Take mercury for an example.
You can dip your hand into a bucket of pure elemental mercury, and you're generally fine (though I wouldn't recommend it if you have open cuts or wounds).
On the other hand, dimethylmercury (an organic compound with mercury in it) will readily absorb through your skin (and latex gloves!) and is so potently toxic that a few drops of the stuff will kill you.
None of this is tob say that synthetic, non-orhanic compounds are in any way certain to be safer. However, organic compounds certainly do have advantages in integrating in the body.