The Construction Of The Periodic System
Gerhardt's unitary perspective also provided the foundation for the construction of the periodic system. Dmitry Ivanovich Mendeleev (1834–1907) was certainly not the first chemist who tried to classify elements on the basis of atomic weights. Van Spronsen reasonably argued that the periodic system was codiscovered by six chemists in the 1860s. However, Mendeleev adopted a different strategy. He pointed to the Karlsruhe Conference, held in 1860, as the first step toward the periodic law. This first international meeting of chemists adopted atomic weights based on Avogadro's and Gerhardt's views. From the general agreement on the distinction between atoms and molecules, Mendeleev derived another crucial distinction, between simple body and element. Since Lavoisier's famous definition of elements as nondecomposed bodies, most chemists did not distinguish between the two terms. Mendeleev, on the contrary, stressed the difference between the simple substances, empirical residues of decomposition characterized by their physical properties and molecular weights, and the abstract element defined as an invisible ingredient and characterized by its atomic weight. Mendeleev chose to classify elements rather than simple bodies because he considered them as responsible for the properties of simple and compound substances. Mendeleev was thus the first chemist who really worried about a clear definition of what was to be classified. Although abstract and unobservable, Mendeleev's elements were material entities and true individuals. Mendeleev was a staunch opponent of the hypothesis formulated by William Prout (1785–1850), which asserted that the multitude of simple substances derived from one single primary element, usually identified as hydrogen. Throughout the nineteenth century this reductionist mainstream stimulated attempts at classifying elements. Classifications were mainly aimed at tracing genealogical relations or families by grouping analogous elements in triads or families according to the numerical ratios of their atomic weights. Starting from the assumption that elements would never be divided or transmuted into one another, Mendeleev took an opposite direction. He sought unity in a natural law ruling the multiple elements, rather than in matter itself. He compared the most dissimilar elements and firmly relied on the order of increasing atomic weights. Whereas grouping elements on the basis of valences always faced the difficulty of multiple valences, Mendeleev strictly relied on the order of increasing atomic weight, which he regarded as the constant criterion of the individuality of chemical elements.
Indeed, this criterion revealed some deficiencies. It sometimes blurred strong chemical analogies (for instance between Li and Mg, Be and Al, B and Si) or induced unexpected proximities (for example, in Mendeleev's eighth group). Mendeleev struggled with these difficulties and published thirty different tables in order to better suit his system with chemical analogies. However, his abstract notion of elements allowed predictions of unknown elements. Many of these elements were confirmed during Mendeleev's lifetime and served to validate his system while guarding it from further challenges. The discovery of a dozen rare earth elements in the late nineteenth century created difficulties because they lack individual properties and have strong mutual analogies. The inert gases were not welcomed either, first, because they had not been predicted, but mainly because they did not exhibit chemical properties. The atomic weights determined by William Ramsay (1852–1916) and Lord Rayleigh (John William Strutt; 1842–1919) for helium (4) and argon (40) would result in the placement of the latter between potassium and calcium according to its atomic weight. This was impossible, and the whole periodic system was in danger to collapse. Mendeleev raised doubts about the elemental nature of argon. Thanks to their sound belief in the periodic law, Ramsey and Rayleigh found a place for them at the cost of a new reversal of increasing atomic weight values and a new prediction of an intermediate element between neon and argon. A zero group was opened up that first undermined Mendeleev's notion of individual elements defined by their atomic weight. A second blow came from the discovery of electrons and radioactivity. Mendeleev desperately tried to explain this apparent anomaly with the hypothesis of confused movements of ether around heavy atoms. In this grandiose attempt to unify mechanics and chemistry, Mendeleev admitted ether in the periodic system in the 0 group. This error is instructive because it reveals the intellectual roots of Mendeleev's system. It is thus impossible to consider Mendeleev's system as a kind of precursor of quantum chemistry. Mendeleev dealt with elements and not with atoms.
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