What is the crystal structure of $Na_2O$?

In the rock salt structure,the $ccp$ arrangement of $Cl^{-}$ ions creates tetrahedral voids,which are....

$(i)$ What is meant by the term 'coordination number'?
$(ii)$ What is the coordination number of atoms:
$(a)$ in a cubic close-packed structure?
$(b)$ in a body-centred cubic structure?

An alloy of copper,silver,and gold is found to have copper atoms in an $fcc$ lattice. If silver atoms occupy the edge centers and gold atoms are present at the body center,what is the formula of the alloy?

The coordination number of an atom in a body-centered cubic $(BCC)$ structure is:
[Assume that the lattice is made up of atoms.]

The ratio of the effective number of atoms in a unit cell of $fcc$ and $bcc$ lattices is