The number of tetrahedral voids per atom in a crystal is ......

Which of the following metals crystallizes in a hexagonal close-packed $(hcp)$ structure?

What does a tetrahedral void in a crystal represent?

In a crystal lattice,anions $A$ form an $hcp$ array,and $2/3$ of the tetrahedral voids are occupied by cations $C$. What is the formula of the crystal?

In a simple cubic lattice,what percentage of the space is left unoccupied by the spheres?

Explain three-dimensional close packing from two-dimensional hexagonal close packing. Explain the formation of face-centered cubic $(FCC)$ unit cell and hexagonal close packing $(HCP)$ in three dimensions.