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? Give one example of such a set of elements.

(N/A) The arrangement of these elements is known as a $Dobereiner$ triad.
According to $Dobereiner's$ law of triads,when elements are arranged in groups of three in the order of increasing atomic masses,the atomic mass of the middle element is approximately the arithmetic mean of the atomic masses of the other two elements.
An example of such a set is the triad of Lithium $(Li)$,Sodium $(Na)$,and Potassium $(K)$.
Atomic mass of $Li = 6.9$,Atomic mass of $K = 39.1$.
Average mass = $(6.9 + 39.1) / 2 = 46 / 2 = 23$,which is the atomic mass of Sodium $(Na)$.