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Atoms were considered the smallest possible division of matter until 1897, when English physicist J.J. Thomson found that there was something smaller: what he termed corpuscles, the subatomic particle now called the electron. Several scientists before Thomson had suggested that atoms were built up from a more fundamental unit but conjectured that this unit was about the size of the smallest atom, hydrogen.
Thomson found that cathode rays could be deflected electrically, and figured that subatomic corpuscles emerged from gas atoms. He concluded that atoms were divisible into constituent corpuscles, whereby he concocted a plum-pudding model of atomic structure.
To explain the overall neutral charge of an atom, as contrasted to the corpuscle (electron) negative charge, Thomson proposed in 1903 that corpuscles floated in a sea of positive charges, with electrons embedded like plums in a pudding, though Thomson’s model posited rapidly moving corpuscles instead of plopped plums.
Japanese physicist Hantaro Nagaoka rejected Thomson’s model on the grounds that opposite charges are impenetrable. Nagaoka proposed a planetary model in 1904, in which a positively charged nucleus was surrounded by revolving negatively charged electrons. Nagaoka had in mind Saturn, with its satellite rings.
One of Thomson’s pupils, English physicist and chemist Ernest Rutherford, disproved the atomic plum pudding in 1909. At the behest of Rutherford, German physicist Hans Geiger and New Zealand physicist Ernest Marsden performed their famous “gold foil experiment”: shooting a beam of radium alpha particles at gold foil, whereupon they measured a widespread deflection of radioactive decay. If Thomson’s plum-pudding atomic model had been correct, the deflection would have been at most a few degrees.
Following Nagaoka, Rutherford proposed his planetary atomic model in 1911: a cloud of negatively charged electrons swirling in orbits over a compact positively charged nucleus. Only a concentrated charge could have accounted for the heavy deflection found in the gold foil experiment. This subatomic particle was the proton, which Rutherford identified in 1918.
Rutherford was working with Niels Bohr, who conjectured in 1913 that electrons moved in specific orbits which were regulated via Planck’s quantum of action. (Planck’s quantum of action, also known as the Planck constant, is the essential granularity of existence.)
In 1919, Rutherford became the first to transmute one element into another, converting nitrogen into oxygen through the nuclear reaction 14N + α → 17O + proton.
Rutherford speculated in 1921 about how atomic nuclei stayed together rather than flying apart. Rutherford concocted neutrons, which could, via some attractive nuclear force, somehow compensate for the repelling effect of the positively charged protons. Neutrons account for much of the extra mass of atoms heavier than hydrogen.
Rutherford’s neutron hypothesis was experimentally shown in 1932 by his associate, English physicist James Chadwick. Chadwick would later join the American Manhattan Project which developed the atomic bombs dropped on Hiroshima and Nagasaki, to close World War II with a most spectacular war crime. For his valiant effort in developing the first weapon of mass destruction, Chadwick was knighted in 1945.
Non-cooperation in military matters should be a vital part of the moral code of basic scientists. ~ Albert Einstein
