Position of the elements in the periodic table:
How is the problem regarding the position of cobalt `(.^(59)Co)` and nickel `(.^(59)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).