CBSE Class 10 Science 2019 · Paper 31/1/2 · Question 3(b)

(a) No. Na, Si and Cl cannot be classified as a Dobereiner's triad. The mean of the atomic masses of Na and Cl, $\frac{23+35}{2}=29$, is close to that of Si ($28$), but the three elements do not have similar properties: sodium is a reactive metal, silicon a metalloid and chlorine a non-metal. (b) Yes. Be, Mg and Ca form a Dobereiner's triad. They are similar metals of valency $2$, and the atomic mass of Mg ($24$) is roughly the mean of those of Be and Ca: $\frac{9+40}{2}=24.5$.