The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. This mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called as ? Give on example of such a set of elements.

Explanation

The arrangement of elements in which the atomic mass of middle element is almost the mean of atomic masses of first and thrid elements is known as Dobereiner's triads.
e.g., `Ca(40),Sr(88),Ba(137)`
Atomic mass of `Sr=(40+137)/(2)=88.5`
Other examples is Li (7), Na (23), K (39).