Question

How were the position of Cobalt and Nickel resolved in the modern periodic table

Answer 1

as the atomic no of cobalt comes before that of nickel so cobalt was placed before nickel in the periodic table.so it could resemble rhodium in properties,if atomic mass was taken as the basis,it would not have been possible

Answer 2

In modern periodic table elements are placed on the basis of atomic number and not atomic mass.
Atomic number of Cobalt and Nickel is 27 and 28 respectively. Hence cobalt will come before nickel though its atomic mass is greater then nickel