For a tetrahedral structure,the limiting radius ratio $r_c^+ / r_a^-$ is .......

How many nearest neighbours are there for $Si$ and $O$ atoms in quartz crystals?

If $AB$ crystallizes in a zinc blende structure with $B^{-}$ ions at lattice points,what fraction of tetrahedral voids is occupied by $A^{+}$ ions? (Express as a percentage)

The ionic radii of $A^{+}$ and $B^{-}$ ions are $0.98 \times 10^{-10} \ m$ and $1.81 \times 10^{-10} \ m$. The coordination number of each ion in $AB$ is.......

In $CsCl$ lattice,the coordination number of $Cs^{+}$ ion is:

Copper crystallizes in $f.c.c.$ with a unit cell length of $361 \ pm$. What is the radius of copper atom? (in $pm$)