CBSE Class 10 Science 2020 · Paper 31/5/2 · Question 17

(a) Any two: Mendeleev’s table is based on atomic mass, the modern table on atomic number; Mendeleev’s table has 8 groups with subgroups A and B, the modern table has 18 groups and no subgroups (other correct differences also earn credit). (b) Döbereiner grouped elements with similar properties in threes (triads), in which the atomic mass of the middle element is about the average of the other two: Li ($6.9$), Na ($23.0$), K ($39.0$), since $\frac{6.9+39.0}{2}=22.95\approx 23.0$. (c) The properties of elements are a periodic function of their atomic number.