In 1803, John Dalton proposed a theory of matter in which the smallest particle was an atom. Dalton viewed the atom as solid and indivisible. For nearly a century, that model of the atom proved to be satisfactory for scientific work. Then, in 1897, Joseph J. Thomson discovered the first particle smaller than an atom, the electron. It was immediately apparent that atoms were not indivisible, but consisted of at least two parts. One part was the electron and the other, some positively charge particle or particles. The search for a positively-charged counterpart to the electron was not immediately successful, however.
Following up on Thomson's discovery, Ernest Rutherford bombarded a thin film of gold metal with alpha particles. He found that most alpha particles were deflected through rather small angles, as he had expected. Some, however, were deflected through relatively large angles. Indeed, a few were deflected at 180°. These particles had, in effect, recoiled off something in the gold and returned to the source from which they came. Rutherford took this result to mean that the positive charge in an atom is concentrated in a small volume at the atom's center. He called that concentration of charge the atom's nucleus.
Shortly after this experiment, the particle carrying the smallest unit of positive charge had been identified. The particle was much more massive than was the electron. In 1914, Rutherford proposed calling the particle a proton and suggested that the positive charge on the nucleus was the result of protons located there.
Some confirmation of this view was obtained by the first experiments on the artificial transmutation of nuclei. In 1919, Rutherford bombarded nitrogen gas with alpha particles. He found that both oxygen gas and protons were produced in the reaction. He concluded that in the interaction between an alpha particle and a nitrogen nucleus, a proton was ejected, leaving behind a new nucleus, that of oxygen.
A second important discovery about the nucleus was also made in 1919 by Francis Aston. Aston had recently invented the mass spectrograph, a device that can be used to separate atoms from each other on the basis of their masses. With the spectrograph, Aston demonstrated that atoms of the same element can have different masses. These atoms corresponded to the isotopes discovered five years earlier by Frederick Soddy. The inference from this discovery was that oxygen, for example, might consist of at least two kinds of atoms that differ from each other only in their masses.
At this point, it was possible to draw some conclusions about the structure of the nucleus. A nucleus with a positive charge of 6, for example, must contain 6 protons. But that picture was incomplete. The atom whose nucleus carries a positive charge of 6 (carbon) also has a mass equivalent to that of 12 protons.
To explain this discrepancy, scientists assumed that the nucleus also contains electrons. There must be enough electrons, they reasoned, to balance the positive charge of some of the electrons. The carbon nucleus, for example, was thought to contain 12 protons and 6 electrons. The six electrons would balance the positive charge of 6 protons, leaving a net positive charge of 6 on the nucleus. And the mass of the nucleus would be the mass of 12 protons and 6 electrons. Since one electron weighs only 1/1840 as much as a proton, the total mass of the electrons can be disregarded, and the total mass of the nucleus is equal to that of 12 protons.
The existence of isotopes could further be explained by assuming that some atoms of an element had an extra proton and electron in their nuclei. A carbon atom with 13 protons and 7 electrons would still have a nuclear charge of +6, but a mass of 13. The proton-electron model of the nucleus seemed to explain, therefore, the known data on nuclear charge and mass.
At least one piece of experimental information appeared to support this model. Many radioactive isotopes decay by the emission of a beta particle. A beta particle is an electron that has been emitted from a nucleus. The existing proton-electron nuclear model would easily explain the phenomenon of beta decay.
The presence of electrons in the nucleus would solve another problem too. It seemed obviously impossible for there to be nothing other than protons in the nucleus. Protons are all positively charged, and there seemed to be no way of packing a number of like-charged particles close together in a small volume. The presence of electrons would make available unlike charges that would help hold the nucleus together.
Two events soon showed this model to be inadequate, however. First, on a theoretical model, Heisenberg's uncertainty principle suggested that electrons could not remain within the nucleus. Second, experimental results suggested that yet a third particle might be present in the nucleus. This particle was first recognized in experiments on the alpha bombardment of beryllium by Irène Joliot-Curie and Frédérick Joliot-Curie in 1932 and confirmed and identified by James Chadwick in the same year. Chadwick proposed the name neutron for this third particle, a particle with a mass equal to that of a proton, but with no electrical charge.
With the discovery of the neutron, the modern theory of the nucleus was in place. According to this theory, a nucleus contains a number of protons equal to the nuclear charge (the atomic number) and a number of neutrons equal to the atomic mass number minus the number of protons.
A number of questions about the nucleus remained. For example, although the neutrons in a nucleus provide some "shielding" effect, protons should still feel a net force of repulsion. To explain the fact that nuclei do not fly apart, the Japanese physicist Hideki Yukawa proposed in 1935 the existence of a particle that would hold the nuclear particles together. Yukawa's pi meson was discovered in 1947 by the English physicist Cecil Powell.
Another problem of continuing interest is the structure of the nucleus. The fact that most nuclei are stable suggests that there may be some specific arrangement of nucleons (protons and neutrons) that accounts for this stability. Two major theories have been proposed.
In one, the liquid-drop model, nucleons are compared to the molecules that make up a drop of liquid. The nucleons (molecules) move about randomly with the nucleus (drop), but strong forces between particles hold them together. Only on relatively rare occasions do individual nucleons break loose from the surface of the nucleus, as molecules do during evaporation in a liquid. The liquid-drop model has been especially successful in explaining the process of nuclear fission, in which the addition of a single neutron causes a nucleus to break apart into two roughly equal parts.
In the second theory, referred to as the shell model, nucleons are thought to occupy specific energy levels within the nucleus, similar to the energy levels occupied by electrons in an atom. The special stability of certain elements with "magic" numbers of nucleons--2, 8, 20, 50, 82, or 126 protons, neutrons, or nucleons--has been used as support for this theory. The most useful model currently appears to be one that attempts to integrate the most useful features of both the liquid drop and shell models.
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