Neutron
The discovery of the nuclear atom by Ernest Rutherford in 1911 was a critical step forward in improving our understanding of the fundamental nature of matter. However, it was immediately clear to scientists that Rutherford's atomic model was incomplete. The simplest and most obvious data showed without question that atoms must consist of more than nuclei, which contain protons, and electrons, located outside the nucleus.
Those data come from measurements of atomic number and atomic mass. An element's atomic number is equal to the total positive charge on--the number of protons in--the nuclei of atoms of which the element is made. The element's atomic mass is equal to the total mass of all particles that make up the atom. Since the mass of a single proton on the atomic mass scale is 1, and that of the electron is 0.0055, the mass of the atom must be very nearly equal to the mass of the protons present in the atom if protons and electrons are the only particles present in the atom. In such a case, an element's atomic number and atomic mass should be about equal. However, with the exception of hydrogen, this is never the case.
Scientists considered a number of ways in which this dilemma could be resolved. One popular notion was that a nucleus might contain both protons and electrons. The electrons in the nucleus could balance the electrical charge of some of the protons without adding any significant mass to the atom. Unfortunately, there were a number of rather clear reasons that electrons could not exist in the nucleus, and this theory was abandoned.
Rutherford himself suggested that some sort of neutral particle might exist in the nucleus and, during the 1920s, he and a graduate student, James Chadwick, searched for such a particle. They were unsuccessful, however, largely because neutrons (the particles for which they were seeking) do not readily ionize atoms and are not, therefore, detected by any standard tools such as cloud chambers or Geiger counters. The clue that led to the resolution of this problem showed up in the research of Irène Joliot-Curie and her husband, Frédéric Joliot-Curie, in France and of Walther Bothe (1891-1957) in Germany in the early 1930s. The Joliot-Curies and Bothe all observed that the bombardment of light elements, such as beryllium and lithium, with alpha particles resulted in the production of a strange, intense form of radiation. They were unable, however, to identify that radiation.
Chadwick, in England, repeated the experiments of the Joliot-Curies and Bothe with greater success. He directed the beam of "strange radiation" at a piece of paraffin and observed that protons were ejected from the paraffin. He concluded that the radiation must consist of particles with no charge and a mass about equal to that of the proton. That particle was the neutron.
Chadwick's discovery was a bonanza for nuclear physicists. Unlike the proton, electron, and other charged subatomic particles, the neutron is not repelled by either the nucleus or the orbital electrons in an atom. It is a much more efficient "bullet," then, in initiating nuclear reactions.
For the next three decades, the neutron was regarded as a fundamental particle that could not be broken down into anything simpler. In the early 1960s, however, the American physicist Robert Hofstadter was able to observe internal structure in both neutrons and protons. Hofstadter passed a beam of electrons close to each type of particle and studied the patterns that resulted from the electrons' deflection. He discovered that both protons and neutrons contain a central core of positively charged matter that is surrounded by two shells. In the neutron, one shell is negatively charged, just balancing the positive charge in the particle's core.
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