Answer
All $\mathrm {Au}$ atoms must be $^{197} \mathrm{Au}$ (with no isotopes) in case all Au atoms have the mass $196.967 \mathrm{g}$ $\mathrm{Au} / N_{\mathrm{A}}$.
$\mathrm{Ag}$ atom has two naturally ocurring isotopes: $^{107} \mathrm{Ag} \text { and } ^{109} \mathrm{Ag}$. This is because there is no silver atom whose the mass $107.868 \mathrm{g}$ $\mathrm{Ag} / N_{\mathrm{A}},$
Work Step by Step
All $\mathrm {Au}$ atoms must be $^{197} \mathrm{Au}$ (with no isotopes) in case all Au atoms have the mass $196.967 \mathrm{g}$ $\mathrm{Au} / N_{\mathrm{A}}$.
$\mathrm{Ag}$ atom has two naturally ocurring isotopes: $^{107} \mathrm{Ag} \text { and } ^{109} \mathrm{Ag}$. This is because there is no silver atom whose the mass $107.868 \mathrm{g}$ $\mathrm{Ag} / N_{\mathrm{A}},$
