X + O3 → XO + O2 XO + O3 → X + 2 O2 2 O3 → 3 O2 The proposed mechanism can be written in a more gene — Kinetics Chemistry Question

STEPS:

1. Analyze the chemical role of species $X$ in the generalized mechanism: In the given reaction mechanism, species $X$ is consumed in the first step and regenerated in the second step:
$$X + \text{O}_3 \rightarrow XO + \text{O}_2$$
$$XO + \text{O}_3 \rightarrow X + 2\ \text{O}_2$$
Because $X$ facilitates the conversion of ozone to oxygen gas but is not consumed by the overall reaction, it acts as a homogeneous catalyst.
2. Understand how chemical similarity relates to the periodic table: Elements in the same periodic group (vertical column) share identical valence electron configurations. Consequently, they exhibit highly similar chemical properties, bonding patterns, and reactivity.
3. Analyze the original catalyst: The original catalyst in the atmospheric ozone decomposition mechanism is chlorine ($\text{Cl}$). Chlorine is a member of Group 17 (the halogens) and has 7 valence electrons, allowing it to easily form highly reactive free radicals ($\text{Cl}^\bullet$) that attack ozone molecules.
4. Identify the matching group member from the choices:
* Looking at the options, bromine ($\text{Br}$) is also a halogen belonging to Group 17, located directly below chlorine on the periodic table.
* Because they belong to the same group, bromine has the same valence shell structure as chlorine, readily forms highly reactive free radicals ($\text{Br}^\bullet$), and is chemically capable of decomposing ozone through an identical catalytic pathway (forming a $\text{BrO}$ intermediate).
5. Conclude the correct option: Bromine is the most suitable alternative catalyst, making Option B the correct choice.

*

WHY_OTHERS_WRONG:

• Option A is incorrect: Helium ($\text{He}$) is a Group 18 noble gas. Because it has a stable, closed-shell electron configuration, it is chemically inert and will not react with ozone to form an intermediate oxide.
• Option C is incorrect: Nitrogen gas ($\text{N}_2$) is a highly stable, chemically unreactive diatomic molecule held together by an extremely strong nitrogen-nitrogen triple bond. It does not exist as reactive single atoms under these atmospheric conditions and cannot act as a radical catalyst.
• Option D is incorrect: Oxygen gas ($\text{O}_2$) is a highly stable molecular product of the overall decomposition reaction, not an active radical catalyst that can initiate the cleavage of ozone bonds in this mechanism.