The number of atoms in 0.004 g of magnesium will be

 Mass of Mg$=4\times {10}^{-3} \mathrm{grams}$

1 mole Mg=24 gm Mg$=6\times {10}^{23} \mathrm{grams}$

$4\times {10}^{-3} \mathrm{gram} \mathrm{Mg}=\frac{6\times {10}^{23}}{24}\times 4\times {10}^{-3}$

$={10}^{20 }\mathrm{atoms}$