Ozone: Ozone (O₃) is a molecule formed by three atoms of oxygen.
Formation: UV radiation splits O₂ molecules into free oxygen atoms, which then combine with another O₂ molecule:
$$O_2 \xrightarrow{\text{UV}} O + O$$
$$O + O_2 \rightarrow O_3$$
Effect on ecosystem: Ozone shields Earth's surface from harmful UV radiation. UV rays are damaging to organisms and can cause skin cancer in humans.
Source: Chapter 13, Section 13.2.1
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Although this question has three parts, it is only 2 marks, so keep each part to one short sentence/equation. Examiners look for: (1) correct definition, (2) the two-step equation for formation, and (3) the protective role against UV radiation. Avoid padding — the equation counts as your "formation" explanation.