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A necessary prerequisite to the construction of the periodic table was the discovery of the individual elements Although elements such as gold silver tin copper lead and mercury have been known since antiquity the first scientific discovery of an element occurred in 1649 when Hennig Brand discovered phosphorous During the next 200 years a vast body of knowledge concerning the properties of elements and their compounds was acquired by chemists By 1869 a total of 63 elements had been discovered As the number of known elements grew scientists began to recognize patterns in properties and began to develop classification schemes In 1817 Johann Dobereiner noticed that the atomic weight of strontium fell midway between the weights of calcium and barium elements possessing similar chemical properties In 1829 after discovering the halogen triad composed of chlorine bromine and iodine and the alkali metal triad of lithium sodium and potassium he proposed that nature contained triads of elements the middle element had properties that were an average of the other two members when ordered by the atomic weight the Law of Triads This new idea of triads became a popular area of study Between 1829 and 1858 a number of scientists Jean Baptiste Dumas Leopold Gmelin Ernst Lenssen Max von Pettenkofer and JP Cooke found that these types of chemical relationships extended beyond the triad During this time fluorine was added to the halogen group oxygen sulfur selenium and tellurium were grouped into a family while nitrogen phosphorus arsenic antimony and bismuth were classified as another Unfortunately research in this area was hampered by the fact that accurate values of were not always available First Attempts At Designing a Periodic Table If a periodic table is regarded as an ordering of the chemical elements demonstrating the periodicity of chemical and physical properties credit for the first periodic table published in 1862 probably should be given to a French geologist AEBeguyer de Chancourtois De
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