The most abundant inert gas in the atmosphere is Argon (Ar). The most abundant gas in the atmosphere is N2, which makes up about 78% of air. O2 is the second most abundant gas at about 21%. The inert gas Ar is the third most abundant gas at 0.93%. There are also trace amounts of CO2, Ne, He, CH4, Kr, H2, NO, Xe, O3, I2, CO, and NH3 in the atmosphere.