Question

Inmendele′ ev '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

Because he had also taken another criteria for his classification that is [similar properties] of elements. So he placed Co before Ni because they may match with properties of its following elements.