|Alchemist's symbol for Arsenic|
Organometallic chemistry is the mystical, covalent union of metallic elements with non-metallic elements--mainly carbon with metals. Saying that Arsenic is a metal is a bit like saying that Barack Obama is black. Arsenic, like phosphorus and sulfur, has different allotropes which is more a mark of non-metallic character.
The element has been known since at least 1250 A.D, but it was known much earlier as its sulfide and its affinity for sulfur is the chief cause for its toxicity. It binds to vital sulfur-containing proteins in cells and shuts them down.
Only about a dozen or so elements are poisonous (not including radioactive ones); the dirty dozen are (in alphabetical order): Aluminum, Antimony, Arsenic, Barium, Beryllium, Cadmium, Chromium (hexavalent), Lead, Mercury, Osmium, Thallium, and Vanadium.
Each name has its own story and its own LD50--some are much worse than others.
*He because the word Arsenic derives from a Greek word corresponding to male potency. Link