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Chemistry

Periodic Table

Dmitri Mendeleev, creator of the Periodic TableDmitri Mendeleev, creator of the Periodic TableThe Periodic Table of elements is a list of all known elements arranged in order of their atomic numbers (the number of protons in each atom). It was designed by Russian scientist Dmitri Mendeleev (1834–1907). He originally arranged the known elements in order of their atomic mass. Those with similar chemical properties were arranged into vertical columns. This resulted in some gaps in his horizontal rows (“periods”), but Mendeleev decided that these would be filled by elements still to be discovered. He predicted the existence of germanium, gallium and scandium, which were all discovered in his lifetime. Of the 118 elements known so far, 94 occur naturally on Earth; the rest are artificially created.



Iodine crystalsIodine crystals

Ordering the elements

Ordering elements according to their atomic mass has some problems. In order to place iodine in the same group as other elements with similar properties (e.g. fluorine, chlorine and bromine), Mendeleev had to put it after tellurium, even though tellurium has a higher atomic mass than iodine. Using the atomic number instead of atomic mass as the organizing principle (first proposed by the British chemist Henry Moseley in 1913) solved the problem—iodine has a higher atomic number than tellurium. In fact in most cases atomic mass and atomic number give the same order.

Element number 101, mendelevium, is named after Dmitri Mendeleev, creator of the Periodic Table.

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