What is the two-dimensional coordination number of a molecule in a square close-packed layer?

Potassium crystallizes in a $bcc$ lattice. What is the coordination number of potassium in the potassium metal?

In the zinc blende structure,zinc atoms fill up:

In $NaCl$ lattice,the coordination number of $Cl^{-}$ ion is:

What is the coordination number of $Cu$ in a face-centered cubic $(fcc)$ lattice?

If the length of the body diagonal of a $FCC$ unit cell is $x \ \mathring{A}$, the distance between two nearest octahedral voids in the cell in $\mathring{A}$ is