What is the most dangerous substance in the world?

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Difficult question. I’d say it’s a tossup between chlorine trifluoride, dioxygen difluoride, and dimethylmercury.

Chlorine trifluoride is hideously, insanely, exuberantly reactive stuff. It will cause almost anything you can think of to burst into flame on contact. Steel, ice, asbestos, sand, anything. About the only material you can store it in is copper, because you get a thin layer of copper fluoride on the surface and it won’t react any more than that. Be careful, though! Handle it wrong and it’ll change its mind about playing nice in copper.

Oh, and the byproducts of it burning on contact with—well, anything, really—are chlorine gas and hot hydrofluoric acid. Fun!

I think it can be ruled out of the running for Most Dangerous Substance, though, because it’s actually used in industry. The semiconductor industry uses it to clean circuit boards.

As exuberantly reactive as chlorine trifluoride is, dioxygen difluoride is worse. It will not only burn vigorously on contact with just about anything you can name, it will do so even at absurdly low temperatures, like going on liquid nitrogen temperatures. It’s a molecule that really really really really doesn’t want to be a molecule, and will without provocation dissociate with vigor, yielding rather a lot of excess energy in the process.

I also rule it out of the #1 spot because no more than a handful of molecules of it have ever existed at any one time. It’s not something you can cart around. It’s not something you’ll ever be exposed to, unless you’re one of maybe two people who’s ever synthesized it, and it’s not something you’ll ever have enough of to kill you because you just can’t keep the stuff around.

If you want pure evil incarnate, the kind of substance that nightmares are made of, look no further than dimethylmercury. The existence of this stuff is incompatible with the idea of a benevolent creator who loves his creation.

Dimethylmercury is an organomercury compound that at room temperature is a clear liquid that looks like water. But understanding its properties will give you nightmares for weeks.

It’s not explosive. It doesn’t spontaneously burst into flame. It doesn’t do anything dramatic. It just sits there.

But…

Inhale just a little tiny bit of it or let a tiny drop of it fall on your skin and you’re done. That’s it. Game over.

Oh, but it’s not a quick death, oh my no. You’re dead, and nothing can save you, but it will take months for you to die. And during those months, it’s in your brain, destroying the myelin sheath around your neurons, slowly causing your brain to disintegrate. You’ll die very slowly, in agony, as your mind goes to pieces.

And it gets worse. It will pass right through materials like latex. Researchers have died handling it in a fume hood and glove box just by spilling a drop of it on their gloves.

As far as I know, nobody will work with it any more. It’s just too damn dangerous. People make chlorine trifluoride in industrial quantities, you can buy it from chemical supply houses, but nobody, and I mean nobody, wants to work with dimethylmercury.