How is oxygen converted into ozone?

a) Ozone is an allotropic form of oxygen. It is too reactive to remain for long in the atmosphere at sea level. At a height of about 20 kilometres, it is formed from atmospheric oxygen in the presence of sunlight. This ozone layer protects the earth’s surface from an excessive concentration of ultraviolet (UV) radiations.

I) In the fist step oxygen molecule break down to give oxygen radical.
O₂    +hv→ O^* +O^*
II) These oxygen radical react with oxygen molecule to form ozone.
O₂  +O^*  →  O₃

When a slow dry stream of oxygen is passed through a silent electrical discharge, conversion of oxygen to ozone (10%) occurs. The product is
known as ozonised oxygen.

3O₂ → 2O₃$∆{\mathrm{H}}^{-}$ (298 K) = +142 kJ mol^–1

Since the formation of ozone from oxygen is an endothermic process, it is necessary to use a silent electrical discharge in its preparation to
prevent its decomposition.