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THEORY OF ATOM
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nucleus of which there are 92 protons, and an equal number of electrons are rotating round its centre As there is only one electron in a hydrogen atom, there is only one orbit in which it rotates. In the other atoms all the protons unite together and form the central nucleus, but the electrons form different groups and rotate in different but well defined orbits
• There is, however, onc important fact to be borne in mind, vz, the nucleus of an 'atom' contains some other particles besides protons Consider, for exemple, the nucleus of an oxygen atom Since oxygen is the eighth element in the natural sequence, its atom must contain 8 electrons and its nucleus must carry 8 positive charges But oxygen atoms are 16 times heavier than those of hydrogen. Thus, if ve assume that an oxygen nucleus is formed from 8 protons, we would get a correct charge, but a wrong mass (both 8), and assuming 16 protons we get correct mass, but wrong charge (both 16).
Neutrons And Positron
It is clear, therefore, that some of the particles forming complex atomic nuclei are electrically neutral, but have the same mass as a proton The existence of such chargeless protons or neutrons as they are now called, was suggested by Rutherford as early as 1920; but it was twelve years later in 1932 that they were discovered experimentally by Chadwick, an assistant of Rutherford The derivative
