The correct option for the number of tetrahedral and octahedral voids in a hexagonal primitive unit cell is:

What is the formula for packing efficiency?

$A$ 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 $.....$

Calculate the total number of tetrahedral and octahedral voids formed in $0.6 \ mol$ of a compound if it forms $hcp$ structure.

The total number of voids (octahedral + tetrahedral) in $1$ mole of a compound forming a hexagonal close-packed $(HCP)$ structure is:

Atoms of element $X$ form $hcp$ lattice and those of element $Y$ occupy $\frac{2}{3}$ of its tetrahedral voids. The percentage of element $X$ in the lattice is ..... . (Nearest integer)