Book Reading Update - Fontana History of Chemistry Chap 9

As the author stated at the end of the chapter, "The periodic classification of elements is one of the greatest and most valuable generalizations in science.  Important in its late 19th century construction for the clarification it brought, and the guidance it gave to chemical research, it was able to adapt to the deeper analysis of the structure of matter revealed by nuclear physics in this century.”

Although there were similar, some concurrent attempts at organizing the elements, the majority of the effort and the most significant initiative and push are attributed to Mendeleev.  Mendeleev used as his primary organizing principle the fundamental property of each unique elements its mass.  In "sorting the elements", Mendeleev was guided by atomic weights (relative scale with C=8), valency, and isomorphism.  In February 1869, he came up with the law “elements placed according to the value of their atomic weights present a clear periodicity of properties”. It is notable that Mendeleev had such confidence in his organization that it warranted redoing of experiments when data did not conform to his prediction as with the case of the measured atomic weight of tellurium and its “misplacement” in Mendeleev’s periodic table.  His predictions ranged from spot on to luck guesses to incorrect outcomes.

The identification of the rare earths proved to be the most difficult to chemists who have devoted their work to the study of these elements.  The rare earths have very similar physical properties which made their resolution into different elements from a mixture and characterization very difficult.  These rare earth discoveries were befuddling to Mendeleev because of the seeming lack of periodicity.  There was debate as to where to place them within Mendeleev’s periodic table and it was suggested they should be placed in their own table until more research has been done.  In 1913, Moseley confirmed that there were exactly 14 rare earth elements.  It was not until Bohr’s electron shell arrangement and atomic number concept did it become clear that “the rare earth group formed a miniature periodic system in which the relationships of the main system are reflected”.

Because chemists were mainly using chemical methods to identify elements, measure weights, and observe properties, the inert gases were not discovered till later, principally by Ramsay who collaborated with Lord Rayleigh in the beginning.  By 1900, 5 of the 6 inert gases have been discovered but it was not until the 1960's that a chemical compound was synthesized using xenon.  Spectroscopy played a big role in the verification of these new elements.

Other methods of discovering the missing elements in Mendeleev's periodic table included manufacturing them in the lab and observing their productions in radioactive decays and fission processes.

As the author concluded in the end, after Lavoisier’s definition of the chemical element, chemists became convinced that matter was built up from these different substances.  Mendeleev sought to find a way to organize these elements using the fundamental nature of mass as his organizing principle, encouraged by observations in "organic chemistry of the dependence of physical properties on molecular weights". "Mendeleev’s system, in which atomic weight became the fundamental feature, or chemical standard, for identifying and defining elements, was sufficiently elastic to allow the prediction of missing elements, or the inclusion of whole new groups such as the noble gases and actinides.”  The discoveries of other elements relied on physicists’ kinetic theory rather than chemical methods.  Spectroscopy played a big role in the identification and verification of many of these, showing “how the explanation of the (periodic) law lay in a theory of atomic structure.  “Following Moseley’s concept of atomic number, and its correlation with Rutherford’s idea of a nuclear charge, it became possible to place a finite figure on the number of possible elements, and to redefine atomic weights as an averaging out of isotopic masses.”