Correct Option
The correct option is Molecular oxygen .
Explanation
The formation of the ozone layer in the stratosphere is a natural photochemical process described by the Chapman Cycle. It involves the interaction of solar ultraviolet (UV) radiation with atmospheric gases to continuously create and destroy ozone, maintaining a dynamic equilibrium.
Mechanism of Formation
- Initiation via Photodissociation: The process begins when high-energy ultraviolet radiation (specifically UV-C, wavelength < 240 nm) strikes a molecule of Molecular Oxygen ($O_2$).
- Splitting of Oxygen: The energy from the UV photon breaks the chemical bond within the oxygen molecule, splitting it into two highly reactive individual oxygen atoms (atomic oxygen, $O$).
Reaction: $O_2 + h\nu \rightarrow 2O$ - Formation of Ozone: These free oxygen atoms ($O$) rapidly combine with other intact molecular oxygen ($O_2$) molecules in the presence of a third body (M) to form Ozone ($O_3$).
Reaction: $O + O_2 + M \rightarrow O_3 + M$
Consequently, the cycle is initiated by the breakdown of Molecular Oxygen ($O_2$), not Ozone ($O_3$) itself, Carbon dioxide ($CO_2$), or Nitrogen ($N_2$).
Key Takeaway
Molecular Oxygen ($O_2$) is the parent molecule for stratospheric ozone; its photodissociation by UV light provides the atomic oxygen required to synthesize Ozone ($O_3$).