An atomic substance A of molar mass 12g mol ^ – 1 has a cubic...

The density of crystal can be calculated as follows,

$ρ = \dfrac{Z \times M}{{N}_{A} \times {a}^{3}}$

$ρ =$density of the unit cell

$Z =$Effective atomic number

$M =$Molar mass

${N}_{A} =$Avogadro's number

$a =$Edge length of the unit cell

$3 = \dfrac{Z \times 12}{6 . 02 \times {10}^{23} \times ( 3{)}^{3} \times {10}^{- 24}}$

$Z = \dfrac{6 . 02 \times {10}^{- 1} \times 27}{4} = 4$