What is the coordination number of the $Ca^{2+}$ ion in the fluorite structure?

In the fluorite structure,the coordination number of $Ca^{2+}$ ion 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? ........... $\, pm$

The number of nearest neighbours in a $BCC$ unit cell is

If the cation is present in the tetrahedral void,what is the radius ratio ${r_ + }/{r_ - }$?

An element crystallizes in a $bcc$ lattice. The number of nearest neighbors of the element is