Question

How were the positions of cobalt and nickel resolved in the modern periodic table?

Answer 1

As per Mendeleev's classification, elements are arranged in an increasing order of atomic masses, but cobalt, with a higher atomic mass was placed before nickel. Cobalt has an atomic mass of 58.9 and nickel has an atomic mass of 58.7, which is slightly lower than that of cobalt.  Mendeleev could not explain this point. This problem was resolved in the modern periodic table. According to the modern periodic law, elements are arranged in the increasing order of their atomic numbers. The atomic numbers of cobalt and nickel are 27 and 28 respectively. So, cobalt with a lower atomic number should come before nickel.