Which gas forms about 78% of the Earth's atmosphere?

The story behind the answer

Nitrogen makes up about 78% of Earth’s atmosphere, far more than oxygen (~21%), argon (~0.93%), or carbon dioxide (~0.04%). Most atmospheric nitrogen is N2, whose strong triple bond makes it relatively inert, so it cushions the atmosphere without reacting readily. This stability helps regulate combustion and biological processes.

Despite being unreactive in the air, nitrogen is essential for life—bacteria (and lightning) “fix” N2 into compounds plants can use to build proteins and DNA. Memory tip: the “78-21-1 rule”—about 78% nitrogen, 21% oxygen, 1% argon; CO2 is tiny but climatically important. Or think “Nearly Eighty is Nitrogen.”