A compound is formed by two elements A and B. The element B forms cubic close packed structure and atoms of A occupy $1/3$ of tetrahedral voids. If the formula of the compound is ${\text{A}}_x{\text{B}}_y$, then the value of $x+y$ is:

In a CCP (cubic close packed) structure, if the number of atoms of element B is $n$, then:Number of tetrahedral voids $=2n$.Number of octahedral voids $=n$.For a unit cell, $n=4$ for B.Number of tetrahedral voids $=2\times 4=8$.Atoms of A occupy $1/3$ of these voids:Number of A $=\frac{1}{3}\times 8=\frac{8}{3}$.Ratio A : B $=\frac{8}{3}:4=8:12=2:3$.Formula is ${\text{A}}_2{\text{B}}_3$.Here $x=2,y=3$. Therefore, $x+y=2+3=5$.