Question

In a compound, atoms of element $Y$ form ccp lattice and those of element $X$ occupy $2 / 3\, r d$ of tetrahedral voids. How many atoms of $X$ are present in one formula unit of the compound?

Answer 1

No. of $Y$ atoms per unit cell in ccp lattice $=4$
No. of tetrahedral voids $=2 \times 4=8$
No. of tetrahedral voids occupied by $X$
$=\frac{2}{3} \times 8=\frac{16}{3}$
Therefore, formula of the compound
$=X_{16 / 3} Y_{4}$
$=X_{16} Y_{12} $
$=X_{4} Y_{3}$