For an ionic crystal of the type $AB$,the value of the limiting radius ratio is $0.40$. This value suggests that the crystal structure should be:

The space lattice of $CaF_2$ is:

$A$ cubic structure is formed where atoms of element $X$ are occupied at the corners of the cube and also at the face centers. Atoms of element $Y$ are present at the body center and at the edge centers. If all the atoms are removed along a plane passing through the middle of the cube (bisecting the four edges), the formula will become

In the rock salt structure,the number of formula units per unit cell is equal to

An example of fluorite structure is

How many formula units are there in the unit cell of sodium chloride $(NaCl)$?