History of the periodic table

The development of the Periodic Table of the Elements began in 1817 with Johann Wolfgang Döbereiner triads and ended in 1869 with the still widely accepted systematic arrangement of Dmitri Mendeleev and Lothar Meyer.

Johann Wolfgang Dobereiner ( Triad System)

Johann Wolfgang Dobereiner compared 1817-1829 the properties of chemical elements, without knowing the actual internal structure of atoms. On July 16, 1817, he mentioned in a letter to Councilor Ferdinand Wurzer for the first time the name of triads for his under development system. Dobereiner noticed that the atomic masses (then " atomic weights " ) of the three mutually similar elements barium ( 137.327 u), strontium ( 87.62 u), and calcium ( 40.078 u) the average of 88.5 u were, so the approximate value of strontium. From this he drew initially suspected would consist of strontium barium and calcium, which he found but not confirmed in similar experiments for this purpose.

In 1829 Dobereiner published a typeface named test to a group of elementary substances according to their analogy and thus the first science-based classification system of chemical elements.

Döbereiner created groups of three elements, the so-called " triad ":

He succeeded in 30 of the past 53 known elements with the Triad system to classify. Vertical triads: the alkali ( lithium, sodium, potassium), alkaline earth (calcium, strontium, barium ), the salt former (chlorine, bromine, iodine) and the acid generator (sulfur, selenium, tellurium ). Fluorine would be easily classified with the triad of salt formers. According to Dobereiner but it should form the first element of a triad, of which the other two elements were not yet discovered. Hydrogen, oxygen, nitrogen and carbon were isolated from Döbereiner considered although oxygen, nitrogen and carbon would form an accidental triad. Furthermore, he failed to classify the earth metals and the elements cadmium, antimony, bismuth, gold, tungsten and tantalum in triads. The platinum metals form two triads, platinum, iridium, osmium and palladium, rhodium, pluran. The existence of the element pluran has been doubted to Döbereiner times of Johann Christian Poggendorff. Johann Wolfgang Dobereiner dropped the Triad usually the foundation for the development of the periodic table.

The Triad system Döbereiner was expanded in the years that followed, inter alia, by Leopold Gmelin ( new triads) and Ernst Lenssen. Lenßen put only the atomic weight of the elements to reason and let the other properties outside before (accidental triads). In this way he was able in 1857 to classify all the known elements according to the Triad principle.

J. A. R. Newlands ( law of octaves)

John Alexander Reina Newlands found out in 1864 that repeat themselves in order of the elements in order of increasing atomic mass, the chemical properties in every eighth position, which he likened to the octaves of music. He called his discovery of the law of octaves. See also: Alexandre -Emile de Béguyer Chancourtois

The noble gases were JAR Newlands not yet known.

Dmitri Mendeleev and Lothar Meyer (Periodic Table )

1869 constituted nearly simultaneously and independently by the German chemist Lothar Meyer and Dmitri Mendeleev, the Russian chemist, the first periodic table before. In Russia, the periodic table is called Tabliza Mendelejewa in memory of Mendeleev even today.

You have the known elements in order of increasing atomic mass (then atomic weights called ) and one below the other on the behavior of similarity at different intervals, with them, the similarities appeared important than the exact order of atomic masses, as it can be seen from the list of Mendeleev in tellurium and iodine. Their theory already went so far that they let some positions available for which they postulated the existence of previously unknown elements.

That this arrangement was properly confirmed, first, by the realization that not increase the atomic masses continuously, because most elements are composed of isotopes with different atomic masses. Their random frequency distribution results in argon with potassium, cobalt, nickel, and tellurium with iodine exchange their places against the mass order ( see list of isotopes). Secondly, it was provided by the discovery of the atomic structures, in particular the orbital model, an explanation for the periodicity.

Discovery of predicted elements

Mendeleev said in 1871, the existence of three elements before because they would fill in the corresponding gaps in his periodic table. For this reason, "invented" it, the prefix Eka (Sanskrit = one) and called these elements with Ekasilizium, Ekaaluminium and Ekabor after each lighter, similar in properties element.

• Ekaaluminium was discovered in 1875 by Paul Emile Lecoq de Boisbaudran and referred to France as gallium.
• Ekabor was discovered in 1879 by Lars Fredrik Nilson and referred to Scandinavia as scandium.
• Ekasilizium was discovered in 1886 by Clemens Winkler and referred to Germany than Germanium.

Henry Moseley ( Moseley's law )

H.G.J. Moseley discovered in 1913 Moseley's law, in which a relationship between the atomic number of an element and the frequency of a spectral line of the characteristic X- radiation is formulated.

Moseley returned with his law a confirmation of the correctness of the order of the elements in the periodic table and could simultaneously predict the exact number of then-unknown elements.