# The density of cr atoms is 7.2g/cm. If the unit cell has edge length 289 pm. Calculate the number of chromium atoms per unit cell. (at mass of cr = 52)

$(a)\;1\qquad(b)\;2\qquad(c)\;3\qquad(d)\;4$

Density $\rho = \large\frac{Z\times at.wt}{a^3\times N_A}$
$Z = \large\frac{\rho\times a^3\times N_A}{at.wt}$
$=\large\frac{7.2\times(289\times10^{-10})^3\times6.023\times10^{23}}{52}$
$\therefore n = 2$