In a face-centered cubic lattice,a unit cell is shared equally by how many unit cells?

$A$ 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

$Na$ and $Mg$ crystallise in $b.c.c.$ and $f.c.c.$ type crystals respectively. The number of atoms of $Na$ and $Mg$ present in the unit cell of their respective crystals is:

$A$ solid $AB$ crystallises in $NaCl$ structure. The anion $B$ forms $fcc$ lattice,while the cation $A^{+}$ occupies all the octahedral holes. If all the particles along a diagonal plane (as shown in the figure) are removed,then the formula of the solid becomes

Find the radius of an atom in $fcc$ unit cell having edge length $393 \ pm$. (in $pm$)

What is the radius ratio $r^+/r^-$ for $NaCl$?