Which gas forms 21% of the Earth's atmosphere?

The gas that forms 21% of the Earth's atmosphere is oxygen. The Earth's atmosphere is the layer of gases. The most abundant gases in the Earth's atmosphere are nitrogen (78%) and oxygen (21%). The remaining 1% belongs to small amounts of other gases.

The atmosphere of the Earth is comprised of different gases. One of which is the most abundant gas in the atmosphere at 79% h is named nitrogen. The second one is the oxygen which is about 21% of the Earths atmosphere. Other trace elements are also present but the amount are not very significant.