Can the following groups of elements be classified as Dobereiner's triads?
(a) Na, Si, Cl
(b) Be, Mg, Ca
Give reason for your answer.
(Atomic masses : Be 9 ; Na 23 ; Mg 24 ; Si 28 ; Cl 35.5 ; Ca 40)

(a) No, the elements Na, Si, and Cl do not form Dobereiner's triads. This is because the elements Na, Si, and Cl do not have similar properties even though the atomic mass of middle element Si is almost equal to the average atomic mass of the first element Na and third element Cl. This is because Sodium (Na) is a metal, Silicon (Si) is a metalloid, and Chlorine (Cl) is a nonmetal, hence, their properties are different.

(b) Yes, the elements Be, Mg, and Ca form Dobereiner's triads. This is because the elements Be, Mg, and Ca have similar properties and the atomic mass of middle element Mg is almost equal to the average atomic mass of the first element Be and the third element Ca.

The arithmetic mean of the atomic masses of  Be and Ca =(9+40)/2=24.5 which is approximately equal to 24 (atomic mass of Mg).