How does oxygen occur in nature. Explain oxygen-cycle in nature.
Oxygen exists in two different forms in nature. These forms occur as oxygen gas 21% and combined form in the form of oxides of metals and nonmetals, in the earth's crust, atmosphere and water. It is also present in carbohydrates, fats, proteins. Oxygen from the atmosphere is used up in three processes, namely combustion, respiration and in the formation of oxides of nitrogen. Oxygen is returned to the atmosphere by the process called as photosynthesis.
