Proton
The discovery of the electron by Joseph J. Thomson in 1897 immediately posed a new problem with regard to atomic structure. Since all atoms are neutral, the existence of a negatively-charged electron meant that the atom must also contain positive charges. How were those positive charges carried within the atom?
Thomson tried to answer this question with a "plum pudding" model of the atom. In this model, the positive charges were smeared uniformly throughout the total volume of the atom with the electrons embedded in the positive charge. Ernest Rutherford's gold foil experiment a decade later showed, however, that the positive charge of the atoms was concentrated in its core (the nucleus) so that Thomson's model could not be correct. But what was the structure of the nucleus?
A possible answer to this question lay in the discovery of " canal rays" by Eugen Goldstein in 1886. These canal rays had properties similar to those of cathode rays (streams of electrons) but consisted of positively-charged particles many times heavier than the electron.
Rutherford reasoned that the simplest of all possible atoms would contain only a single electron and, therefore, only a single positive charge. Canal rays of this atom (hydrogen) would consist of a stream of particles, each carrying a single unit of positive charge. He proposed calling these particles protons.
Atoms must consist, then, of one or more electrons with an equal number of protons in the nucleus to balance the negative charge of the electrons. An oxygen atom that contains eight electrons must, therefore, also contain eight protons in its nucleus.
The model of an atom consisting of two kind of elementary particles, protons and electrons, survived for twenty years until the discovery of the neutron by James Chadwick in 1932. During all that time, and for another 30 years afterwards, the proton was regarded as an elementary particle, one that can not be broken down into anything simpler.
That view proved to be insupportable as a result of experiments performed by Robert Hofstadter in the early 1960s. Hofstadter studied the scattering pattern produced when nuclei are bombarded with high energy electron beams. He concluded that protons (as well as neutrons) have an internal structure. He reported that protons appear to consist of a central core surrounded by shells of some other material.
A possible explanation of these results was provided in 1964 by Murray Gell-Mann. Gell-Mann suggested that protons and neutrons are made of even smaller particles which he called quarks. Gell-Mann's theory has been largely confirmed by experiments, and the proton is now believed to consist of two "up" quarks and one "down" quark.
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