$A$ compound made up of elements $A$ and $B$ (with a general formula $A_x B_y$), where $B$ forms an $hcp$ lattice and $A$ occupies $2/3$ of the tetrahedral voids. The formula of the compound is

In an ionic solid,anions $(B)$ are arranged in an $hcp$ array and cations $(A)$ occupy $\frac{2}{3}$ of the octahedral voids. What is the formula of the ionic compound?

Explain tetrahedral and octahedral voids.

How will you distinguish between the following pairs of terms:
$(iii)$ Tetrahedral void and octahedral void?

$A$ metal forms an $fcc$ structure. Calculate the volume of the $fcc$ unit cell in $\text{cm}^3$ if the void volume is $1.66 \times 10^{-23} \text{ cm}^3$.

$A$ two-dimensional solid is made by alternating circles with radius $a$ and $b$ such that the sides of the circles touch. The packing fraction is defined as the ratio of the area under the circles to the area under the rectangle with sides of length $x$ and $y$. The ratio $r=b/a$ for which the packing fraction is minimised is closest to $.....$