Here’s a bit of trivia to stump even your most science-savvy friends: What is the strongest possible acid, so reactive you cannot even measure it on the pH scale? Or try this one: What is the very first molecule that formed in the universe, before water, before even molecular hydrogen? While your victim is scouring his or her brain, you could offer a taunting clue: The two compounds are one and the same.
The unexpected answer is … helium hydride, HeH+. Jerome Loreau of the Free University of Brussels calls it the mystery molecule (or rather the “mystery ion,” since it carries an electric charge), and for good reason. Your chemistry teacher probably taught you that helium, a noble gas, never reacts with anything, but that turns out to be entirely untrue — at least, under certain highly exotic conditions. Helium hydride is so obscure that many astronomers have never heard of it, even though it marked a key turning point in the evolution of the universe. It was step one in the birth of chemistry and the emergence of stars, planets and life itself.
Which brings me to the last and most mysterious thing about helium hydride. “We can’t observe it,” Loreau says sheepishly. “It just seems to be somehow invisible in space.”
