A compound is formed by two elements X and Y. The element Y forms cubic close packed arrangement and those of element X occupy one third of the tetrahedral voids. What is the formula of the compound?

Element Y forms a cubic close packed (CCP/FCC) arrangement. In a CCP structure, the number of atoms of Y per unit cell is 4, and the number of tetrahedral voids is $$2 \times 4 = 8$$.

Now, element X occupies one-third of the tetrahedral voids:

$$\text{Number of X atoms} = \frac{1}{3} \times 8 = \frac{8}{3}$$

The ratio of X to Y atoms is:

$$X : Y = \frac{8}{3} : 4 = \frac{8}{3} : \frac{12}{3} = 8 : 12 = 2 : 3$$

Hence, the correct answer is X$$_2$$Y$$_3$$.