Elementary Order - Mendeleev's Periodic System by I. Petryanov, D. Trifonov

By I. Petryanov, D. Trifonov

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That is (and this was Rutherford's bold conjecture), it cannot be anything but the nucleus of an atom. Thus the nuclear, or planetary, model of the atom was evolved. At its centre it had a massive positively charged nucleus surrounded by electrons travelling along orbits like planets around the sun. In May 1911, Rutherford published a paper in which this idea was clearly deduced. To what numerically were the charges of atomic nuclei of various elements equal? Experiments and calculations indicated that for the light elements it was equal to approximately one half of the atomic weight.

Instead of UI we write 238 U; instead of UX,, 234Th. Try to compile a table of the isotopesformedin the decay of uranium-? IX only indirectly determine to which place an element is moved as a result of a P decay. It turned out to be one space to the right in the periodic system. All of these observations were generalized at the beginning of 1913 by Soddy and the Polish chemist and educator Kasimir Fajans, who formulated the law of radioactive displacement: in an a decay, the radioactive element is transformed into an element two spaces to the left of the initial element in the periodic system, whereas in a p decay, one space to the right.

In July 1898 they announced the discovery of polonium (named after Mme. Curie's native Poland), and in December, of radium (from the Latin meaning "a ray"), two new radioactive elements. A year later the French chemist Andre Louis Debierne, a collaborator of Mme. Curie, discovered actinium, still another radioactive substance. It was not by chance that we used the word "substance". The discoveries of new elements enumerated above were unusual. When, for instance, Lecoq de Boisbaudran discovered gallium, he soon had radius, 36 at his disposal such amounts of its compounds that they could be weighed by rough scales.

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