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Question

In Mendeleev’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with an atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give a reason for the same.


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Solution

Mendeleev's Periodic Table:

  • In Mendeleev's periodic table the elements were arranged on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties.
  • There were 63 elements known at the time of Mendeleev's periodic table.
  • 63 elements were arranged in the increasing order of their atomic masses and found that the elements with similar chemical properties occur at regular intervals.
  • Mendeleev's periodic law states that “the properties of elements are a periodic function of their atomic masses”.

Exceptions in Mendeleev's Table:

  • There are a few exceptions when the elements with lower atomic mass are placed after higher atomic mass.
  • In order to keep elements with similar properties in the same group, this was done.
  • Hence, cobalt(Co) with an atomic mass of 58.93 amu was placed before nickel(Ni) having an atomic mass of 58.71 amu.

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