What is the main gas in the air we breathe?

Air is about 78 per cent nitrogen and only about 21 per cent oxygen. Every breath is mostly nitrogen, and almost all of it comes straight back out again, because nitrogen gas is remarkably reluctant to react with anything. Its two atoms are held together by one of the strongest bonds in chemistry.

That inertness is the central problem of biology. Living things need nitrogen for proteins and DNA, and cannot get it out of the air. For most of history the only routes were lightning and certain bacteria in soil and root nodules, which is why crop rotation with legumes was such an enormous discovery.

Then, between 1909 and 1913, Fritz Haber and Carl Bosch found a way to force nitrogen and hydrogen together under great pressure and heat. It is not an exaggeration to say the result reorganised the planet: synthetic fertiliser feeds something like half the people alive, and roughly half the nitrogen atoms in your body have passed through that industrial process. The same chemistry also made explosives, and Haber went on to direct Germany's chemical weapons programme, which is a difficult legacy to summarise on a plaque.