· just now. 1  The third most abundant gas in the earth's atmosphere is a rare gas 'argon' (which constitutes 0.9% of the air). 2  The thin envelope of air that surrounds our planet is a mixture of gases, each with its own physical properties. 3  Nitrogen 78.084%. 4  The third most abundant gas element is argon at 0.93% but there is about 1% water vapor. In absolutely dry air it is argon at 0.93 % of the volume. In very humid air it is watervapour but the amount of watervapour vaires greatly while the level of argon is pretty much constant.