Free radicals are important in many environmentally significant reactions (see the Chemistry in the Environment box on free radicals in this chapter). For example, photochemical smog— smog that results from the action of sunlight on air pollutants— forms in part by these two steps:

The product of this reaction, ozone, is a pollutant in the lower atmosphere. (Upper atmospheric ozone is a natural part of the atmosphere that protects life on Earth from ultraviolet light.) Ozone is an eye and lung irritant and also accelerates the weathering of rubber products. Rewrite the given reactions using the Lewis structure of each reactant and product. Identify the free radicals.

Verified step by step guidance

1. Identify the reactants and products in each reaction: \( \text{NO}_2 \rightarrow \text{NO} + \text{O} \) and \( \text{O} + \text{O}_2 \rightarrow \text{O}_3 \).
2. Draw the Lewis structure for each molecule: \( \text{NO}_2 \), \( \text{NO} \), \( \text{O} \), \( \text{O}_2 \), and \( \text{O}_3 \).
3. For \( \text{NO}_2 \), show the nitrogen atom bonded to two oxygen atoms with one unpaired electron on the nitrogen, indicating it is a free radical.
4. For \( \text{O} \), show the oxygen atom with two lone pairs and two unpaired electrons, indicating it is a free radical.
5. Identify the free radicals in the reactions: \( \text{NO}_2 \) and \( \text{O} \) are free radicals due to their unpaired electrons.

Key Concepts

Free Radicals

Free radicals are highly reactive species that contain unpaired electrons. They can initiate chain reactions in various chemical processes, including combustion and atmospheric reactions. In the context of the question, free radicals play a crucial role in the formation of ozone and other pollutants, as they can react with other molecules to produce new compounds.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They help visualize the arrangement of electrons and the connectivity of atoms, which is essential for understanding chemical reactions. In this question, drawing Lewis structures for the reactants and products will clarify how free radicals are formed and how they interact.

Photochemical Reactions

Photochemical reactions are chemical reactions that are initiated or accelerated by light, particularly ultraviolet (UV) light. In the case of photochemical smog, sunlight triggers the breakdown of nitrogen dioxide (NO2) into free radicals, which then participate in further reactions to produce ozone. Understanding these reactions is vital for grasping the environmental impact of pollutants.