Question

One of the defects of Mendeleev's periodic table was the inversion of elements. Explain with an example.

Answer 1

Mendeleev arranged the elements in the increasing order of atomic mass. This created certain defects in the periodic table.

For example:

• The atomic number of Cobalt (Co) is $27$ and that of Nickel (Ni) is $28$. However, in Mendeleev's periodic table Cobalt was placed after Nickel because the atomic mass of Cobalt is $(58.9)$ more than Nickel $(58.6)$.
• Thus, one of the defects of Mendeleev's periodic table was the inversion of elements.
• This defect is rectified in the Modern periodic table as the elements are classified in the increasing order of atomic number.