unreactive

A currently unexplained phenomenon is the loss of the unreactive gas xenon from our atmosphere more than 2.5 billion years ago.

In addition to being lighter than air, helium is staunchly unreactive, making it useful for manufacturing sensitive products such as computer chips.

"If we then offer a certain molecule, for example nitrogen, which is normally unreactive and doesn't bind, the beast is satisfied with it because it has no other choice," he says.

It relies on marking sugars with a small, biologically unreactive chemical group that can slip undetected past the enzymes that attach glycans to proteins.

But the development of such methods is by no means easy, because CO2 is a stable and relatively unreactive molecule.