This is impressive. The Department of Chemistry at the University of Kentucky has a guide to what elements have appeared in comic books, along with the pages that they appeared on. Not every element has gotten featured, however. A little known Marvel Team-Up between Spidey and the element fermium was planned for issue #151 back in 1985, but when that title got canned, so did the story. Oh well. Better luck next time.
Found via Neatorama.