Question

Can these group of elements be classified as Dobereiner's triads ? Explain your answer.

• Be , Mg , Ca
• N , O , F

Answer 1

Dobereiner's triads include sets of three elements in order of increasing atomic masses such that the atomic mass of the middle element is approximately the arithmetic mean of the atomic masses of other two elements.
Now,
Mass of Be = 9 u
Mass of Mg = 24 u
Mass of Ca = 40 u
Arithmetic mean of masses of Be and Ca = (9 + 40)/2 = 24.5 which is nearly equal to atomic mass of magnesium. Therefore, this set can be classified as Dobereiner's triad.
Mass of N = 14 u
Mass of O = 16 u
Mass of fluorine = 19 u
Arithmetic mean of masses of N and F = (14 + 19)/2 = 16.5 which is nearly equal to atomic mass of oxygen. Therefore, this set can be classified as Dobereiner's triad.
Dobereiner could identify only three triads from the elements known at that time. Hence, this system of classification into triads was not found to be useful.