78% of the earth's atmosphere is composed of which gas?

78% of Earth’s atmosphere is composed of nitrogen. More precisely, dry air contains about 78.08% nitrogen by volume, usually present as diatomic nitrogen gas, N₂. Oxygen accounts for about 20.95%, while argon and carbon dioxide make up most of the remainder.

Nitrogen is relatively unreactive because the two nitrogen atoms in N₂ are joined by a very strong triple bond. That stability helps make it a useful background gas in the atmosphere, preventing oxygen from making many natural combustion and oxidation reactions far more aggressive.

Although abundant in the air, most plants cannot use atmospheric N₂ directly. Bacteria convert it into compounds such as ammonia and nitrates through nitrogen fixation; plants then use those compounds to build proteins and DNA. Animals obtain nitrogen by eating plants or other animals.

A common mix-up is confusing atmospheric abundance with biological importance: oxygen is essential for human respiration, but nitrogen is the atmosphere’s dominant gas. Scottish physician Daniel Rutherford is generally credited with isolating nitrogen in 1772.