Tellurium reversal with iodine

The second innovation Mendeleev made involved the relative placement of tellurium and iodine. If the elements are listed in strict order of their atomic masses, then iodine should be placed before tellurium, since iodine is lighter. That would place iodine in a group with sulfur and selenium and tellurium in a group with chlorine and bromine, an arrangement that does not work for either iodine or tellurium. Therefore, Mendeleev rather boldly reversed the order of tellurium and iodine so that tellurium falls below selenium and iodine falls below bromine. 

Recognition of the fact that elements always displayed the same chemical behavior - regardless of their isotopic composition - led to a reformulation of the periodic law. The idea that each element was characterized by a unique number had already been demonstrated experimentally by Hemy Moseley (1887-1915). By studying the X-ray diffraction patterns produced by a variety of elements, he discovered that the frequencies of the K lines differed from element to element in a predictable and consistent fashion. He went on to show that the frequency of any line in the X-ray spectrum is approximately proportional to A(N-b), where A and b are constants and N is an integer that he termed the atomic number of the element. Moseley was able to identify the number N with the number of protons in the atomic nucleus. Plots of the square root of the frequency for the K and L lines in the X-ray spectra of the elements versus their atomic number, reproduced in Figure 5, show almost straight lines. From this work, it became clear why the order in which certain element pairs appeared in the periodic table needed to be reversed. The pairs in question are argon (39.95) and potassium (39.10) cobalt (58.93) and nickel (58.69) and tellurium (127.60) and iodine (126.91), the... 

One aspect beyond dispute is that Mendeleev made far more successful predictions than any of the codiscoverers of the periodic system. For example, he successfully predicted new elements, corrected the atomic weights of a number of known elements, and correctly reversed the positions of the elements tellurium and iodine. Why was it Mendeleev and not Lothar Meyer or others who was able to make such notable predictions Is it simply that the others lacked the courage to do so, as many historians of science state I suggest that Mendeleev had the advantage of being blessed with a deeply philosophical approach to chemistry, which allowed him to arrive at insights that his less philosophically minded contemporaries could not have reached. 

Recall that at the time the argon problem arose only one pair reversal had yet come to light, that of iodine and tellurium, and that its explanation would not be obtained until nearly 20 years later with the discovery of isotopes and the work of Moseley. 

ATRP), and reversible chain transfer catalyzed polymerizations " and (iii) degenerative transfer-based polymerization (with reversible addition-fragmentation chain transfer (RAFT) polymerization being the most successful technique but also including iodine-mediated polymerizations and polymerizations in the presence of tellurium or antimony compounds ). Most of these techniques are covered in detail in other chapters of this book. 

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