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) In modern periodic table, elements are arranged according to the increasing atomic number. The atomic number of cobalt and nickel is 27 and 28 respectively. Therefore cobalt has higher atomic mass and appears before nickel in the periodic table.

(b) Mendeleev’s period law failed to explain the position of hydrogen as it resembles both the properties of group 1 alkali metals and group 17 halogens. Hydrogen reacts metal to form ionic compounds called hydrides and also with non-metals to form covalent compounds.