Are there any good examples of commonly ingested molecules that contain particular toxic individual elements?
Table salt! What would be worse than putting sodium (it can spontaneously combust if you get it wet) and chlorine (used as a war gas in WWI) all over your food?

Then there's water which always contains some $\ce{OH-}$ (an active ingredient in Drano) and $\ce{H+}$ (that stuff eats through metal, man). Although those aren't elements.

And hiding inside every $\ce{O2}$ molecule are two deadly oxygen atoms; once freed, they'll wreak havoc on just about anything they come into contact with.