The word nucleus is a derivative of the Latin word nux, meaning nut or kernel. Around 1909, Ernest Rutherford gave that name to the dense, central part of the atom that he and his colleagues Hans Geiger (1882-1945) and Ernest Marsden discovered by bombarding thin sheets of gold foil with alpha particles. The results of the experiment led Rutherford to propose that all the positive charge of an atom and almost all its mass are located in the nucleus. He developed this model based on the behavior of the positively charged alpha particles as they hit the gold foil. Although most of them passed right through, some were deflected at various angles, including angles that directed the alpha particles back toward their source. Since most of the alpha particles passed through undeflected, Rutherford concluded that the positive part of the atom was not evenly distributed throughout the atom. He explained the deflections as the result of the electric repulsion between like charges--a positively charged alpha particle and the positive part of the atom. He used measurements of the deflection angles and the number of particles deflected at that angle to calculate the size of the nucleus. Its diameter is about 10-15 meters (one femtometer, now known as a fermi) which is ten thousand times smaller than the diameter of the atom itself. About half a century later, Robert Hofstadter (1915-1990) won the 1961 Nobel Prize in physics for his work verifying that the radius of an atomic nucleus is about one fermi (10-15 meters). In his experiments, Hofstadter collided electrons with atoms at high energies.
How the positive charges are able to stay in such close proximity in the confines of the nucleus is another question that was raised. The force, labeled the strong nuclear force, would have to be attractive between the nuclear particles and act at very small distances. At that time only two forces were recognized. The first, gravity, is only significant when large masses are involved. The second, electric force, acts only between charged particles. It is an attractive force between particles of opposite charge, and a repulsive force between particles of same charge. The nuclear force is about one hundred times stronger than this repulsive force between the protons. Rutherford suggested that there is another particle in the nucleus that has mass but no charge. He called this particle the neutron, and postulated that the strong nuclear force acts between the protons and the neutrons to keep the nucleus together. This particle was not discovered until 1932 by English physicist James Chadwick (1891-1974) Chadwick won the 1935 Nobel Prize in physics for this discovery. In 1934, Hideki Yukawa (1907-1981) was the first to attempt to explain the way in which this nuclear force operates. He suggested that the strong nuclear force results from the exchange of a particle between the neutrons and the protons; he named that exchange particle a meson. In 1947, English physicist Cecil Powell (1903-1969) observed Yukawa's mesons, now called pi-mesons or pions, in the upper atmosphere, where they were produced by cosmic ray collisions.
Maria Goeppert Mayer shared in the 1963 Nobel Prize in physics for developing the shell model theory of the structure of atomic nuclei. This theory suggests that just like electrons in an atom, protons and neutrons within the nucleus are arranged in quantum shells that fill according to specific magic numbers.
It has been proved that the mass of an atomic nucleus is less than the sum of the masses of the protons and neutrons that it contains. Using Einstein's mass-energy equivalency equation, this mass difference multiplied by the speed of light squared gives the binding energy of the nucleus. When the sum of the masses of the particles in a nucleus is less than the mass of the nucleus itself, the nucleus will not be stable and that atom will be radioactive (unstable).
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