The two forms of oxygen found in the atmosphere are:
(i) Elemental oxygen is normally found in the form of diatomic molecule (O2) in the lower part of atmosphere. It is about 21% in the air and non-poisonous.
(ii) Ozone is found in the stratosphere part of atmosphere. It contains three atoms of oxygen(O3). It is the poisonous form of oxygen.
(iii) Some other forms of oxygen are also found in the combined state. In Earth’s crust, it is found as the oxides of most metals and silicon and also as carbonate, sulphate, nitrate and other minerals. In other forms, it is the part of biological molecules like carbohydrates, proteins, fats and nucleic acids, etc.
