A metal M crystallizes into two lattices :- face centred cubic (fcc) and body centred cubic (bcc) with unit cell edge length of $$2.0$$ and $$2.5\,\mathop A\limits^o $$ respectively. The ratio of densities of lattices fcc to bcc for the metal M is ____________. (Nearest integer)

A metal M forms hexagonal close-packed structure. The total number of voids in 0.02 mol of it is _________ $$\times$$ $$10^{21}$$ (Nearest integer).

(Given $$\mathrm{N_A=6.02\times10^{23}}$$)

Ionic radii of cation $$\mathrm{A}^{+}$$ and anion $$\mathrm{B}^{-}$$ are 102 and $$181 \,\mathrm{pm}$$ respectively. These ions are allowed to crystallize into an ionic solid. This crystal has cubic close packing for $$\mathrm{B}^{-}$$. $$\mathrm{A}^{+}$$ is present in all octahedral voids. The edge length of the unit cell of the crystal AB is __________ pm. (Nearest Integer)