Whats the most abundant element in the atmosphere?

The atmosphere is 78% Nitrogen, 29% Oxygen, 0.9% argon, and the remaining gasses (methane, CO2, Ozone ...) only appear in trace amounts * this does not mean that there isn't a lot of these "trace" gases, the atmosphere is very very very big, and a few millionths of CO2 can wreck the whole thing.