(a) In the modern Periodic Table, why does cobalt with higher atomic mass of 58.93 appear before nickel having lower atomic mass of 58.71?
(b) Why could no fixed position be given to hydrogen in Mendeleev’s periodic table?
(a) Modern periodic table arranges the elements according to increasing atomic numbers. So, the atomic number of cobalt (27) comes first whereas the atomic number of nickel (28) comes later.
(b) In Mendeleev’s periodic table, hydrogen has been placed in group I since like alkali metals, hydrogen also combines with halogens, oxygen and sulphur to form compounds having similar formulae. This means that hydrogen resembles alkali metals in some of the properties. Hydrogen also resembles halogens in some of the properties. So, hydrogen could also be placed in group VII of halogen elements.
Thus, Mendeleev’s periodic law could not assign a correct position to hydrogen in the periodic table.