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1.2 The Periodic Table_(9_8_09)

1.2 The Periodic Table_(9_8_09) - Chem 43 Therien F 09 1.2...

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Chem 43 Therien F 09 1 1.2 The Periodic Table In the early 19 th Century Johann Dobereiner observed several groups of three elements (triads) with similar chemical properties. [Dobereiner’s definition of an element was that of Boyle’s: a pure substance that cannot be decomposed into any simpler substance.] He also noted that the atomic mass of 1 of the three was ~ the average of the other two Cl, Br, I; S, Se, Te; Ca, Sr, Ba; Li, Na, K In the mid 19 th Century John Newlands organized elements from lightest to heaviest, and noted that the properties of the 1 st resembled the 9 th , the 2 nd the 10 th , and so on: H Li Be B C N O F Na Mg Al Si P S Cl K Ca Cr Ti Mn Fe He compared his rule of octaves to music Comparison is flawed; had he known of noble gases his periodicity would have been nine, but the notion of periodicity was established. Other problems: no room for new elements; some didn’t seem to be in right place, e.g. , Cr and Al; Mn and P Periodic Table as we know it today is usually credited to the Russian Chemist Dimitri Mendeleev (1834-1907), although the German Chemist Lothar Meyer devised the same system independently and contemporaneously (as far as we know, neither knew of Newland) o Several improvements over Newland’s system were incorporated: Long periods were introduced for elements now
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Chem 43 Therien F 09 2 known as transition metals; this avoided putting metals under non-metals o When an element did not seem to fit in place, a blank space was left o Improved measures of atomic weights were devised Still problems existed; atomic weights alone did not always give appropriate insight into where a particular element fitted in the periodic table: o Te (127.6 amu) has higher atomic weight than I (126.9 amu); but based on reactivity comparisons, I clearly belongs with Br and Cl, while Te belongs with S and Se. o Mendeleev ordered the elements by reactivity and assigned numbers, 1-92 (now 105). In 1912, Henry Moseley discovered that the frequency of X-rays emitted from elements correlated with atomic number, not atomic weight. (When atoms are bombarded with electrons, it is possible to eject electron from the inner shells; when an electron drops down from an outer shell, an X-ray is emitted)
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