Question

How many lithium atoms are present in a unit cell with edge length $3.5A$ and density $0.53{gcm}^{-3}$ ? (Atomic mass of $Li=6.94)$

• A. $2$
• B. $1$
• C. $4$
• D. $6$

Answer 1

The correct option is A $2$

Density of a crystal is given by: $d=\frac{Z\times M}{N_A\times a^3}$

$⟹Z=\frac{a^3\times d\times N_A}{M}$

$⟹Z=\frac{(3.5\times {10}^{-8}{)}^3\times 0.53\times 6.023\times {10}^{23}}{6.94}=1.97\approx 2$

Hence, Option "A" is the correct answer.