Question 3 - Past Year - 2 Mark Questions - Chapter 5 Class 10 - Periodic Classification of Elements

Can the following groups of elements be classified as Dobereiner’s triad?

Answer

Dobereiner triads are those in which elements are arranged in such a way that the atomic mass of the middle element is the mean of the atomic masses of the first and third element.  

(a) Na, Si, Cl

Atomic mass of Na = 23 amu

Atomic mass of Si = 28 amu

Atomic mass of Cl = 35.5 amu

The mean of first and third elements i.e Na and Cl= (23 + 35.5)/2 = 29.25 

Since the atomic mass of Si is not equal to the mean of atomic masses of first and third elements , this is not a Dobereiner’s Triad.

(b) Be, Mg, Ca

Atomic mass of Be=9 amu

Atomic mass of Mg=24 amu

Atomic mass of Ca= 40 amu

The mean of first and third elements i.e. Be and Ca= (40 + 9)/2 = 24.5

Since the atomic mass of Mg is equal to the mean of atomic masses of first and third elements, this is a Dobereiner’s Triad.