What is the coordination number of sodium ions in the case of sodium chloride structure?

In a $ccp$ lattice of $X$ and $Y$,$X$ atoms are present at the corners while $Y$ atoms are at face centers. What would be the formula of the compound if one of the $X$ atoms from a corner is replaced by a $Z$ atom?

The vacant space in the $bcc$ unit cell is .............. $\%$

Which of the following is correct for the crystal lattices mentioned below?

An alloy made up of $A$ and $B$ metals crystallises in a cubic lattice, where $B$ atoms occupy the corners and $A$ atoms occupy the face centers. The formula of the alloy formed is

$A$ metal $X$ crystallises in a face-centred cubic arrangement with the edge length $862 \, pm$. What is the shortest separation of any two nuclei of the atom? ........... $\, pm$