(a) In the modern periodic Table, why does cobalt with a higher atomic mass of 58.93 appear before nickel having a lower atomic mass of 58.71?
(c) Why could no fixed position be given to hydrogen in Mendeleev's periodic table?
(a) In the modern periodic table, elements are arranged according to their increasing atomic number. The atomic numbers of cobalt (Co) and nickel (Ni) are 27 and 28, respectively. Therefore, although cobalt (Co) has higher atomic mass, it appears before nickel (Ni) in the periodic table.
(b) Mendeleev's periodic law could not assign a fixed position to hydrogen in the periodic table because hydrogen resembled both alkali metals (Group 1) and halogens (Group 17) in some of its properties. Hydrogen reacts with metals to form ionic compounds called hydrides and also with non-metals to form covalent compounds.