Can the following groups of elements be classified as Dobereiner's triad ?
(a) Na, Si, Cl
(b) Be, Mg, Ca
Atomic mass of Be 9 , Na 23 , Mg 24 , Si 28 , Cl 35 , Ca 40
Explain by giving reason.

Explanation

(a) Na, Si and Cl have different properties, therefore, they do not form a Dobereiner's triad even though the atomic mass of the middle atom (Si) is approximately the average of the atomic masses of Na and Cl i.e.,
Na (23) , Si (28) , Cl (35)
Atomic mass of `Si=(23+35)/(2)=(58)/(2)=29`
(b) Be, Mg and Ca have many similar properties and also the atomic mass of the middle element Mg is approximately the average of the atomic masses of Be and Ca i.e.,
Be (9) , Mg (24) , Ca (40)
Atomic mass of `Mg=(9+40)/(2)=(49)/(2)=24.5`
Therefore, they form a Dobereiner's triad.