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ATOM IN MODERN SCIENCE 23 unfavourable to the acceptance of this bold hypo-' thesis Isotopes were not discovered and the chemical atomic weight of chlorine, for instance, being 35 5 was in direct contradiction to Prout's hypothesis He died without ever learning how right he actually was
In the year 1919 the British physicist F W Aston discovered that ordinary chlonne was actually a mixture of two different kinds of chlorine possessing identical chemical properties but having different atomic weights 35 & 37 Further study revealed the striking fact that a mixture of several components identical in chemical properties but differing in atomic weights made up most of the elements They were called isotopes Prout's forgotten hypothesis was given a new life and was reformulated by saying that nuclei of various atoms are composed of various number of hydrogen nuclei called protons
Actually the nuclei of various elements are composed of protons as well as neutrons which are electrically neutral and not positively charged like protons The existence of neutrons was suggested by Rutherford in 1920 but they were found experimentally only in 1932
Thus, the nucleus of an oxygen atom which is the eighth element in the atomic table and has 16 units of mass and 8 units of charge must be composed of 8 protons and 8 neutrons whereas the heavy nucleus of uranium, atomic weight 238 atomic number 92 is formed by 92 protons and 146 neutrons An important fact to keep constantly in mind about protons and neutrons is that the two are interchangeable À proton, under certain conditions loses its positive charge by cmitting a positive electron (positron) and thus becomes a neutron Similarly, a neutron when agitated emits a negative electron and becomes a
