![]() The cathode rays were later found to be a beam of electrons. Roentgen had discovered X-rays in 1895 when using a cathode ray tube. ![]() He was investigating X-rays given off by metals. Henry Moseley, then aged 26, worked in Rutherford’s team at the University of Manchester. In 1913 Geiger suggested that the number of positive charges on the nucleus was half the atomic weight but he had no evidence for this idea. Then Rutherford and Geiger showed that atoms had a tiny, positively charged nucleus surrounded by electrons. Another example appeared with the discovery of argon which has heavier atoms than, but is placed before, potassium. Mendeleev knew that tellurium had heavier atoms than iodine but put it in the group before insisting that the measurements were wrong. Atomic masses did not seem to follow a very regular pattern and in any case, some elements were in the wrong order. This produced a lot of gaps lower down in the table and meant that there should be a lot more rare earths to fill them. Some people suggested that atomic weights should increase in proportion by approximately two units for each successive element. Lower down, however, amongst the rare earth metals, it was a bit of a mess. New elements had indeed been discovered and the top of the table looked fairly complete. Thanks to Mendeleev’s inspired guess that there were more elements to discover, his Periodic Table had become an accepted and useful tool for chemists. He had started with a list of the known elements in order of the atomic weights – what we now call Relative Atomic Mass. Fifty years had passed since Mendeleev had designed his Periodic Table. What determines the order of the elements in the Periodic Table? How do we know how many elements there are? The first we shall explore is Henry Moseley’s work on atomic number. ![]() A number of important ideas were developed during that one year that are important in the chemistry learned in school today. Anniversaries of chemical discoveries are celebrated every year, but the centenary of 1913 is quite special.
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