Air was originally a classical element and thought to be a pure substance, but it was only until the 1800s that air was found to be a mixture of several gases. The most common gas in air is nitrogen, with nitrogen taking up 78% of any volume of air. The second is oxygen, which will take up 21% of any volume of air. The remaining 1% would be made up of other gases like carbon dioxide, neon, and helium (which is rare because of its light mass, as most helium would rise into space). That reason also applies to hydrogen, which takes up a mere 0,000055% of any volume of air!
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