it was deflected slightly. An alpha particle is positively charged and is one of three forms of radioactivity. Since like charges repel and opposite charges attract, the deflection of alpha particles indicated that the center of the atom was also positively charged. If an alpha particle actually collided with the nucleus of a gold atom, the alpha particle was deflected at a ninety degree angle or larger. When Rutherford measured the fraction of alpha particles that were deflected by large angles, he was able to estimate the size of the nucleus. The gold foil experiment proved that most of the volume of an atom is empty space. A gold foil was used in this experiment because it could be made very thin, 0.00004 centimeters thick. The gold foil was about 2000 atoms thick. Ernest Rutherford reflected on his observation of how alpha particles passed through a gold foil by saying, It was quite the most incredible event that has ever happened 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.
Rutherford also observed that the alpha particle looked to have a mass about the size of a helium atom. He observed that the alpha particle also acted like a helium atom.
By 1932 further experiments showed that the positive charge in the nucleus comes from protons and that most of the mass in the nucleus comes from neutrons. The negatively charged electron was found to be orbiting the nucleus. Rutherford was not able to explain why the nucleus of an atom was so much heavier than the electron portion of the atom. The discovery of the neutron explained the stark differences Rutherford observed. Rutherford calculated that the nucleus was 10,000 times smaller than the atom itself. It was found that the alpha particle is made up of two protons and two neutrons; the alpha particle is the same as the nucleus of a helium atom.