Question

Which of the following statements is not correct?
(AIPMT-2008)

Answer 1

In the diamond cubic unit cell, there are eight corner atoms, six face centered atoms and four more atoms at tetrahedral voids.
∴ Number of atoms present in a diamond cubic unit cell
$=1+3+4=8\text{atoms}$
The number of Bravais lattices in which a crystal can be categorized is 14.
Molecular solids are generally volatile.

For simple cubic (SC):
$Z_{eff}\text{for SC = 1}$
So,
$\text{The total volume occupied by sphere}V=1\times \frac{4}{3}\times \pi r^3$
$\text{Volume of the cube}=a^3=(2r{)}^3$
$\text{Packing efficiency (P.E.)}=\frac{1\times \frac{4}{3}\times \pi r^3}{(2r{)}^3}\times 100\%P.E.=\frac{\pi \times 100}{6}\%=52.33\%$
i.e., 52% of unit cell is occupied by atoms and 48% is empty, i.e., the fraction of the total volume occupied by the atoms in a primitive cell is $0.52$