CBSE Class 10 Answered

Mendeleev's periodic table was based on the law that the properties of elements are the periodic function of their atomic masses.

So, cobalt with atomic mass 58.93 should be placed after nickel with atomic mass 58.71.

But to maintain similarity in properties, it had to be placed before nickel in Mendeleev's Periodic Table. 

Modern periodic table the classification of elements is based on the law that the properties of elements are the periodic function of their atomic numbers.

So, the problem was resolved because cobalt has a lower atomic number (27) than that of nickel (28). 

Therefore nickel with atomic number 28 is placed after cobalt with atomic number 27.