Question

The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. Such an arrangement of elements called as :

• A. Dobereiner triad
• B. Mendeleev triad
• C. Newland triad
• D. None of the above.

Answer 1

The correct option is A Dobereiner triad

Dobereiner observed in a few atoms that the mass of one atom is equal to the average of two atoms. He defined almost all properties basing on atomic weight. Hence, this is called Dobereiner triad.

Lithium has 7, sodium has 23 whereas potassium has 39 as atomic masses.
$23=(7+39)/2=23$

Here, we could observe that atomic mass of sodium is the average of atomic masses of potassium and lithium.