Answer
$2.476 \times 10^{19} Mg \space atoms $
Work Step by Step
We can find the number of magnesium atoms in 1.00 mg crystal of magnesium as
$1.00\times 10^{-3}g \times \frac{1 mol\space Mg}{24.31 g\space Mg}\times \frac{6.02\times 10^{23}Mg\space atoms}{1 mol Mg}=2.476 \times 10^{19} Mg \space atoms $
