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

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, in order to maintain the similarity in chemical properties of elements belonging to same group. It means, the properties of cobalt are similar to those of rhodium (Rh) and iridium (Ir) and the properties of nickel are similar to those of palladium (Pd) and platinum (Pt).