CBSE Class 10 Science 2022 · Paper 31(B)/6 · Question 2(a)

Döbereiner's triads are groups of three elements with similar chemical properties, identified by the German chemist Johann Wolfgang Döbereiner. When the three elements of a triad are written in the order of increasing atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two. Three elements of such a triad: lithium (Li), sodium (Na) and potassium (K). Their atomic masses are $6.9$, $23.0$ and $39.0$, and $\frac{6.9+39.0}{2}=22.95\approx 23.0$, the atomic mass of sodium. (Other triads: calcium, strontium and barium; chlorine, bromine and iodine.)