What is the coordination number of sodium in $Na_2O$?

An element occurring in the $bcc$ structure has $12.08 \times 10^{23}$ unit cells. The total number of atoms of the element in these cells will be:

The number of unit cells present in $39 \ g$ of potassium if it crystallizes as a body-centered cubic $(BCC)$ structure is ($N = \text{Avogadro number}$, $\text{At. wt. of potassium} = 39$).

If three elements $A, B, C$ crystallize in a cubic solid lattice with $B$ atoms at the cubic centers, $C$ atoms at the center of edges, and $A$ atoms at the corners, then the formula of the compound is

The crystal structure of $AB$ is of $NaCl$ type. If the radius of the $A^+$ ion is $x \ pm$ and the radius of the $B^-$ ion is $y \ pm$, then the edge length of the unit cell will be ..........

In calcium fluoride,having the fluorite structure,the coordination numbers for calcium ion $(Ca^{2+})$ and fluoride ion $(F^{-})$ are