The accurate percentage of Oxygen in the atmosphere is around 20.8%. The earth’s atmosphere consists of various different gases all of which are present in different quantities. For instance, the composition of nitrogen in the air is about 78.09% and carbo dioxide is only present in trace amounts of 0.04%.
Apart from the gases, air also contains some amount of water vapor. The earth’s atmosphere could be considered as a mixture of various different gases. Please note that it is not a compound, rather a mixture of gases. Air also contains some amount of noble gases that are chemically unreactive.