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

Explanation

This is regarded as a defect in the Mendeleev's periodic table. However, the main reason for making the arrangement was that the elements with similar characteristics must be grouped together. In this case,
Cobalt `(Co)` should be in the company of the elements Rhodium `(Rh)` and Iridium `(Ir)`.
Nickel `(Ni)` should be in the company of the elements Palladium `(Pd)` and Platinum `(Pt)`.