Ans: The topic of Mendeleev’s Periodic Table covered in Chapter 5 of NCERT Solutions for Class 10 Science includes the following points: Elements are arranged in increasing order of their atomic masses. Eight groups are present. Each group is subdivided into subgroups ‘a’ and ‘b’ Noble gases were not discovered at that time, so the groups for noble gases were not present. Isotopes have no place in this periodic table.