How is the problem regarding the position of cobalt("_59CO) and nickel ("_NI)` in Mendeleev's periodic table resolved in Modern periodic table ?

Explanation

In Mendeleev's periodic table, the elements are arranged in the order of increasing atomic masses. So, there was an ambiguity regarding the sequence (or order) in which `.^(59)Co` and `.^(59)Ni`, having same whole number atomic mass, are placed . However, this problem was resolved in the modern periodic table. In the modern periodic table, the elements are arranged in the order of increasing atomic number . The atomic number of cobalt is 27 and that of nickel is 28 . Thus, cobalt which has lower atomic number (27) is placed before nickel which has a higher atomic number (28).