State two forms in which the elemental oxygen is found in the earth’s atmosphere. Mention in brief the essential function performed by each at the place they are found.  What are CFCs and why are these considered harmful for the life on the earth?

Elemental oxygen is found in the earth’s atmosphere are oxygen, O2, and ozone, O3.
Ozone is present as a thin layer in a region of Earth’s stratosphere.
The Ozone shield protects the earth from much of the sun's harmful radiation as it absorbs the medium frequency ultraviolet light.  The UV light saves us from diseases like skin cancer.  The defensive role of the ozone layer is so vital that scientists believe life on land most likely would not have evolved.

Oxygen is present in the troposphere, the layer of atmosphere closest to the earth. Oxygen is one of the most important elements required to sustain life. Generation and maintenance of all our life processes are regulated by oxygen.  


Chlorofluorocarbons   are also known as CFCs. They are used in refrigerators, air conditioners, and aerosol sprays. CFCs   are responsible for  damaging the ozone layer and have led to the formation of the ozone hole in the atmosphere. A rapidly depleting ozone layer allows the harmful UV radiations of the sun to reach the Earth, which is responsible for an increase in the cases of skin cancers in humans.


The Cl atoms of chlorofluoro carbons can destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. The chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O3) to yield another chlorine atom and two molecules of oxygen. The chemical shorthand for these gas-phase reactions is:

Cl + O3 → ClO + O2 – The chlorine atom changes an ozone molecule to ordinary oxygen

ClO + O3 → Cl + 2O2 – The ClO from the previous reaction destroys a second ozone molecule and recreates the original chlorine atom, which can repeat the first reaction and continue to destroy ozone.

The overall effect is a decrease in the amount of ozone.

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in the form on oxygen O2 and ozone O3
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