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, Cobalt (Co) with a higher atomic mass of 58.93 u is placed before nickel (Ni) due to the following reasons
(i) The properties of cobalt are similar to those of rhodium (Rh) and iridium (lr) (same group) and
(ii) The properties of nickel are similar to those of palladium (Pd) and platinum (Pt) (same group).