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The PeriodicTable.

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The Periodic Table Greek thinkers from around 400BC used the words 'element' and 'atom' to describe the differences and smallest parts of matter. These ideas are still used universally in modern times although it wasn't until much later that anything as complex as a 'periodic table' was devised. By the 19th century chemistry had progressed to the point of defining an element as a substance that could get no smaller. The number of elements discovered increased rapidly, but there was no way of telling how many remained to be discovered or where they might be found. As the number increased, however, it became evident that certain groups of elements could be classified into 'families' with similar chemical properties. The actual discovery of the periodic law came in the years between 1868 and 1870 and was made almost simultaneously by 'Lothar Meyer' in Germany and 'Dmitry Ivanovich Mendeleyev' in Russia. ...read more.


Mendeleyev's predictions were soon confirmed and new atomic weight determinations corrected the values he had questioned, and the discovery of the actual elements gallium, scandium, and germanium showed that these had almost exactly the properties that he had anticipated. From that time on, the periodic table assumed the basic form that, in spite of some modifications, it has retained ever since. Hydrogen, which was recognized as an anomalous element, was placed by itself at the beginning of the chart. The periodic table can be read either horizontally or vertically. If horizontally, the elements are arranged in a series not only by atomic weight, but also by 'valence'. Originally there were seven vertical columns below each element in the first period, designated by the Roman numerals I to VII. Further down the table appeared the 'transition elements' (iron, cobalt, and nickel). Because the properties of these transition elements set them apart, another column, VIII, was designated for them. ...read more.


It was quickly realized that the atomic number represented the positive charge of the atomic nucleus, and that this charge increased by one as the elements advanced along the table. The atomic number was therefore a more fundamental value than the atomic weight. In fact, it was found that the atomic weight was not a unique value for each element. Studies of radioactive elements and their end products showed that elements from different locations sometimes had different atomic weights, yet showed identical chemical properties. These were named 'isotopes'. Gilbert Lewis and Irving Langmuir between 1916 and 1920 pointed out that the chemical properties of an element are determined by the number of electrons in the outermost shell. The noble gases do not normally form compounds, because their outermost shells are filled. Other elements tend to form the ideal number to attain this maximum, either by losing electrons or by gaining electrons. Chemistry 2002 Tom Woodgate 10T ...read more.

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