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According to Mendeleev's periodic law, elements are arranged in the order of their increasing atomic masses. When Co and Ni were put in the correct group on the basis of their chemical properties it was found later that Co having higher mass of 58.9 comes first and nickel with slightly lower atomic mass of 58.7 comes later. Mendeleev's periodic law could not explain this situation of wrong order of atomic masses! Modern periodic table resolved this problem. As according to the modern periodic law, elements are arranged in their increasing order of atomic nos. The atomic no of cobalt is 27 comes first(lower at. no) and nickel with 28 comes next(higher at. no) even if their atomic masses are in the wrong order...
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