Silver $(\text{at}wt=108\mathrm{g}{\mathrm{mol}}^{-1})$ has a density of $10.5\mathrm{g}/{\mathrm{cm}}^3$ The number of silver atoms on a surface area of${10}^{-12}{m}^2$ can be expressed in scientific notations as $Y\times {10}^x$ the value of ‘x’ is   

Volume of 1 unit cell =

$a^3=\frac{M\times Z}{N_0\times d}=\frac{108\times 4}{6.023\times {10}^{23}\times 10.5}$

$=6.83\times {10}^{-23}c{m}^3=6.83\times {10}^{-29}{m}^3$

Hence $a=4.09\times {10}^{-10}m$

$a^2=1.678\times {10}^{-19}$

The number of silver atoms on the surface area of ${10}^{-12}{m}^2$

$$\begin{gathered} =\frac{{10}^{-12}}{1.678\times {10}^{-19}}=Y\times {10}^7=Y\times {10}^x \\ X=7 \end{gathered}$$