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DP IB Chemistry: HL

Revision Notes

Home / IB / Chemistry: HL / DP / Revision Notes / 14. HL Chemical Bonding & Structure / 14.1 More Structures & Shapes / 14.1.4 Formal Charge


14.1.4 Formal Charge


Formal Charge

  • A limitation of the model of covalent bonding is that when drawing Lewis structures for molecules, it is sometimes possible to come up with more than one structure while still obeying the octet rule
  • This leads to the problem of deciding which structure is appropriate and is consistent with other information such as spectroscopic data on bond lengths and electron density
  • One approach to determining which is the preferred structure is to determine the formal charge (FC) of all the atoms present in the molecule
  • It is a kind of electronic book keeping involving the bonding, non-bonding and valence electrons
  • Formal charge is described as the charge assigned to an atom in a molecule, assuming that all the electrons in the bonds are shared equally between atoms, regardless of differences in electronegativity
  • The formula for calculating FC is

FC= (number of valence electrons) - ½(number of bonding electrons) - (number of non-bonding electrons)

or

FC= V - ½B - N

  • The Lewis structure which is preferred is the one which:
    • the difference in FC of the atoms is closest to zero
    • has negative charges located on the most electronegative atoms
  • The process of drawing a Lewis structure has been covered previously, but here is a reminder of how to draw the Lewis structure of tetrachloromethane, CCl4,

Steps in drawing the Lewis structure for CCl4

  • To work our the formal charge of the C and Cl atoms in the structure simply apply the FC formula:

FC for carbon = (4) - ½(8) - 0 = 0

FC for chlorine = (7) - ½(2) - 6 = 0

  • Notice that formal charge is calculated for one of each type of atom and does not count the total number of atoms in the molecule

Worked Example

What is the formal charge on boron in the BH4- ion?

Answer

    • Boron is a group 13 element, so has 3 valence electrons. Hydrogen has one valence electron and the charge on the ion is -1, so there are 8 electrons in the diagram. The Lewis structure is therefore:

BH4 anion Lewis structure, downloadable IB Chemistry revision notes

Lewis structure of BH4-

    • The number of bonded electrons is 8 and the number of non-bonded electrons is zero. So the formal charge on B is:

FC (B) = (3) - ½(8) - 0 = -1

Applying Formal Charge

  • It is possible to draw three resonance structures for sulfur dioxide, SO2:

Three resonance structures for SO2, downloadable IB Chemistry revision notes

The three resonance structures of sulfur dioxide

  • The first structure is an illustration of the expansion of the octet as the sulfur has 10 electrons around it
  • Formal charge can be used to decide which of the Lewis structures is preferred
  • The FC on the first structure is as follows:

FC on sulfur = (6) - ½(8) -(2) = 0

FC on oxygen = (6) - ½(4) -(4) = 0

Difference in FC = ΔFC = FCmax- FCmin = 0

  • The FC on the second (and third) structures is as follows:

FC on sulfur = (6) - ½(6) -(2) = +1

FC on left side oxygen = (6) - ½(2) -(6) = -1

FC on right side oxygen = (6) - ½(4) -(4) = 0

Difference in FC = ΔFC = FCmax- FCmin = 2

Worked Example

What is the formal charge on the two resonance structures shown?

Resonance Structures of CO2, downloadable IB Chemistry revision notes

Resonance structures of carbon dioxide

Deduce which is the preferred structure.

Answer

Structure I

FC on carbon = (4) - ½(8) -(0) = 0

FC on oxygen = (6) - ½(4) -(4) = 0

Difference in FC = ΔFC = FCmax- FCmin = 0

Structure II

FC on carbon = (4) - ½(8) -(0) = 0

FC on left oxygen = (6) - ½(6) -(2) = +1

FC on right oxygen = (6) - ½(2) -(6) = -1

Difference in FC = ΔFC = FCmax- FCmin = 2

Structure I is the preferred structure as the difference is zero



  • 1. Stoichiometric Relationships
    • 1.1 Matter, Chemical Change & the Mole Concept
      • 1.1.1 Elements, Compounds & Mixtures
        • 1.1.2 Equations
          • 1.1.3 State Changes
            • 1.1.4 The Mole Concept
              • 1.1.5 Moles-Mass Problems
                • 1.1.6 Empirical Formulae
                • 1.2 Reacting Masses & Volumes
                  • 1.2.1 Reacting Masses
                    • 1.2.2 Reaction Yields
                      • 1.2.3 Avogadro's Law & Molar Gas Volume
                        • 1.2.4 The Ideal Gas Equation
                          • 1.2.5 Gas Law Relationships
                            • 1.2.6 Real Gases
                              • 1.2.7 Standard Solutions
                                • 1.2.8 Concentration Calculations
                                  • 1.2.9 Titrations
                                • 2. Atomic Structure
                                  • 2.1 Atomic & Electronic Structure
                                    • 2.1.1 The Nuclear Atom
                                      • 2.1.2 Deducing Subatomic Particles
                                        • 2.1.3 Relative Atomic Mass Calculations
                                          • 2.1.4 The Electromagnetic Spectrum
                                            • 2.1.5 Emission Spectra
                                              • 2.1.6 Energy Levels & Sublevels
                                                • 2.1.7 Sublevels & Orbitals
                                                  • 2.1.8 Writing Electron Configurations
                                                • 3. Periodicity
                                                  • 3.1 The Periodic Table & Periodic Trends
                                                    • 3.1.1 The Periodic Table
                                                      • 3.1.2 Periodic Trends: Physical - Atomic & Ionic Radius
                                                        • 3.1.3 Periodic Trends: Physical - Ionisation Energy
                                                          • 3.1.4 Periodic Trends: Physical - Electron Affinity
                                                            • 3.1.5 Periodic Trends: Physical - Electronegativity
                                                              • 3.1.6 Periodic Trends: Chemical
                                                              • 3.2 Oxides, Group 1 & Group 17
                                                                • 3.2.1 Periodic Trends: Oxides Across a Period
                                                                  • 3.2.2 Periodic Trends: Group 1 - The Alkali Metals
                                                                    • 3.2.3 Periodic Trends: Group 17 - The Halogens
                                                                  • 4. Chemical Bonding & Structure
                                                                    • 4.1 Ionic & Covalent Bonding
                                                                      • 4.1.1 Forming Ions
                                                                        • 4.1.2 Ionic Compounds
                                                                          • 4.1.3 Formulae & Names of Ionic Compounds
                                                                            • 4.1.4 Covalent Bonds
                                                                              • 4.1.5 Bond Polarity
                                                                                • 4.1.6 Lewis Structures
                                                                                • 4.2 Resonance, Shapes & Giant Structures
                                                                                  • 4.2.1 Resonance Structures
                                                                                    • 4.2.2 Shapes of Molecules
                                                                                      • 4.2.3 Predicting Molecular Shapes
                                                                                        • 4.2.4 Molecular Polarity
                                                                                          • 4.2.5 Giant Covalent Structures
                                                                                          • 4.3 Intermolecular Forces & Metallic Bonding
                                                                                            • 4.3.1 Types of Intermolecular Forces
                                                                                              • 4.3.2 Deducing Intermolecular Forces
                                                                                                • 4.3.3 Properties of Covalent Compounds
                                                                                                  • 4.3.4 Metallic Bonding
                                                                                                    • 4.3.5 Trends in Melting Points of Metals
                                                                                                      • 4.3.6 Alloys & their Properties
                                                                                                    • 5. Energetics/ Thermochemistry
                                                                                                      • 5.1 Energetics
                                                                                                        • 5.1.1 Energy Changes in Reactions
                                                                                                          • 5.1.2 Standard Enthalpy Change
                                                                                                            • 5.1.3 Calorimetry
                                                                                                              • 5.1.4 Calorimetry Experiments
                                                                                                              • 5.2 Hess's Law
                                                                                                                • 5.2.1 Hess's Law
                                                                                                                  • 5.2.2 Using Hess's Law - Cycles
                                                                                                                    • 5.2.3 Using Hess's Law - Equations
                                                                                                                      • 5.2.4 Using ∆Hf° to Find Enthalpy Changes
                                                                                                                      • 5.3 Bond Enthalpy
                                                                                                                        • 5.3.1 Bond Enthalpy
                                                                                                                          • 5.3.2 Bond Enthalpy Calculations
                                                                                                                            • 5.3.3 Energy Profiles
                                                                                                                              • 5.3.4 Case Study: Ozone
                                                                                                                            • 6. Chemical Kinetics
                                                                                                                              • 6.1 Chemical Kinetics
                                                                                                                                • 6.1.1 Collision Theory
                                                                                                                                  • 6.1.2 Rate of Reaction
                                                                                                                                    • 6.1.3 Measuring Rates
                                                                                                                                      • 6.1.4 Activation Energy
                                                                                                                                        • 6.1.5 Rate Experiments
                                                                                                                                          • 6.1.6 Explaining Rates
                                                                                                                                            • 6.1.7 Maxwell-Boltzmann Curves
                                                                                                                                              • 6.1.8 Energy Profiles & Catalysis
                                                                                                                                            • 7. Equilibrium
                                                                                                                                              • 7.1 Equilibrium
                                                                                                                                                • 7.1.1 The State of Equilibrium
                                                                                                                                                  • 7.1.2 The Equilibrium Law
                                                                                                                                                    • 7.1.3 Equilibrium Constant Relationships
                                                                                                                                                      • 7.1.4 The Reaction Quotient
                                                                                                                                                        • 7.1.5 Le Chatelier's Principle
                                                                                                                                                          • 7.1.6 Catalysts & Equilibrium
                                                                                                                                                        • 8. Acids & Bases
                                                                                                                                                          • 8.1 Theories & Reactions of Acids & Bases
                                                                                                                                                            • 8.1.1 Brønsted–Lowry Acids & Bases
                                                                                                                                                              • 8.1.2 Conjugate Acid-Base Pairs
                                                                                                                                                                • 8.1.3 Characteristic Reactions of Acids
                                                                                                                                                                  • 8.1.4 Neutralization
                                                                                                                                                                  • 8.2 More About Acids
                                                                                                                                                                    • 8.2.1 Acid-base Titrations
                                                                                                                                                                      • 8.2.2 pH & [H⁺]
                                                                                                                                                                        • 8.2.3 Interpreting pH
                                                                                                                                                                          • 8.2.4 The Ionic Product of Water
                                                                                                                                                                            • 8.2.5 Acid-Base Calculations
                                                                                                                                                                              • 8.2.6 pH Meters & Universal Indicator
                                                                                                                                                                                • 8.2.7 Strong & Weak Acids & Bases
                                                                                                                                                                                  • 8.2.8 Comparing Strong & Weak Acids
                                                                                                                                                                                  • 8.3 Acid Deposition
                                                                                                                                                                                    • 8.3.1 Acid Deposition
                                                                                                                                                                                      • 8.3.2 Effects of Acid Deposition
                                                                                                                                                                                        • 8.3.3 Reducing Sulfur Oxide Emissions
                                                                                                                                                                                      • 9. Redox Processes
                                                                                                                                                                                        • 9.1 Redox Processes
                                                                                                                                                                                          • 9.1.1 Oxidation & reduction
                                                                                                                                                                                            • 9.1.2 Deducing Oxidation Numbers
                                                                                                                                                                                              • 9.1.3 Oxidizing & Reducing agents
                                                                                                                                                                                                • 9.1.4 Naming Transition Metal Compounds
                                                                                                                                                                                                  • 9.1.5 Half Equations
                                                                                                                                                                                                    • 9.1.6 The Activity Series
                                                                                                                                                                                                      • 9.1.7 Redox Titrations
                                                                                                                                                                                                        • 9.1.8 The Winkler Method
                                                                                                                                                                                                          • 9.1.9 Voltaic Cells
                                                                                                                                                                                                            • 9.1.10 Electrolytic Cells
                                                                                                                                                                                                          • 10. Organic Chemistry
                                                                                                                                                                                                            • 10.1 Fundamentals of Organic Chemistry
                                                                                                                                                                                                              • 10.1.1 Homologous Series
                                                                                                                                                                                                                • 10.1.2 Understanding Organic Molecules
                                                                                                                                                                                                                  • 10.1.3 Nomenclature
                                                                                                                                                                                                                    • 10.1.4 Organic Families - Hydrocarbons
                                                                                                                                                                                                                      • 10.1.5 Organic Families - Halogenoalkanes
                                                                                                                                                                                                                        • 10.1.6 Organic Families - Alcohols & Ethers
                                                                                                                                                                                                                          • 10.1.7 Organic Families - Carbonyls
                                                                                                                                                                                                                            • 10.1.8 Organic Families - Carboxylic Acids & Esters
                                                                                                                                                                                                                              • 10.1.9 Organic Families - Organic Nitrogen Compounds
                                                                                                                                                                                                                                • 10.1.10 3-D modelling
                                                                                                                                                                                                                                  • 10.1.11 Primary, Secondary & Tertiary atoms
                                                                                                                                                                                                                                    • 10.1.12 Benzene
                                                                                                                                                                                                                                    • 10.2 Functional Group Chemistry
                                                                                                                                                                                                                                      • 10.2.1 Alkanes - Combustion
                                                                                                                                                                                                                                        • 10.2.2 Alkanes - Halogenation
                                                                                                                                                                                                                                          • 10.2.3 Alkenes - Reactivity
                                                                                                                                                                                                                                            • 10.2.4 Alkenes - Hydrogenation
                                                                                                                                                                                                                                              • 10.2.5 Alkenes - Halogenation
                                                                                                                                                                                                                                                • 10.2.6 Alkenes - Hydrohalogenation
                                                                                                                                                                                                                                                  • 10.2.7 Alkenes - Hydration
                                                                                                                                                                                                                                                    • 10.2.8 Addition Polymers
                                                                                                                                                                                                                                                      • 10.2.9 Alcohols - Combustion
                                                                                                                                                                                                                                                        • 10.2.10 Alcohols - Oxidation
                                                                                                                                                                                                                                                          • 10.2.11 Alcohols - Esterification
                                                                                                                                                                                                                                                            • 10.2.12 Halogenoalkanes
                                                                                                                                                                                                                                                              • 10.2.13 Reactions of Benzene
                                                                                                                                                                                                                                                            • 11. Measurements & Data Processes
                                                                                                                                                                                                                                                              • 11.1 Spectroscopic Identification
                                                                                                                                                                                                                                                                • 11.1.1 Index of Hydrogen Deficiency
                                                                                                                                                                                                                                                                  • 11.1.2 Mass Spectrometry
                                                                                                                                                                                                                                                                    • 11.1.3 Nuclear Magnetic Resonance Spectroscopy
                                                                                                                                                                                                                                                                      • 11.1.4 Infrared Spectroscopy
                                                                                                                                                                                                                                                                      • 11.2 Synoptic Data Handling & Graphical Skills
                                                                                                                                                                                                                                                                        • 11.2.1 Recording Data
                                                                                                                                                                                                                                                                          • 11.2.2 Processing Errors
                                                                                                                                                                                                                                                                            • 11.2.3 Reducing the Impact of Errors
                                                                                                                                                                                                                                                                              • 11.2.4 Percentage Error
                                                                                                                                                                                                                                                                                • 11.2.5 Drawing Graphs
                                                                                                                                                                                                                                                                                  • 11.2.6 Analysing Graphs
                                                                                                                                                                                                                                                                                • 12. HL Atomic Structure
                                                                                                                                                                                                                                                                                  • 12.1 Electrons in Atoms
                                                                                                                                                                                                                                                                                    • 12.1.1 Emission Spectra & First Ionisation Energy
                                                                                                                                                                                                                                                                                      • 12.1.2 Ionisation Energy Trends
                                                                                                                                                                                                                                                                                    • 13. HL The Periodic Table- Transition Metals
                                                                                                                                                                                                                                                                                      • 13.1 Transition Metals
                                                                                                                                                                                                                                                                                        • 13.1.1 Transition Elements
                                                                                                                                                                                                                                                                                          • 13.1.2 Variable Oxidation States
                                                                                                                                                                                                                                                                                            • 13.1.3 Complex Ions
                                                                                                                                                                                                                                                                                              • 13.1.4 Catalytic & Magnetic Properties
                                                                                                                                                                                                                                                                                                • 13.1.5 Colour in Transition Metals
                                                                                                                                                                                                                                                                                                  • 13.1.6 Factors Affecting Colour
                                                                                                                                                                                                                                                                                                • 14. HL Chemical Bonding & Structure
                                                                                                                                                                                                                                                                                                  • 14.1 More Structures & Shapes
                                                                                                                                                                                                                                                                                                    • 14.1.1 Covalent Bonds
                                                                                                                                                                                                                                                                                                      • 14.1.2 More Lewis Structures
                                                                                                                                                                                                                                                                                                        • 14.1.3 Further VSEPR Theory
                                                                                                                                                                                                                                                                                                          • 14.1.4 Formal Charge
                                                                                                                                                                                                                                                                                                          • 14.2 Further Aspects of Bonding
                                                                                                                                                                                                                                                                                                            • 14.2.1 Delocalisation & Resonance
                                                                                                                                                                                                                                                                                                              • 14.2.2 Ozone Revisited
                                                                                                                                                                                                                                                                                                                • 14.2.3 Hybrid Orbitals
                                                                                                                                                                                                                                                                                                              • 15. HL Energetics / Thermochemistry
                                                                                                                                                                                                                                                                                                                • 15.1 Energy Cycles
                                                                                                                                                                                                                                                                                                                  • 15.1.1 Key Enthalpy Terms
                                                                                                                                                                                                                                                                                                                    • 15.1.2 Born-Haber Cycles
                                                                                                                                                                                                                                                                                                                      • 15.1.3 Born-Haber Cycle Calculations
                                                                                                                                                                                                                                                                                                                        • 15.1.4 Dissolution Energy Cycles
                                                                                                                                                                                                                                                                                                                          • 15.1.5 Energy Changes in Aqueous Solutions
                                                                                                                                                                                                                                                                                                                          • 15.2 Entropy & Spontaneity
                                                                                                                                                                                                                                                                                                                            • 15.2.1 Entropy
                                                                                                                                                                                                                                                                                                                              • 15.2.2 Calculating Entropy Change
                                                                                                                                                                                                                                                                                                                                • 15.2.3 Gibbs Free Energy
                                                                                                                                                                                                                                                                                                                                  • 15.2.4 Spontaneous Reactions
                                                                                                                                                                                                                                                                                                                                    • 15.2.5 Free Energy & Equilibrium
                                                                                                                                                                                                                                                                                                                                  • 16. HL Chemical Kinetics
                                                                                                                                                                                                                                                                                                                                    • 16.1 Rate Expression & Reaction Mechanism
                                                                                                                                                                                                                                                                                                                                      • 16.1.1 Rate Equations
                                                                                                                                                                                                                                                                                                                                        • 16.1.2 Rate Constants
                                                                                                                                                                                                                                                                                                                                          • 16.1.3 Graphical Representations
                                                                                                                                                                                                                                                                                                                                            • 16.1.4 Rate Determining Step
                                                                                                                                                                                                                                                                                                                                              • 16.1.5 Mechanism Problems
                                                                                                                                                                                                                                                                                                                                              • 16.2 Activation Energy
                                                                                                                                                                                                                                                                                                                                                • 16.2.1 The Arrhenius Equation
                                                                                                                                                                                                                                                                                                                                                  • 16.2.2 Graphing the Arrhenius Equation
                                                                                                                                                                                                                                                                                                                                                    • 16.2.3 Activation Energy from Rate Constants
                                                                                                                                                                                                                                                                                                                                                  • 17. HL Equilibrium
                                                                                                                                                                                                                                                                                                                                                    • 17.1 The Equilibrium Law
                                                                                                                                                                                                                                                                                                                                                      • 17.1.1 Applying the Equilibrium Law
                                                                                                                                                                                                                                                                                                                                                        • 17.1.2 Gibbs Free Energy & the Equilibrium Constant
                                                                                                                                                                                                                                                                                                                                                      • 18. HL Acids & Bases
                                                                                                                                                                                                                                                                                                                                                        • 18.1 Further Aspects of Acids & Bases
                                                                                                                                                                                                                                                                                                                                                          • 18.1.1 Lewis Theory
                                                                                                                                                                                                                                                                                                                                                            • 18.1.2 pH Curves
                                                                                                                                                                                                                                                                                                                                                              • 18.1.3 Indicators
                                                                                                                                                                                                                                                                                                                                                                • 18.1.4 Buffers
                                                                                                                                                                                                                                                                                                                                                                  • 18.1.5 Salt Hydrolysis
                                                                                                                                                                                                                                                                                                                                                                  • 18.2 Calculations Involving Acids & Bases
                                                                                                                                                                                                                                                                                                                                                                    • 18.2.1 Acid & Base Dissociation Constants
                                                                                                                                                                                                                                                                                                                                                                      • 18.2.2 Acid & Base Problem Solving
                                                                                                                                                                                                                                                                                                                                                                    • 19. HL Redox Processes
                                                                                                                                                                                                                                                                                                                                                                      • 19.1 Electrochemical Cells
                                                                                                                                                                                                                                                                                                                                                                        • 19.1.1 Electrochemical Cells
                                                                                                                                                                                                                                                                                                                                                                          • 19.1.2 The Standard Hydrogen Electrode
                                                                                                                                                                                                                                                                                                                                                                            • 19.1.3 Electrolysis of Aqueous Solutions
                                                                                                                                                                                                                                                                                                                                                                              • 19.1.4 Quantitative Electrolysis
                                                                                                                                                                                                                                                                                                                                                                                • 19.1.5 Free Energy & Eº
                                                                                                                                                                                                                                                                                                                                                                                  • 19.1.6 Electroplating
                                                                                                                                                                                                                                                                                                                                                                                • 20. HL Organic Chemistry
                                                                                                                                                                                                                                                                                                                                                                                  • 20.1 Types of Organic Reactions
                                                                                                                                                                                                                                                                                                                                                                                    • 20.1.1 Nucleophilic Substitution Reactions
                                                                                                                                                                                                                                                                                                                                                                                      • 20.1.2 Electrophilic Addition Reactions
                                                                                                                                                                                                                                                                                                                                                                                        • 20.1.3 Electrophilic Substitution Reactions
                                                                                                                                                                                                                                                                                                                                                                                          • 20.1.4 Reduction Reactions
                                                                                                                                                                                                                                                                                                                                                                                          • 20.2 Synthetic Routes
                                                                                                                                                                                                                                                                                                                                                                                            • 20.2.1 Synthesis
                                                                                                                                                                                                                                                                                                                                                                                            • 20.3 Stereoisomerism
                                                                                                                                                                                                                                                                                                                                                                                              • 20.3.1 Stereoisomers
                                                                                                                                                                                                                                                                                                                                                                                                • 20.3.2 Cis-Trans & E/Z Isomers
                                                                                                                                                                                                                                                                                                                                                                                                  • 20.3.3 Optical Isomers
                                                                                                                                                                                                                                                                                                                                                                                                • 21. Measurement & Analysis
                                                                                                                                                                                                                                                                                                                                                                                                  • 21.1 Spectroscopic Identification of Organic compounds
                                                                                                                                                                                                                                                                                                                                                                                                    • 21.1.1 High Resolution NMR
                                                                                                                                                                                                                                                                                                                                                                                                      • 21.1.2 X-Ray Crystallography
                                                                                                                                                                                                                                                                                                                                                                                                        • 21.1.3 Structure Identification Problems
                                                                                                                                                                                                                                                                                                                                                                                                      Stewart Hird

                                                                                                                                                                                                                                                                                                                                                                                                      Author: Stewart

                                                                                                                                                                                                                                                                                                                                                                                                      Stewart has been an enthusiastic GCSE, A Level and IB Chemistry teacher for more than 30 years in the UK as well as overseas, and has been an examiner for IB and A Level, also. As a long-standing Head of Department, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams.


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