Dmitry Ivanovich Mendeleyev formulated a law describing a recurrent pattern in the properties of chemical elements that became apparent when they were organized according to their atomic weights. He devised a periodic table depicting this pattern that enabled him both to understand the relationships of elements and to predict the characteristics of unknown elements.
For more than two thousand years, most natural philosophers believed that only four elements—earth, air, fire, and water—existed, and they attempted to fit all newly discovered substances into this quadripartite scheme. During the eighteenth century, Antoine-Laurent Lavoisier
The first person to discover some order among the elements was Johann Wolfgang Döbereiner
The actual periodic system of the elements was discovered independently in Russia by Dmitry Ivanovich Mendeleyev and in Germany by Julius Lothar Meyer. During the late 1860’s, both Mendeleyev and Meyer were preparing textbooks on chemistry. Meyer based his on the atomic theory and the systematization of elemental properties. Mendeleyev, skeptical of atomism, initially organized his Osnovy khimii (1868-1871; The Principles of Chemistry, 1891) around chemical practice rather than any theories of the classification of elements. It was not until early in 1869, when he was writing the second volume of the textbook, that he realized that he needed a better way of organizing the fifty-five elements, which he had not yet discussed.
During the first few weeks of February, 1869, while he was writing his second volume’s first two chapters on the alkali metals, Mendeleyev listed sodium and potassium along with the recently discovered rubidium and cesium in order of their increasing “elemental” (his preferred term), or atomic, weights. He compared this arrangement with a similar one for the halogens (fluorine, chlorine, bromine, and iodine) and for the alkaline earths (magnesium, calcium, strontium, and barium). This comparison revealed to Mendeleyev an important pattern governed by elements’ atomic weights.
Precisely how Mendeleyev arrived at his recognition of the importance of atomic weight as a classificatory tool is controversial, since the paucity of contemporary documents leaves room for various interpretations. In some accounts, a dream played a role. Some scholars claim that Mendeleyev put the elements and their properties on cards, arranged them in rows according to increasing atomic weights, and then noticed regular repetitions of physical and chemical properties. Other scholars believe that he grouped elements into natural families, such as the halogens, and then noticed their dependence on atomic weight. Still other scholars hold that he was acting as a pedagogue and he was looking for a method of discovery rather than a system of classification. However Mendeleyev arrived at his breakthrough, his account of it was made public when a friend presented a paper describing it on his behalf at a meeting of the Russian Chemical Society on March 6, 1869. It was soon published both in Russian and, in abbreviated form, in German.
Over the next three years, Mendeleyev gradually realized the deficiencies of his early formulations of the periodic law and table, and he grasped how to remove many of these defects while increasing the power of his systematization of the elements to make useful predictions. When arranged strictly in order of increasing atomic weights, some elements appeared to be out of place, and Mendeleyev assumed that erroneous atomic weights were responsible for these anomalies. Some of his adjusted atomic weights proved helpful, but others were simply wrong (three atomic weight inversions exist in the modern periodic table, which is based on atomic number rather than atomic weight).
By November of 1870, Mendeleyev had heard of Meyer’s work, published eight months earlier, in which Meyer had arranged fifty-six elements in vertical columns according to their increasing atomic weights. Meyer’s horizontal families clearly showed the periodic recurrence of elemental properties. In the same paper, he presented a graphical illustration of periodicity by plotting atomic volumes (the space occupied by atoms) as a function of atomic weights. In this illustration, the analogous relationships of elemental properties occurred as waves, with the alkali metals at the peaks of each curve.
Mendeleyev became so convinced of the lawlike nature of his system that he published a periodic table with empty spaces for unknown elements, and because his law of periodicity brought out the dependence of properties on atomic weight, he was able to characterize these unknown elements with precision. For example, he reasoned that the unknown element in the empty space following calcium should be related to boron, and he gave this unknown element the provisional name “eka-boron” (after the Sanskrit eka, meaning “first”). He predicted the properties of two other unknown elements, eka-aluminum and eka-silicon.
When, late in 1871, Mendeleyev published an improved and expanded periodic table, he was convinced that he had discovered a new law of chemistry. This table, which had twelve horizontal rows and eight vertical columns, showed that most of the elements’ properties had a periodic dependence on their atomic weights. Nevertheless, perplexing problems remained. For example, Mendeleyev failed to understand the few rare-earth elements that were then known, since these abundant metals have closely similar properties (once all fourteen were discovered, they needed a separate section in the periodic table). However, when Lecoq de Boisbaudran found eka-aluminum in 1875 and named it gallium, the power of Mendeleyev’s formulation of his periodic table and law began to generate admirers, including Meyer. With the discovery of scandium (eka-boron) in 1879 and germanium (eka-silicon) in 1886, both of which had properties that Mendeleyev had predicted, chemists became satisfied that Mendeleyev’s periodic table was much more than the simple teaching tool that he had initially envisioned. It was a new way of making sense of chemistry’s rich past and of creating a fertile future.
Although it took time for the periodic table of elements to assume its modern form, the table provided throughout its history a way for physicists and chemists to understand accumulated information about the elements. It also helped teachers to communicate to students the nature and properties of the basic building blocks of matter, and it enabled researchers to make discoveries of new elements. A good early example of its value as a research tool was the discovery by William Ramsay
New advances in chemistry, such as the discovery of atomic numbers by Henry Moseley, helped scientists more rationally to organize the periodic table. When scientists discovered the spin of electrons and the shared-electron-pair bond, these new ideas helped deepen their understanding of the periodic law. Mendeleyev himself gave his estimate of the significance of his discovery by emphasizing the table’s ability to elucidate unexplained phenomena and to make verifiable predictions. The periodic law’s value to modern scientists has increased with the increased understanding of the nature of the atom brought about by the theories of quantum mechanics. What was a ridiculed idea during the nineteenth century has become a much-valued part of the foundations of modern science.
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Gordin, Michael D. A Well-Ordered Thing: Dmitrii Mendeleev and the Shadow of the Periodic Table. New York: Basic Books, 2004. The author, a historian, elucidates Mendeleyev and his achievements by situating him in the complex political, cultural, and intellectual contexts of nineteenth century Europe. Extensive set of notes, a thirty-nine-page bibliography, and an index. -
Morris, Richard. The Last Sorcerers: The Path from Alchemy to the Periodic Table. Washington, D.C.: Joseph Henry Press, 2003. Morris, a physicist and science writer, emphasizes the human beings behind the discoveries that led to chemists’ modern understanding of elements in the periodic table. An appendix on the elements, a “further reading” section, and an index. -
Strathern, Paul. Mendeleyev’s Dream: The Quest for the Elements. New York: Berkeley Books, 2000. The sometimes quixotic quest to understand the world’s elements by Mendeleyev and others is the theme that unifies this popular history of chemistry. A “further reading” section and an index. -
Van Spronsen, J. W. The Periodic System of Chemical Elements: A History of the First Hundred Years. Amsterdam: Elsevier, 1969. Published to commemorate the centennial anniversary of the discovery of the periodic system, this narrative account, based largely on original sources, reveals how periodicity was discovered, how it absorbed old knowledge, and how it helped create new knowledge. References at the ends of chapters, name and subject indexes. -
Weeks, Mary Elvira. Discovery of the Elements. 7th ed. Easton, Pa.: Journal of Chemical Education, 1968. This classic text on how each of the elements was discovered has been updated and reorganized by the historian of chemistry Henry M. Leicester, who consolidated chapters around each element or groups of elements. Chapter 14 is specifically on the periodic system of the elements. A “chronology of element discovery,” and name and subject indexes.
Dalton Formulates the Atomic Theory of Matter
Clausius Formulates the Second Law of Thermodynamics
Articulation of Quantum Theory
Dmitry Ivanovich Mendeleyev.