Question

A compound is formed by two elements $X$ and $Y$. Atoms of the element $Y$ (as anions) make ccp and those of the element $X$ (as cations) occupy all the octahedral voids. What is the formula of the compound?

Answer 1

Since, atoms of $Y$ forms CCP (cubic close packing) which is analogous to FCC (face centered cubic)

So, effective number of atoms of $Y$ in lattice $=4$

Number of octahedral voids $=$ Number of atoms forming CCP $=4$

So, effective number of atoms of $X$ in the lattice $=4$

Now, $X:Y\Rightarrow X:Y$

$4:4$ $1:1$

So, the formula of the compound is $XY$