Three chlorine atoms can destroy how many ozone molecules?

• A. 3,000
• B. 30,000
• C. 3,000,000
• D. 300,000

Answer: (D)

Chlorine atoms destroy ozone through catalytic cycles in the stratosphere. A chlorine atom isn’t consumed in the reaction; it keeps triggering ozone-destroying steps over and over. For example, one cycle converts ozone and an oxygen atom into two oxygen molecules while regenerating the chlorine atom: Cl + O3 → ClO + O2, then ClO + O → Cl + O2, net: O3 + O → 2 O2. Because the chlorine atom is regenerated, it can continue destroying many ozone molecules until it’s tied up into stable reservoir species (like HCl or ClONO2). Under typical conditions, a single chlorine atom can destroy on the order of 100,000 ozone molecules. Three chlorine atoms therefore can destroy about 300,000 ozone molecules, which is why the figure around 300,000 is used. Remember this is an approximate, order-of-magnitude estimate; the exact number varies with atmospheric conditions.