Question

33.
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. What is such an arrangement of elements
called as? Give one example of such a set of elements.​

Answer 1

Answer:

Such an arrangement of elements is called as triad. Triad of elements was proposed by Dobereiner. He arranged elements in triads. In a triad, the atomic mass of the middle element was equal to the average of atomic massed of the other two elements. 

Lithium, Sodium and Potassium make a triad. Atomic massed of Li. Na and K are 6.9, 23.0 and 39.0 respectively. Average of masses of Li and K is approximately 23.0 which is equal to the atomic mass of Na.