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Explain Modern Periodic Law and The present form of the periodic table. |
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Answer» Solution :(A) Work of Moseley and Modern PERIODIC Rules: When Mandeleev developed his periodic table, chemists knew nothing about the internal STRUCTURE of atom. The beginning of the 20th century witnessed profound developments in theories about sub-atomic particles. In 1913, the ENGLISH Physicist, Henry Moseley observed regularities in the characteristic X-ray spectra of the elements. Where, v = Frequency of X-ray emitted. Z = atomic number of element. (i) `sqrt v to Z`straight line obtained (ii) `v to Z`not obtained straight line He thereby showed that the atomic number is a more fundamental property of an element than its atomic mass. Modern Periodic Law : "The physical and chemical properties of the elements are periodic functions of their atomic numbers." (B) Modern Periodic table : Modern periodic table contains 7 period and 18 Groups. Period : Horizontal line is known as period. First period contain two elements (Hydrogen and Helium).  Groups : "Vertical columns in periodic table is known as Groups." According to the recommendation of INTERNATIONAL union of PURE and applied chemistry (IUPAC) the groups are numbered from 1 to 18 replacing the older notation of groups. In to VIII-A, VII-m, IB to VII-B. |
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