Question

how is the problem regarding the position of Cobalt and nickel in mendeleev's periodic table resolved in modern periodic table?

Answer 1

In the mendeleev periodic table the elements were arranged , classified on the basis of their atomic masses.But in the modern periodic table the elements were arranged in order of their atomic number . The mass number of cobalt was greater than the mass of nickel but the number of electrons were less .  Therefore, there was confusion in arrangement of these two elements.

Answer 2

In the Mendeleev periodic table the elements were arranged on the basis of their atomic masses.Yet in the modern periodic table the elements were arranged in order of their atomic number . The mass number of cobalt was greater than the mass of nickel but the number of electrons were less . So there was  a bit of confusion in arrangement of these two elements.