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A Quick Look at the History of the Periodic Table Things are different from each other, and each can be reduced to very small parts of itself. - Ancient knowledge This was noticed early by people, and Greek thinkers, about 400BC, used the words "element', and `atom' to describe the differences and smallest parts of matter. These ideas survived for 2000 years while concepts such as `Elements' of Earth, Fire, Air, and Water to explain `world stuff' came and went. Much later, Boyle, an experimenter like Galileo and Bacon, and who was influenced much by Democritus, Gassendi, and Descartes, lent important weight to the atomic theory of matter in the 1600s. The familiar periodic table that adorns many science classrooms is based on a number of early efforts to identify and classify the elements. In the 1790’s, one of the first lists of elements and their compounds was compiled by French chemist Antioine-Laurent Lavioser. It was Lavoisier who divided the few elements known in the 1700's into four classes, and then John Dalton made atoms even more convincing, suggesting that the mass of an atom was it's most important property. "The chemical elements are composed of... indivisible particles of matter, called atoms... atoms of the same element are identical in all respects, particularly weight." - Dalton In the early 1800's Dobereiner noted that similar elements often had relative atomic masses, and DeChancourtois made a cylindrical table of elements to display the periodic reoccurrence of properties. Cannizaro determined atomic weights for the 60 or so elements known in the 1860s, then a table was arranged by Newlands, with the elements given a serial number in order of their atomic weights, beginning with Hydrogen. This made evident that "the eighth element, starting from a given one, is a kind of repetition of the first", which Newlands called the Law of Octaves. Both Meyer and Mendeleyev constructed periodic tables independently, Meyer more impressed by the periodicity of physical properties, while Mendeleyev was more interested in the chemical properties. "...if all the elements be arranged in order of their atomic weights a periodic repetition of properties is obtained." Mendeleyev History of the Periodic Table 1 C. Pace, Instructor Mendeleyev published his periodic table & law in 1869 and Dimitri Mendeleyev forecast the properties of missing elements, and chemists began to appreciate it when the discovery of elements predicted by the table took place. The periodic tables have always been related to the way scientists thought about the shape and structure of the atom, and has changed accordingly. The `modern' periodic table is very much like a later table by Meyer, arranged, as was Mendeleyev's, according to the size of the atomic weight, but with Group 0 added by Ramsay. Later, the table was reordered by Mosely according to atomic numbers (nuclear charge) rather than by weight. Although Mendeleev's table demonstrated the periodic nature of the elements, it remained for the discoveries of scientists of the 20th Century to explain why the properties of the elements recur periodically. In 1911 Ernest Rutherford published studies of the scattering of alpha particles by heavy atom nuclei which led to the determination of nuclear charge. He demonstrated that the nuclear charge on a nucleus was proportional to the atomic weight of the element. Also in 1911, A. van den Broek in a series of two papers proposed that the atomic weight of an element was approximately equal to the charge on an atom. This charge, later termed the atomic number, could be used to number the elements within the periodic table. Ernie Rutherford In 1913, the results of his measurements of the wavelengths of the xray spectral lines of a number of elements which showed that the ordering of the wavelengths of the x-ray emissions of the elements coincided with the ordering of the elements by atomic number. With the discovery of isotopes of the elements, it became apparent that atomic weight was not the significant player in the periodic law as Mendeleev, Meyers and others had proposed, but rather, the properties of the elements varied periodically with atomic number. Henry Moseley Harry D. Hubbard, of the United States National Bureau of Standards, modernized Mendeleyev's periodic table, and his first work was published in 1924. This was known as the "Periodic Chart of the Atoms". Into the 1930s the heaviest elements were being put up in the body of the periodic table, and Glenn Seaborg "plucked those out" while working with Fermi in Chicago, naming them the Actinide series, History of the Periodic Table 2 C. Pace, Instructor Glenn Seaborg which later permitted proper placement of subsequently 'created' elements - the Transactinides, changing the periodic table yet again. These elements were shown separate from the main body of the table. When Seaborg examined the Alexander Arrangement, he said that it was correct, and later told a photographer that it was his favorite periodic table. The last major changes to the periodic table resulted from Glenn Seaborg's work in the middle of the 20th Century. Starting with his discovery of plutonium in 1940, he discovered all the transuranic elements from 94 to 102. He reconfigured the periodic table by placing the actinide series below the lanthanide series. In 1951, Seaborg was awarded the Nobel Prize in chemistry for his work. Element 106 has been named seaborgium (Sg) in his honor. Glenn Seaborg During Glenn Seaborg's work on nuclear energy in the Manhattan Project during WWII he first placed the then new radioactive elements in a separate location. Before he died he had an Alexander Arrangement and determined that this 3-D method was now the correct way to show the Rare Earths. The Alexander Arrangement of the Elements, a three-dimensional periodic chart designed and patented by Roy Alexander and introduced in 1994, retains the separate Lanthanide and Actinide series, but integrates them at the same time, made possible by using all three dimensions. Further improvement provided by the Alexander Arrangement of the Elements is location of all the element data blocks in a continuous sequence according to atomic numbers while retaining all accepted property interrelationships. This eases use & understanding of the immense correlative power of the periodic chart in teaching, learning, and working with chemistry. History of the Periodic Table 3 C. Pace, Instructor