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Ch.10 - Chemical Bonding I: The Lewis Model
All textbooksTro 5th EditionCh.10 - Chemical Bonding I: The Lewis ModelProblem 102
Chapter 10, Problem 102

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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Step 1: Count the total number of valence electrons. Nitrogen has 5 valence electrons, Carbon has 4, Oxygen has 6, and Hydrogen has 1. Since there are 2 Nitrogen atoms, 1 Carbon atom, 1 Oxygen atom, and 4 Hydrogen atoms, the total number of valence electrons is 2*5 + 4 + 6 + 4*1 = 24.
Step 2: Draw the skeleton structure of the molecule. The problem states that the central Carbon atom is bonded to both Nitrogen atoms and to the Oxygen atom. Each Nitrogen atom is also bonded to two Hydrogen atoms. So, the skeleton structure is H-N-C-O-N-H with two Hydrogen atoms attached to each Nitrogen atom.
Step 3: Distribute the remaining electrons as lone pairs. Start by placing pairs of electrons between atoms to form chemical bonds. Each bond consists of 2 electrons. After forming the bonds, distribute the remaining electrons as lone pairs on the outer atoms (Nitrogen and Oxygen) to fulfill the octet rule.
Step 4: Check if all atoms satisfy the octet rule. If not, form double or triple bonds as necessary. In this case, a double bond is formed between the Carbon and Oxygen atoms.
Step 5: Determine the polarity of the molecule. A molecule is polar if it contains polar bonds and is not symmetrical. In urea, the bonds between Nitrogen and Hydrogen, Carbon and Nitrogen, and Carbon and Oxygen are all polar because of the difference in electronegativity between these atoms. The most polar bond would be the one with the greatest difference in electronegativity, which is the bond between Carbon and Oxygen.

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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 help visualize the arrangement of electrons and the connectivity of atoms, allowing for a better understanding of molecular geometry and reactivity. In the case of urea, drawing its Lewis structure involves placing the central carbon atom with appropriate bonds to the nitrogen and oxygen atoms while ensuring that all atoms satisfy the octet rule.
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Polarity of Bonds

Polarity in chemical bonds arises from the difference in electronegativity between the atoms involved. When two atoms with different electronegativities form a bond, the shared electrons are pulled closer to the more electronegative atom, creating a dipole moment. In urea, the bonds between carbon and nitrogen, as well as carbon and oxygen, can be analyzed for polarity based on the electronegativity values of these elements.
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Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. The Pauling scale is commonly used to quantify electronegativity values, with higher values indicating a stronger attraction for electrons. In urea, the bond between carbon and oxygen is expected to be the most polar due to oxygen's higher electronegativity compared to carbon and nitrogen, leading to a significant dipole moment in that bond.
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Related Practice
Textbook Question

Use Lewis structures to explain why Br3- and I3- are stable, while F3- is not.

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

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. d. CH3OO (unpaired electron on terminal oxygen)