Answer
1.87 $\times$ 10$^{21}$atoms
Work Step by Step
23.5 mg $\times$ $\frac{1 g}{1000mg}$ $\times$ $\frac{1 mol cpd}{339.8 g}$ $\times$ $\frac{45 mol atoms}{1 mol cpd}$ $\times$ $\frac{6.022 \times 10^{23} atoms}{mol}$ = 1.87 $\times$ 10$^{21}$atoms