A metal ( $M$ ), crystallises in fcc lattice with edge length of $4.242 \mathop {\rm{A}}\limits^{\rm{o}}$. What is the radius of $M$ atom (in $\mathop {\rm{A}}\limits^{\rm{o}}$ )?

Sodium metal crystallises in a body centred cubic lattice with edge length of $x \mathop {\rm{A}}\limits^{\rm{o}}$. If the radius of sodium atom is $1.86 \mathop {\rm{A}}\limits^{\rm{o}}$ the value of $x$ is

The incorrect statement about crystals with schottky defect is

A metal crystallises in two cubic phases, fcc and bcc with edge lengths $3.5 \mathop {\rm{A}}\limits^{\rm{o}}$ and $3 \mathop {\rm{A}}\limits^{\rm{o}}$ respectively. The ratio of densities of fcc and bcc is approximately