Correct Option
The correct option is Splitting of molecular oxygen by UV radiation followed by combination with atomic oxygen.
Explanation
The formation of ozone ($O_3$) in the upper atmosphere (stratosphere) is a natural photochemical process driven by the sun's ultraviolet (UV) radiation. This process is scientifically known as the Chapman Cycle.
Mechanism of Formation:
- Step 1 (Photodissociation): High-energy UV radiation strikes a molecule of oxygen ($O_2$), splitting it into two individual oxygen atoms (atomic oxygen, $O$).
Reaction: $O_2 + \text{UV radiation} \rightarrow 2O$ - Step 2 (Combination): The reactive atomic oxygen ($O$) then collides with another molecular oxygen ($O_2$) in the presence of a third body (usually $N_2$ or $O_2$ which absorbs excess energy) to form an ozone molecule ($O_3$).
Reaction: $O + O_2 \rightarrow O_3$
Analysis of Options:
- Splitting of molecular oxygen by UV radiation followed by combination with atomic oxygen is Correct: It accurately describes the two-step sequence: the splitting of molecular oxygen by UV radiation followed by the combination of the resulting atomic oxygen with molecular oxygen.
- Direct recombination of two oxygen molecules without energy input is Incorrect: Oxygen molecules ($O_2$) do not spontaneously recombine to form ozone without the initial dissociation provided by high-energy radiation.
- Combination of nitrogen and oxygen under low-energy radiation is Incorrect: Nitrogen is not a reactant in the synthesis of ozone, although nitrogen oxides can participate in ozone depletion cycles.
- Photolysis of water vapour followed by oxidation of hydrogen is Incorrect: While photolysis of water vapour occurs in the atmosphere, it is not the primary mechanism responsible for the formation of the ozone layer.
Key Takeaway: Stratospheric ozone is produced continuously when UV radiation dissociates molecular oxygen ($O_2$) into atomic oxygen ($O$), which subsequently combines with $O_2$ to form $O_3$.