DEVELOPMENT OF THE PERIODIC TABLE
Although Dmitri Mendeleev is often considered the "father" of the periodic table, the work of many scientists contributed to its present form. Elements such as gold, silver, tin, copper, lead and mercury have been known since earliest times.
In 1649 the first scientific discovery of an element occurred. Hennig Brand, a German chemist, treated urine to a series of processes that resulted in the production of the element phosphorus. Over the next 200 years, a great deal of knowledge about elements and compounds was gained. By the middle of the 19th century, about 60 elements had been discovered. Scientists began to recognise patterns in the properties of these elements and set about developing 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 J.P. Cooke) found that these types of chemical relationships extended beyond the triad. During this time fluorine was added to the halogen group; oxygen, sulphur, 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.
In 1862, French geologist Alexandre-Emile Béguyer de Chancourtois listed the elements on paper tape and wound...
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