Role of the measurable properties of the elements in the discovery and development of the periodic law of D. I. Mendeleev

Abstract

UDC 389.0 : 541.9 Mendeleev In contrast to many of his fellow chemists, D. I. Mendeleev always gave first place to those properties of matter which could be measured and their values expressed exactly and quantitatively. In his first article of the Periodic Law "The Relation of the Properties of Elements to Their Atomic Weights," Mendeleev wrote that in constructing the system of elements he had tried to be guided by some specifically precise origin. Up to that time there had been an almost complete lack of numerical relationships in setting up previous systems of elements "every system based on accurately observed numbers will of course deserve preference over other systems without a numerical basis in that it allows less place for arbitrary considerations" [1]. Later Mendeleev notes that numerical data regarding the chemical elements were at that time limited and only related to a very small number of elements. Principally these data characterized not the chemical elements themselves but simple substances, i.e., elements in their free form. Many elements may exist in the form of various simple substances, the properties of which are extremely diverse. "Nevertheless," says Mendeleev,"every one of us knows that in all changes in the properties of simple substances in their free state something remains constant, and when an element passes into a compound this something material also constitutes a characteristic of the compounds incorporating the particular element. In this connection at present we know only one numerical factor, that is, the atomic weight, characteristic of the element. The value of the atomic weight, in the very nature of things, is a factor relating not to the actual state of an individual simple substance but to that material part which is common both to the free simple substance and to all its compounds. The atomic weight relates not to coal or diamond but to carbon. It was for this reason that I strove to base the system on the value of the atomic weight" [2]. Thus at the very beginning of Mendeleev's discovery the fundamental principle was that of basing the future system of elements on an exactly measurable property of the chemical elements. The measurability of the properties became a peculiar criterion for deciding whether a particular property was suitable in constructing a system, and especially in developing it further. The ink records found in the scientific archives of the scientist, made on the actual day of the discovery (February 17/March 1, 1869),* indicate that from the first note made on this day up to the last Mendeleev worked primarily with the values of the atomic weights, striving to find numerical relationships between them [3]. The first such record was a comparison of chlorine with potassium. It is well known that at the time the atomic weights of these elements came immediately next to one another despite the sharp qualitative difference in the actual elements, chlorine being an active nonmetal ("metalloid" as it was then said) and potassium just as active a metal. Nevertheless, these two elements came very close to one another in the quantitative values of their atomic weights: C1 = 35.5 and K = 39. The difference of 3.5 between them served Mendeleev as a guide in his subsequent search for a new law of nature. Mendeleev started to compare different chemical elements in whole groups with respect to the atomic weights of their members, as he had done chlorine and potassium, and quickly found that in very many cases the differences in the atomic weights of the elements so compared were in fact of approximately the same order, fluctuating within a few atomic units. This observation led Mendeleev to the conclusion that Nature contained a fundamentally new principle, which he called the Periodic Law.