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Introductory Chemistry - 1st Canadian Edition: Violations of the Octet Rule

Introductory Chemistry - 1st Canadian Edition
Violations of the Octet Rule
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table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Acknowledgments
  6. Dedication
  7. About BCcampus Open Education
  8. Chapter 1. What is Chemistry
    1. Some Basic Definitions
    2. Chemistry as a Science
  9. Chapter 2. Measurements
    1. Expressing Numbers
    2. Significant Figures
    3. Converting Units
    4. Other Units: Temperature and Density
    5. Expressing Units
    6. End-of-Chapter Material
  10. Chapter 3. Atoms, Molecules, and Ions
    1. Acids
    2. Ions and Ionic Compounds
    3. Masses of Atoms and Molecules
    4. Molecules and Chemical Nomenclature
    5. Atomic Theory
    6. End-of-Chapter Material
  11. Chapter 4. Chemical Reactions and Equations
    1. The Chemical Equation
    2. Types of Chemical Reactions: Single- and Double-Displacement Reactions
    3. Ionic Equations: A Closer Look
    4. Composition, Decomposition, and Combustion Reactions
    5. Oxidation-Reduction Reactions
    6. Neutralization Reactions
    7. End-of-Chapter Material
  12. Chapter 5. Stoichiometry and the Mole
    1. Stoichiometry
    2. The Mole
    3. Mole-Mass and Mass-Mass Calculations
    4. Limiting Reagents
    5. The Mole in Chemical Reactions
    6. Yields
    7. End-of-Chapter Material
  13. Chapter 6. Gases
    1. Pressure
    2. Gas Laws
    3. Other Gas Laws
    4. The Ideal Gas Law and Some Applications
    5. Gas Mixtures
    6. Kinetic Molecular Theory of Gases
    7. Molecular Effusion and Diffusion
    8. Real Gases
    9. End-of-Chapter Material
  14. Chapter 7. Energy and Chemistry
    1. Formation Reactions
    2. Energy
    3. Stoichiometry Calculations Using Enthalpy
    4. Enthalpy and Chemical Reactions
    5. Work and Heat
    6. Hess’s Law
    7. End-of-Chapter Material
  15. Chapter 8. Electronic Structure
    1. Light
    2. Quantum Numbers for Electrons
    3. Organization of Electrons in Atoms
    4. Electronic Structure and the Periodic Table
    5. Periodic Trends
    6. End-of-Chapter Material
  16. Chapter 9. Chemical Bonds
    1. Lewis Electron Dot Diagrams
    2. Electron Transfer: Ionic Bonds
    3. Covalent Bonds
    4. Other Aspects of Covalent Bonds
    5. Violations of the Octet Rule
    6. Molecular Shapes and Polarity
    7. Valence Bond Theory and Hybrid Orbitals
    8. Molecular Orbitals
    9. End-of-Chapter Material
  17. Chapter 10. Solids and Liquids
    1. Properties of Liquids
    2. Solids
    3. Phase Transitions: Melting, Boiling, and Subliming
    4. Intermolecular Forces
    5. End-of-Chapter Material
  18. Chapter 11. Solutions
    1. Colligative Properties of Solutions
    2. Concentrations as Conversion Factors
    3. Quantitative Units of Concentration
    4. Colligative Properties of Ionic Solutes
    5. Some Definitions
    6. Dilutions and Concentrations
    7. End-of-Chapter Material
  19. Chapter 12. Acids and Bases
    1. Acid-Base Titrations
    2. Strong and Weak Acids and Bases and Their Salts
    3. Brønsted-Lowry Acids and Bases
    4. Arrhenius Acids and Bases
    5. Autoionization of Water
    6. Buffers
    7. The pH Scale
    8. End-of-Chapter Material
  20. Chapter 13. Chemical Equilibrium
    1. Chemical Equilibrium
    2. The Equilibrium Constant
    3. Shifting Equilibria: Le Chatelier’s Principle
    4. Calculating Equilibrium Constant Values
    5. Some Special Types of Equilibria
    6. End-of-Chapter Material
  21. Chapter 14. Oxidation and Reduction
    1. Oxidation-Reduction Reactions
    2. Balancing Redox Reactions
    3. Applications of Redox Reactions: Voltaic Cells
    4. Electrolysis
    5. End-of-Chapter Material
  22. Chapter 15. Nuclear Chemistry
    1. Units of Radioactivity
    2. Uses of Radioactive Isotopes
    3. Half-Life
    4. Radioactivity
    5. Nuclear Energy
    6. End-of-Chapter Material
  23. Chapter 16. Organic Chemistry
    1. Hydrocarbons
    2. Branched Hydrocarbons
    3. Alkyl Halides and Alcohols
    4. Other Oxygen-Containing Functional Groups
    5. Other Functional Groups
    6. Polymers
    7. End-of-Chapter Material
  24. Chapter 17. Kinetics
    1. Factors that Affect the Rate of Reactions
    2. Reaction Rates
    3. Rate Laws
    4. Concentration–Time Relationships: Integrated Rate Laws
    5. Activation Energy and the Arrhenius Equation
    6. Reaction Mechanisms
    7. Catalysis
    8. End-of-Chapter Material
  25. Chapter 18. Chemical Thermodynamics
    1. Spontaneous Change
    2. Entropy and the Second Law of Thermodynamics
    3. Measuring Entropy and Entropy Changes
    4. Gibbs Free Energy
    5. Spontaneity: Free Energy and Temperature
    6. Free Energy under Nonstandard Conditions
    7. End-of-Chapter Material
  26. Appendix A: Periodic Table of the Elements
  27. Appendix B: Selected Acid Dissociation Constants at 25°C
  28. Appendix C: Solubility Constants for Compounds at 25°C
  29. Appendix D: Standard Thermodynamic Quantities for Chemical Substances at 25°C
  30. Appendix E: Standard Reduction Potentials by Value
  31. Glossary
  32. About the Authors
  33. Versioning History

Violations of the Octet Rule

Learning Objectives

  1. Recognize the three major types of violations of the octet rule.

As important and useful as the octet rule is in chemical bonding, there are some well-known violations. This does not mean that the octet rule is useless — quite the contrary. As with many rules, there are exceptions, or violations.

There are three violations to the octet rule. Odd-electron molecules represent the first violation to the octet rule. Although they are few, some stable compounds have an odd number of electrons in their valence shells. With an odd number of electrons, at least one atom in the molecule will have to violate the octet rule. Examples of stable odd-electron molecules are NO, NO2, and ClO2. The Lewis electron dot diagram for NO is as follows:

\Large \Lewis{0:2:4.,N}\Lewis{2:4:6:,O}

Although the O atom has an octet of electrons, the N atom has only seven electrons in its valence shell. Although NO is a stable compound, it is very chemically reactive, as are most other odd-electron compounds.

Electron-deficient molecules represent the second violation to the octet rule. These stable compounds have less than eight electrons around an atom in the molecule. The most common examples are the covalent compounds of beryllium and boron. For example, beryllium can form two covalent bonds, resulting in only four electrons in its valence shell:

\chemfig{\Lewis{2:4:6:,Cl}-Be-\Lewis{0:2:6:,Cl}}

Boron commonly makes only three covalent bonds, resulting in only six valence electrons around the B atom. A well-known example is BF3:

\chemfig{\Lewis{2:4:6:,F}-[:45]B(-[:90]\Lewis{0:2:4:,F})-[:-45]\Lewis{0:2:6:,F}}

The third violation to the octet rule is found in those compounds with more than eight electrons assigned to their valence shell. These are called expanded valence shell molecules. Such compounds are formed only by central atoms in the third row of the periodic table or beyond that have empty d orbitals in their valence shells that can participate in covalent bonding. One such compound is PF5. The only reasonable Lewis electron dot diagram for this compound has the P atom making five covalent bonds:

\chemfig{\Lewis{2:4:6:,F}-[:-30]P(-[:90]\Lewis{0:2:4:,F})(-[:30]\Lewis{0:2:6:,F})(-[:-45]\Lewis{0:4:6:,F})(-[:-135]\Lewis{0:4:6:,F})}

Formally, the P atom has 10 electrons in its valence shell.

Example 9.9

Identify each violation of the octet rule by drawing a Lewis electron dot diagram.

  1. ClO
  2. SF6

Solution

  1. With one Cl atom and one O atom, this molecule has 6 + 7 = 13 valence electrons, so it is an odd-electron molecule. A Lewis electron dot diagram for this molecule is as follows:

    \Large \Lewis{0:2:4.6:,Cl}\Lewis{0:2:6:,O}

  2. In SF6, the central S atom makes six covalent bonds to the six surrounding F atoms, so it is an expanded valence shell molecule. Its Lewis electron dot diagram is as follows:

    \chemfig{\Lewis{2:4:6:,F}-[:-30]S(-[:90]\Lewis{0:2:4:,F})(-[:30]\Lewis{0:2:6:,F})(-[:-30]\Lewis{0:2:6:,F})(-[:-90]\Lewis{0:4:6:,F})(-[:-150]\Lewis{2:4:6:,F})}

Test Yourself

Identify the violation to the octet rule in XeF2 by drawing a Lewis electron dot diagram.

Answer

\LARGE \Lewis{0:2:4:6:,F}\Lewis{0:2:4.6:,Xe}\Lewis{0:2:4.6:,F}

The Xe atom has an expanded valence shell with more than eight electrons around it.

Key Takeaways

  • There are three violations to the octet rule: odd-electron molecules, electron-deficient molecules, and expanded valence shell molecules.

Exercises

Questions

  1. Why can an odd-electron molecule not satisfy the octet rule?
  2. Why can an atom in the second row of the periodic table not form expanded valence shell molecules?
  3. Draw an acceptable Lewis electron dot diagram for these molecules that violate the octet rule.
    1. NO2
    2. XeF4
  4. Draw an acceptable Lewis electron dot diagram for these molecules that violate the octet rule.
    1. BCl3
    2. ClO2
  5. Draw an acceptable Lewis electron dot diagram for these molecules that violate the octet rule.
    1. POF3
    2. ClF3
  6. Draw an acceptable Lewis electron dot diagram for these molecules that violate the octet rule.
    1. SF4
    2. BeH2

Answers

  1. There is no way all electrons can be paired if there are an odd number of them.
    1. \chemfig{\Lewis{2:4:6:,O}-\Lewis{2.,N}=\Lewis{0:6:,O}}
    2. \chemfig{\Lewis{2:4:6:,F}-\Lewis{0:4:,Xe}(-[:90]\Lewis{0:2:4:,F})(-[:0]\Lewis{0:2:6:,F})-[:-90]\Lewis{0:4:6:,F}}
    1. \chemfig{\Lewis{2:4:6:,F}-P(=[:90]\Lewis{0:2:,O})(-[:0]\Lewis{0:2:6:,F})-[:-90]\Lewis{0:4:6:,F}}
    2. \chemfig{\Lewis{2:4:6:,F}-\Lewis{0:2:,Cl}(-[:-90]\Lewis{0:4:6:,F})-\Lewis{0:2:6:,F}}

Annotate

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Molecular Shapes and Polarity
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Chemistry

Copyright © 2014

                                by Jessie A. Key

            Introductory Chemistry - 1st Canadian Edition by Jessie A. Key is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
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