2023 O Level Chem P1 Q40 — Air / Ozone Layer Solution

Answer: A. Ozone in the stratosphere absorbs ultraviolet radiation from the Sun — this is the headline reason the ozone layer matters environmentally, and the absorption protects living organisms from DNA damage. ✓

B is wrong terminology: \(\text{O}_2\) and \(\text{O}_3\) are allotropes (different forms of the same element), not isomers. Isomers share a molecular formula but differ in structure — \(\text{O}_2\) and \(\text{O}_3\) have different formulas altogether.

C is wrong: isotopes are atoms of the same element with different numbers of neutrons. \(\text{O}_3\) is a molecule of three oxygen atoms — that has nothing to do with isotopes.

D is wrong on the radical: it is the chlorine radical from CFCs (not fluorine) that is photolysed from C–Cl bonds and goes on to destroy ozone catalytically.