How ozone is formed from the oxygen present in atmosphere?

The upper stratosphere consists of considerable amount of ozone $\left(\mathrm{O}_{3}\right)$, which protects us from the harmful ultraviolet ($UV$) radiations coming from the sun.

These radiations cause skin cancer in humans. Therefore, it is important to maintain the ozone shield.

In the stratosphere when $UV$ radiations acting on dioxygen $\left(\mathrm{O}_{2}\right)$ molecules, the $UV$ radiations split apart molecular oxygen into free oxygen $(O)$ atoms. These oxygen atoms combine with the molecular oxygen to form ozone.

$\mathrm{O}_{2(\mathrm{~g})} \stackrel{\mathrm{UV}}{\longrightarrow} \mathrm{O}_{(\mathrm{g})}+\mathrm{O}_{(\mathrm{g})}$

$\mathrm{O}_{(\mathrm{g})}+\mathrm{O}_{2(\mathrm{~g})} \stackrel{\mathrm{UV}}{\longrightarrow} \mathrm{O}_{3(\mathrm{~g})}$

Ozone is thermodynamically unstable and decomposes to molecular oxygen. Thus, a dynamic equilibrium exists between the production and decomposition of ozone molecules. The depletion of this protective ozone layer because of the presence of certain chemicals in the stratosphere.