Define: Packing Efficiency.

In a face-centred cubic arrangement of $A$ and $B$ atoms,$A$ atoms are at the corners of the unit cell and $B$ atoms are at the face centres. If one of the $A$ atoms is missing from one corner of the unit cell,what is the simplest formula of the compound?

Potassium hexachloroplatinate $(K_2PtCl_6)$ adopts an antifluorite crystal structure. The corners and face centers of the cubic unit cell are occupied by :-

Fill in the blanks:
$(a)$ In $ccp$ structure,$a = \dots \dots r$.
$(b)$ In $hcp$ structure,packing efficiency $= \dots \dots \%$.
$(c)$ In $bcc$ structure,percentage of packing efficiency $= \dots \dots \%$.

In a compound,nitrogen atoms $(N)$ make a cubic close-packed lattice and metal atoms $(M)$ occupy one-third of the tetrahedral voids present. Determine the formula of the compound formed by $M$ and $N$.

The fraction of total volume occupied by the atoms present in a simple cubic unit cell is