CBSE Class 10 Science 2020 · Paper 31/3/1 · Question 17

(i) Category A (Li, Na, K): the atomic mass of sodium is the mean of those of lithium and potassium, $\frac{7+39}{2}=23$. For B, $\frac{14+74}{2}=44\ne 31$; for C, $\frac{10.8+69.7}{2}=40.25\ne 27$. (ii) Mendeleev placed elements with similar physical and chemical properties (similar formulae of oxides and hydrides) in the same group. The elements of each category resemble one another, but the physical and chemical properties of categories A, B and C differ from one another, so he placed them in three different groups. (iii) No. Newlands' law of octaves was applicable only up to calcium. It holds for category A, whose elements all come before calcium, but not for B and C, whose heaviest members, arsenic and gallium, come after calcium.