Question

The three elements A, B and C with similar properties .the mass of Y is aplroximately 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

Such arrangement of elements is called Doberiener's triads. One such example is of the alkali metal group which is:
Lithium, Li
Sodium, Na
Potassium, K