(a) Isotopes of an element are species that have the same atomic number but different atomic mass. So they are not given separate place in periodic table and are based on the atomic number of that particular element.

(b) As per the Mendeleev’s classification, elements are arranged in the periodic table in an increasing order of their atomic masses . Cobalt which has a higher atomic mass was placed before nickel. It has atomic mass of 58.9 and Nickel has an atomic mass of 58.7. In the modern periodic table, elements are arranged in the increasing order of their atomic numbers and hence the cobalt and nickel have 27 and 28 atomic number each and cobalt with lower atomic number is placed before nickel.