Natural Resources — Science STD 9 — Question

1. Elemental oxygen is normally found in the form of a diatomic molecule $(O_2)$ in the lower regions of the atmosphere to the extent of $21\%.$ It is non-poisonous form of oxygen.
2. But in the upper reaches of the atmosphere (stratosphere), it occurs in the form of ozone, containing three atoms of oxygen and having the molecular formula $O_3.$ It is the poisonous form of oxygen.

Some other forms of oxygen:
It also Occurs extensively in the combined form in the Earth’s crust as well as also in the air in the form of carbon dioxide. In the crust, it is found as the oxides of most metals and silicon, and also as carbonate, sulphate, nitrate and other minerals. It is also an essential component of most biological molecules like carbohydrates, proteins, nucleic acids and fats (or lipids).