In a compound $XY$,the atoms of element $Y$ form a $ccp$ structure,while the atoms of element $X$ occupy $2/3$ of the tetrahedral voids. The molecular formula of the compound will be ..........

$A$ crystalline solid is made of $X$,$Y$ and $Z$ elements. Atoms of $X$ form $FCC$ packing; atoms of $Y$ occupy octahedral voids while atoms of $Z$ occupy tetrahedral voids. What will be the simplest formula of the solid if atoms along one body diagonal are removed?

What is the packing efficiency of $fcc$ crystal structure (in $\%$)?

Atoms of element $A$ occupy $\frac{1}{3}$ of the octahedral voids,while atoms of element $B$ form an $hcp$ structure. Determine the formula of the compound formed by $A$ and $B$.

An octahedral void is surrounded by how many spheres?

$A$ compound formed by $Mg$,$Al$,and $O$ is found to have a cubic close-packed $(CCP)$ array of oxide ions,in which $Mg^{2+}$ ions occupy $\frac{1}{8}^{th}$ of the tetrahedral voids and $Al^{3+}$ ions occupy $\frac{1}{2}$ of the octahedral voids. The formula for the compound is: