Question

How is the problem regarding the position of cobalt (59Co) and nickel (59Ni) in mendeleev's periodic table resolved in modern periodic table?

Answer 1

The atomic number of cobalt is 27 and that of nickel is 28.
Now, according to modern periodic law, the elements are arranged in the order of increasing atomic numbers.
So, cobalt with lower atomic number (27) should come first and nickel with highest atomic number (28) should come later, even if their atomic masses are in the wrong order. 
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