text: From Wikipedia.org:\n\n"first devised in 1869 by the Russian chemist Dmitri Mendeleev. Mendeleev intended the table to illustrate recurring ("periodic") trends in the properties of the elements."\n\n"The original table was created before the discovery of subatomic particles or the formulation of current quantum mechanical theories of atomic structure. If one orders the elements by atomic mass, and then plots certain other properties against atomic mass, one sees an undulation or periodicity to these properties as a function of atomic mass. The first to recognize these regularities was the German chemist Johann Wolfgang Döbereiner who, in 1829, noticed a number of triads of similar elements:\nSome triads Element Molar mass\n(g/mol) Density\n(g/cm³) Quotient\n(cm³/mol)\nchlorine 35.4527 0.003214 11030\nbromine 79.904 3.122 25.6\niodine 126.90447 4.93 25.7\n \ncalcium 40.078 1.54 26.0\nstrontium 87.62 2.64 33.2\nbarium 137.327 3.594 38.2\n\nThis was followed by the English chemist John Newlands, who noticed in 1865 that the elements of similar type recurred at intervals of eight, which he likened to the octaves of music, though his law of octaves was ridiculed by his contemporaries. Finally, in 1869, the German Julius Lothar Meyer and the Russian chemistry professor Dmitri Ivanovich Mendeleev almost simultaneously developed the first periodic table, arranging the elements by mass. However, Mendeleev plotted a few elements out of strict mass sequence in order to make a better match to the properties of their neighbours in the table, corrected mistakes in the values of several atomic masses, and predicted the existence and properties of a few new elements in the empty cells of his table. Mendeleev was later vindicated by the discovery of the electronic structure of the elements in the late 19th and early 20th century.\n\nIn the 1940s Glenn T. Seaborg identified the transuranic lanthanides and the actinides, which may be placed within the table, or below (as shown above)."