The packing efficiency in a body centered cubic $(bcc)$ structure is closest to $.... \, \%$

Explain the calculation of packing efficiency in $hcp$ and $ccp$ structures with the help of a diagram.

$A$ 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 $.....$

An ionic compound is formed between a metal $M$ and a non-metal $Y$. If $M$ occupies half the octahedral voids in the cubic close-packed arrangement formed by $Y$,the chemical formula of the ionic compound is

Which of the following represents the hexagonal close-packed $(HCP)$ arrangement of spheres?

In the closest packing of atoms,there are