Question

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.

Answer 1

There are instances in Mendeleev's Periodic Table when an element with higher atomic mass is placed before and element with lower atomic mass. This was done to ensure that elements with similar chemical properties were put in the same group. Due to this, cobalt was placed before nickel in spite of the fact that cobalt has higher atomic mass than nickel.