Mendeleev worked on the period length but unlike Meyer, he left empty spaces in his table: they, theoretically, had to correspond to elements that weren’t discovered yet. The theory was confirmed by the use of the spectroscope which allowed the discovery of the new elements imagined by Mendeleev himself.

But who was Medeleev? (Tobolsk, Siberia 1834 - S. Petersburg 1907).

Mendeleev Dimitrij Ivanovic was a Russian chemist who carried out his studies inSt. Petersburg University on the density of different substances. In 1859 he went to Heidelberg University where he finished his experiments on the capillary phenomenon, on the noble gas as on the chemical affinity and he collaborated with the Italian chemist Stanislao Cannizzaro. In 1863 he returned to St. Petersburg and was appointed professor of chemistry in Polytechnic. In 1869 he published the first version of the periodic table. In 1874, another version came out in which the scientist left vacant squares indicating the unknown elements. He wrote the theory of solutions, and studied the theory of the thermal expansion of liquids and the theory of the origin of petroleum. In 1890 he left university teaching and in 1893 he was director of Weights and Measurements Organization of S. Petersburg until his death.

These blank spaces, we mentioned above, were filled, in 1875, by Gallium called eka-aluminium of Mendeleev, discovered by Boisbaudran; in 1879 by, ScandiumofNilson that was the eka-boro of Mendeleev; in 1886 by the Germanium of Winkler that was the eka-siliceous of Mendeleev.

Now that we have outlined the history of chemistry which changed our studies and conception, we have to conclude that the law of the periodicity has suffered two important changes since the original draft of MendeleevandMeyer. The first modification extended the law to three noble gases (helium, argon and neon) discovered between 1894 and 1898 by the British scientists RayleighandRamsay. In particular Ramsay heated a gas blister that didn’t react and saw the shining lines of the spectrum.

The clearer lines didn’t coincide to those of any known element. And so this blister contained a gas heavier than nitrogen and chemically inert; for this it was called argon, a Greek word that means slow or tardy. Ramsay, following the theory of Mendeleev, noted that Argon can’t be a unique case, so he continued his research and discovered Helium, Krypton and Xenon.

The second modification originated from the expansion of the periodicity law because of Bohr’s theory (1913) on the electronic structure of the atom (but it was already in 1900!).

Chemistry, certainly, will not stop at these extraordinary discoveries: in many periods, research has always gone ahead and, for sure, science can’t limited into symbolic borders but it will continue developing until the world exists.

Bibliography

• Isaac Asimov, Breve storia della chimica, Zanichelli, Bologna, 1968
• G. Valitutti, A. Tifi, A. Gentile, La chimica in moduli, Zanichelli, Bologna, 2001
• Giovanni Treccani, Enciclopedia italiana Treccani, Istituto poligrafico dello Stato, Roma, 1970

Iconography

• Hawez, Il bacio, pinacoteca di Brera, Milano, 1859
• Foto di laboratorio dell’Ottocento, Accademia nazionale delle scienze, www.accademiaxl.it, Roma
• Tavola periodica degli elementi, www.cicap.org, Comitato italiano per il controllo delle affermazioni sul paranormale, Padova, last update: 24/03/2006
• IUPACLogo, www.tutms.tut.ac, School of Materials Science, Toyhoshi University of Tecnology
• Humphry Davy, www.corriosion-doctors.org
• Gay-Lussac, John H. Lienhard, www.uh.edu, University of Huston, Huston
• Thenard Eugene Chevreul, www.hdelboy.club.fr, Alchimie Astrologie Statistiques Musique Echecs, last update: 15/03/2006
• Cannizzaro Stanislao, Direttore Filippo Sorbello, www.unipe.it
• Avogadro Amedeo, www.nndb.com
• Meyer Julius Lothar, www.answers.com, enciclopedia on-line
• Mendeleev Dimitrij, www.bbc.co.uk
• Bohr Niels, www.gc.cuny.edu

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