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

The $8:8$ type packing is present in which of the following?

What is the volume occupied by particles in $BCC$ structure if '$a$' is the edge length of the unit cell?

$A$ solid has a structure in which $W$ atoms are located at the corners of a cubic lattice,$O$ atoms at the centre of edges,and $Na$ atom at the centre of the cube. The formula for the compound is

For an ionic crystal with the general formula $AX$,the coordination number is $6$. What is the radius ratio range?

For tetrahedral coordination number,the radius ratio $\frac{r_{c^+}}{r_{a^-}}$ is