Question

In Mendeleev’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with an atomic mass of $58.93$ amu was placed before nickel having an atomic mass of $58.71$ amu. Give a reason for the same.

Answer 1

Mendeleev's Periodic Table:

• In Mendeleev's periodic table the elements were arranged on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties.
• There were $63$ elements known at the time of Mendeleev's periodic table.
• $63$ elements were arranged in the increasing order of their atomic masses and found that the elements with similar chemical properties occur at regular intervals.
• Mendeleev's periodic law states that “the properties of elements are a periodic function of their atomic masses”.

Exceptions in Mendeleev's Table:

• There are a few exceptions when the elements with lower atomic mass are placed after higher atomic mass.
• In order to keep elements with similar properties in the same group, this was done.
• Hence, cobalt$\left(\mathrm{Co}\right)$ with an atomic mass of $58.93$ amu was placed before nickel$\left(\mathrm{Ni}\right)$ having an atomic mass of $58.71$ amu.