For the record, I felt the O[sub]2[/sub] molecule is elemental, as opposed to a multi-element molecule, which then would be a compound.
Diamond is pure elemental crystaline carbon: a macroscopic molecule (let us for the sake of arguement disregard inclusions, impurities, and the tendancy of diamond to adsorb hydrogen to its outer surface 
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BTW, mono-atomic oxygen can be found in the high-vaccuum of the outer fringes of the atmosphere; it is, IIRC even more reactive than flourine gas.
One does sometimes eat elemental iron (by elemental, I mean not chemically combined with any other element). Occasionally, a food fortified with various minerals will include “reduced iron” on its list of ingredients. “Reduced” means “not oxidised”, and the food in this case literally contains powdered metallic iron. I imagine that it turns into some sort of salt in the stomach, before being absorbed, but it is absorbed and used eventually when ingested in this form.
I was about to say this. Apparently (I’ve never bothered to actually try) you can use a magnet to pull the iron filings out of fortified cereal. I’m not sure whether elemental iron is as bioavailable as the forms it naturally takes in food, though.
Interestingly, there’s elements of truth to both points of view. Hydrogen is of course chemically reactive. It is biologically inert when you breath it, in the sense that it doesn’t participate in any chemical reactions when dissolved in your bloodstream. However, under pressure it has a narcotic effect, like nitrogen and indeed like many of the truly inert noble gases such as krypton. The mechanism for this narcosis is thought not to be chemical but physical, since there is a strong correlation between the narcotic effects of the noble gases and their solubility in fat (the Meyer-Overton model).
Hydrogen-oxygen (hydrox) has been used as a diving gas, striking a balance between nitrogen-oxygen (nitrox) which becomes narcotic at quite shallow depths, and helium-oxygen (heliox) which isn’t narcotic enough and leads to high-pressure nervous syndrome (HPNS). The hydrox mixture was pioneered by Zetterstrom, who was eventually killed experimenting with it. (The problem was, to avoid flammability problems the mix was limited to 4-5% oxygen, which doesn’t supply enough oxygen to the body until you get 100 ft deep or so. So Zetterstrom used nitrox down to 100 ft and switched to hydrox. Unfortunately, he was hauled back to the surface without being allowed to switch back, and asphyxiated.)
The more common solution today to the necessary compromise between having a gas mix which isn’t too narcotic, but isn’t too non-narcotic either, is trimix, a mixture of helium, oxygen and just enough nitrogen to prevent HPNS. But for moderately deep diving, hydrox is a viable solution, possibly cheaper, and being less dense is easier to breathe.