According to Mendeleev’s periodic table Co (Atomic mass 58.9) has been placed ahead of _____.

The elements in Mendeleev’s Periodic table have been arranged in order of increasing atomic masses, but in some cases, the element with a higher atomic mass precedes the element with a lower atomic mass.

Co (Atomic mass 58.9) has been placed ahead of Ni (Atomic mass =58.7). No justification has been given for such anomalous positions.