Mendeleev is presented the periodic table containing 63 known elements. This table was arranged on the basis of atomic mass of the elements. The original table consisted eight vertical columns called groups. This table consisted a number of discrepancies such as anomalous pairs; position of isotopes and position of lanthanides and actinides etc. Some of these discrepancies are described below: (i) Anomalous pairs: The original periodic law based on atomic masses is violated in case of four pairs of elements in order to give them positions on the basis of properties. The elements having higher atomic masses have been assigned position before the elements having lower atomic masses at four places as shown below:
|(a) Ar k||(b) Co Ni||(c) Te I||(d) Th Pa|
|At. mass||40 39||59. 58.6||127.5 127||232 2 31|
|At. no.||18 19||27 28||52 53||90 91|
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