A single atom of gold is far too small for any photon in the visible spectrum to interact with.
That's incorrect --- single atoms can, and do, interact with optical photons.
https://arxiv.org/abs/2410.19671 https://www.nature.com/articles/ncomms13716
And the entire field of super resolution microscopy relies on small things (e.g., molecules) interacting with light.
No, they're too small to resolve. You can see small things if they're bright enough: https://www.newscientist.com/article/2161094-a-single-atom-is-visible-to-the-naked-eye-in-this-stunning-photo/