A compound is formed by two elements X and Y. The atoms of element...

Let the number of atoms of element Y in the hcp lattice be $N$.

The number of octahedral voids in a close-packed structure is equal to the number of atoms in the lattice, which is $N$.

Since atoms of element X occupy $\dfrac{1}{3}$ of the octahedral voids, the number of atoms of X is $\dfrac{N}{3}$.

The ratio of the number of atoms of X to that of Y is $\dfrac{N}{3} : N = 1 : 3$.

Therefore, the formula of the compound is $\text{XY}_3$.

Answer: $\text{XY}_3$