Throughout its long history, the periodic table has been disputed, altered and improved as science has progressed and as new elements have been discovered. The story of the periodic system for classifying the elements can be traced back over 200 years. Indeed, nothing quite like it exists in the other disciplines of science. A version hangs on the wall of nearly every chemical laboratory and lecture hall in the world. The periodic table of the elements is one of the most powerful icons in science: a single document that consolidates much of our knowledge of chemistry. The complete version with artwork is available for purchase here (PDF). The information contained in this biography was last updated on December 4, 2017.Editor's note: The following is a text-only version. Upon final Italian unification in 1871 Cannizzaro moved to Rome, where he continued his roles as a public figure and as a chemical scientist and an educator. Subsequently, Cannizzaro took part in the new government centered in Palermo, while simultaneously expanding the program of chemical studies at the university there. In 1860 Cannizzaro supported a second Sicilian revolt, led by General Giuseppe Garibaldi and his forces, who were fighting to achieve a united Italy. After returning to Italy he held academic appointments in Alessandria, where he worked out the “Cannizzaro reaction”-the self-oxidation and self-reduction of aldehydes-and Genoa, where he expounded Avogadro’s hypothesis. When it failed, he fled to Paris, where he resumed his chemical studies. Despite his family’s connections to the royal court in Naples, Cannizzaro joined the antimonarchical 1848 revolution in Sicily. Mendeleev acknowledged that his periodic table was inspired by Cannizzaro’s work.Įdgar Fahs Smith Collection, Kislak Center for Special Collections, Rare Books and Manuscripts, University of Pennsylvania Chemist and Political Activistīorn in Palermo, Sicily, where his father was a magistrate and the minister of police, Cannizzaro later attended medical school there, which kindled his interest in chemistry. Within a decade Cannizzaro’s ideas regarding atomic weights had spread throughout the scientific community and were incorporated by Julius Lothar Meyer and Dmitri Mendeleev, both of periodic-table fame. However, many participants carried away a handout-a printed version of Cannizzaro’s course outline, Sunto di un corso di filosofia chimica-that seemed convincing upon later reading. After much discussion the chemists agreed to return home to decide for themselves how to proceed. In 1860 the first international chemical congress was held in Karlsruhe, Germany, to settle some of the contemporary chemical disputes-how to define molecule and atom, what chemical nomenclature to use, how to determine atomic weights, and so on. Cannizzaro, in his course outline, argued that Avogadro’s theories were the key to creating a standard set of atomic weights, a goal much sought after, but his work went relatively unnoticed. By all accounts Cannizzaro was much clearer in his explanations than Avogadro, and as an organic chemist he also showed how Avogadro’s ideas could be applied to this branch of chemistry. Avogadro also reasoned that simple gases were not formed of solitary atoms but were instead compound molecules of two or more atoms. Elucidating Avogadro’s HypothesisĪvogadro had hypothesized in 1811 that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, from which it followed that relative molecular weights of any two gases are the same as the ratio of the densities of the two gases under the same conditions of temperature and pressure. Edgar Fahs Smith Collection, Kislak Center for Special Collections, Rare Books and Manuscripts, University of Pennsylvania.
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