An element M crystallises in a body centred cubic unit cell with a...

$M$ is body centred cubic, $∴ Z = 2$

Let mass of $1$ atom of $M$ is $A$

Edge length $= 300pm$

Density $= 6g / {cm}^{3}$

$∴ 6g / {cm}^{3} = \dfrac{Z \times A}{{\left(300 \times {10}^{- 10}\right)}^{3}} = \dfrac{2 \times A}{27 \times {10}^{- 24}}$

$A = 81 \times {10}^{- 24}g$

Atoms of $M = \dfrac{Totalmass}{Massofoneatom} = \dfrac{180}{81 \times {10}^{- 24}} = 22 . 22 \times {10}^{23} ≅ 22 \times {10}^{23}$