| Ordering the elements in the Periodic Table (pre-16) |
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Problems with the order by relative atomic mass |
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If you look carefully at a modern Periodic Table you will see that there are three instances where an element is followed by another element of smaller relative atomic mass. Activity 1Find the three cases where this happens. Activity 2Use the Before function of the interactive Periodic Table to view all the elements known in 1869. Which one of the pairs of elements you found in Activity 1 would not have concerned Mendeleev? Why?
One of the other cases is cobalt (Z = 27) and nickel (Z = 28). They have very similar relative atomic masses (58.93 and 58.69 respectively) so a possible explanation was that the values had been measured incorrectly. However tellurium (Z = 52, Ar = 127.6) was listed as heavier than iodine (Z = 53, Ar = 126.9) yet their chemical properties clearly demanded that iodine follow tellurium in the Table. Iodine fits much better in Group 7 and Tellurium in Group 6 rather than the other way round. Mendeleev was so convinced of the relative positions of the two elements that he believed the relative atomic mass of tellurium was wrong. Activity 3Find out some of the properties of iodine and of tellurium – you could use a reference book or search on the web, at www.webelements.com, for example. Explain why tellurium fits into Group 6 and iodine into Group 7 rather than the other way round. You may need to look up the properties of other elements in Groups 6 and 7 as a comparison. Some properties you could look for include the formulae of their compounds with hydrogen and with metals.
Mendeleev’s practical assistant, Bohuslav Brauner, re-measured the relative atomic mass of tellurium but still got the same result. He put this down to impurities he had failed to remove. There is a message for us all here; sometimes there are unexpected results which are nevertheless correct. |
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