Question

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)

Answer 1

(a) The atomic masses of Na, Si and Cl are 23, 28 and 35.5 respectively.
According to Dobereiner's law of triads, when elements are arranged in the increasing order of their atomic masses, a three element group with similar chemical properties is obtained. The atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two.

The above set of elements does not form a triad because arithmetic mean of  the atomic masses of Na and Cl `=(23+35.5)/2=29.25`
This value is approximately equal to the atomic mass of silicon, but they do not form a triad because the elements do not exhibit similar chemical properties.

(b) The set of elements Be, Mg and Ca with atomic masses 9, 24 and 40 respectively, form a triad  because they have similar chemical properties and the atomic mass of magnesium (Mg) is  approximately equal to the average of the atomic masses of beryllium (Be) and calcium (Ca).
The arithmetic mean of the atomic masses of  Be and Ca `=(9+40)/2=24.5` which is approximately equal to 24.