Draw Lewis structures for each free radical implicated in this theory of aging: a. O2- b. O-

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Identify the total number of valence electrons for each species. For O2^-, oxygen has 6 valence electrons, so for two oxygen atoms, it's 12. Add 1 more electron for the negative charge, totaling 13 electrons. For O^-, oxygen has 6 valence electrons, plus 1 for the negative charge, totaling 7 electrons.

For O2^-, arrange the electrons to form a bond between the two oxygen atoms. Start by placing a single bond (2 electrons) between the two oxygens, then distribute the remaining electrons to satisfy the octet rule as much as possible, keeping in mind the odd number of electrons.

For O2^-, after placing the single bond, distribute the remaining 11 electrons around the two oxygen atoms. Each oxygen should have 8 electrons around it, but due to the odd number, one oxygen will have 7 electrons, indicating the presence of an unpaired electron, which is the free radical.

For O^-, place the 7 electrons around the single oxygen atom. Since there is an odd number of electrons, one electron will remain unpaired, indicating the presence of a free radical.

Check the formal charges for each structure to ensure they are minimized. For O2^-, the formal charge should be -1 overall, and for O^-, the formal charge should also be -1.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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 use dots to represent electrons and lines to represent bonds, allowing chemists to visualize the arrangement of electrons and predict molecular geometry and reactivity. Understanding how to draw Lewis structures is essential for analyzing the stability and behavior of molecules, including free radicals.

Free Radicals

Free radicals are atoms or molecules that have unpaired electrons, making them highly reactive. They can be formed through various processes, including the breaking of chemical bonds and exposure to radiation or heat. In the context of aging, free radicals can cause oxidative damage to cells, leading to various age-related diseases. Recognizing the nature of free radicals is crucial for understanding their role in biological systems.

Oxidation and Reduction Reactions

Oxidation and reduction (redox) reactions involve the transfer of electrons between species, where oxidation refers to the loss of electrons and reduction refers to the gain of electrons. Free radicals often participate in these reactions, contributing to oxidative stress in biological systems. Understanding redox chemistry is vital for comprehending how free radicals affect cellular processes and contribute to aging.

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Draw the Lewis structure for urea, H₂NCONH₂, one of the compounds responsible for the smell of urine. (The central carbon atom is bonded to both nitrogen atoms and to the oxygen atom.) Does urea contain polar bonds? Which bond in urea is most polar?

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Textbook Question

Some theories of aging suggest that free radicals cause certain diseases and perhaps aging in general. As you know from the Lewis model, such molecules are not chemically stable and will quickly react with other molecules. According to certain theories, free radicals may attack molecules within the cell, such as DNA, changing them and causing cancer or other diseases. Free radicals may also attack molecules on the surfaces of cells, making them appear foreign to the body's immune system. The immune system then attacks the cells and destroys them, weakening the body. Draw Lewis structures for each free radical implicated in this theory of aging. c. OH

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.

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