Answer
There are $10^{18}$ silicon atoms in a grain of sand.
Work Step by Step
There is one silicon atom in each molecule of $SiO_2$. We can divide the volume of the grain of sand by the volume of a $SiO_2$ molecule to find the number of molecules in the grain of sand:
$\frac{\frac{4}{3}\pi~(0.25\times 10^{-3}~m)^3}{\frac{4}{3}\pi~(0.25\times 10^{-9}~m)^3} = 10^{18}$
There are $10^{18}$ silicon atoms in a grain of sand.
