7.4 Formal Charges and Resonance
3 min read•june 25, 2024
Formal charges and are crucial concepts in understanding and stability. They help explain how electrons are distributed in molecules and why certain structures are more stable than others.
Calculating formal charges allows us to predict the most stable . show how electrons can be delocalized across molecules, leading to enhanced stability and unique chemical properties.
Formal Charges and Resonance
Formal charges in Lewis structures
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- represents the difference between the number of in an isolated atom and the number of electrons assigned to that atom in a
- are the electrons in the outermost shell of an atom (Group 1A: 1, Group 2A: 2, Group 3A-8A: 3-8)
- Assigned electrons include electrons in lone pairs and half of the electrons in bonding pairs
- Calculate using the formula:
- are non-bonding electrons assigned to a specific atom (H2O, NH3)
- are electrons shared between atoms in a (H2, Cl2)
- Carbon monoxide (CO) example:
- Carbon has 4 valence electrons, 2 lone pair electrons, and 2 bonding pair electrons
- Oxygen has 6 valence electrons, 4 lone pair electrons, and 2 bonding pair electrons
- Carbon has 4 valence electrons, 2 lone pair electrons, and 2 bonding pair electrons
Evaluation with formal charges
- The most stable Lewis structure minimizes the sum of the absolute values of formal charges
- Structures with formal charges closer to zero are generally more stable (CH4, H2O)
- If multiple structures have the same sum of absolute formal charges, the most stable structure will have:
- Negative formal charges on more atoms (O, N, F)
- Positive formal charges on less electronegative atoms (C, P, S)
- Comparing two Lewis structures for CO2 example:
- Structure 1: C=O=O
- Sum of absolute formal charges = 0
- Structure 2: C-O=O+
- Sum of absolute formal charges = 2
- Structure 1 is more stable due to lower sum of absolute formal charges
- Structure 1: C=O=O
Resonance forms of molecules
- Resonance occurs when a molecule can be represented by two or more Lewis structures
- The actual structure is a hybrid of these (benzene, nitrate ion)
- Resonance forms differ in the arrangement of electrons, but not in the positions of atoms
- Draw resonance forms using these steps:
- Start with one valid Lewis structure
- Move electrons (as lone pairs or bonding pairs) to create another valid Lewis structure
- Do not change the positions of atoms
- Maintain the for each atom
- Ozone (O3) example:
- Resonance form 1: O=O-O
- Central oxygen has a positive formal charge, terminal oxygen has a negative formal charge
- Resonance form 2: O-O=O
- Central oxygen has a positive formal charge, other terminal oxygen has a negative formal charge
- The actual structure is a hybrid of these two forms, with a of 1.5 between each oxygen atom
- Resonance form 1: O=O-O
- Resonance involves , where electrons are spread over multiple atoms or bonds
Advanced Concepts in Resonance
- provides a more accurate description of electron behavior in resonance structures
- play a crucial role in resonance, as they can be easily delocalized across multiple atoms
- occurs when alternating single and double bonds allow for extended electron delocalization
- is a special case of resonance stability in cyclic, planar molecules with delocalized electrons
Key Terms to Review (28)
Aromaticity: Aromaticity is a chemical property that describes the stabilizing effect of cyclic, planar, and conjugated molecular structures. It is a crucial concept in organic chemistry, particularly in understanding the behavior and reactivity of certain hydrocarbon compounds.
Bond order: Bond order is the number of chemical bonds between a pair of atoms. It indicates the stability and strength of a bond.
Bond Order: Bond order is a concept that describes the strength and stability of a chemical bond between atoms. It is a measure of the number of shared electron pairs between two atoms, and it plays a crucial role in understanding the properties and reactivity of molecules.
Bonding Pair Electrons: Bonding pair electrons are the shared pairs of electrons that hold atoms together in a covalent bond. These electrons are located between the nuclei of the bonded atoms and are responsible for the attractive force that keeps the atoms together in a stable arrangement.
Conjugation: Conjugation refers to the process of delocalization of electrons in a molecule, resulting in the stabilization of the overall structure through the sharing or overlap of p-orbitals. This concept is particularly relevant in the context of understanding formal charges and resonance structures.
Coordinate covalent bond: A coordinate covalent bond is a type of covalent bond in which one atom provides both electrons for the shared pair. This often occurs between a Lewis base and a Lewis acid.
Covalent Bond: A covalent bond is a chemical bond formed by the sharing of one or more pairs of electrons between two atoms. This type of bond is responsible for the stability and structure of many molecules and is a fundamental concept in understanding the topics of 7.2 Covalent Bonding, 7.3 Lewis Symbols and Structures, 7.4 Formal Charges and Resonance, 7.5 Strengths of Ionic and Covalent Bonds, 7.6 Molecular Structure and Polarity, and 8.1 Valence Bond Theory.
Electron delocalization: Electron delocalization refers to the phenomenon where electrons are not confined to a specific bond or atom but are spread out over several atoms, allowing for a more stable arrangement. This concept is crucial in understanding how molecules can have multiple resonance structures, which reflect the shifting nature of electron distribution and enhance stability in certain compounds, especially those with multiple bonds and complex bonding scenarios.
Electronegative: Electronegativity is a chemical property that describes the ability of an atom to attract shared electrons in a chemical bond. Atoms with high electronegativity values are considered electronegative, meaning they have a strong tendency to attract and hold onto electrons.
Formal charge: Formal charge is a theoretical charge assigned to an atom in a molecule, calculated based on the number of valence electrons minus the sum of non-bonding electrons and half of the bonding electrons. It helps determine the most stable Lewis structure for a molecule.
Formal Charge: Formal charge is a concept in chemistry that helps determine the distribution of electrons in a molecule or ion. It is a way to assign a partial charge to each atom in a molecule based on the difference between the number of valence electrons an atom would have in an isolated, neutral state and the number of electrons it appears to have in the molecule.
Lewis Structure: A Lewis structure is a model used in chemistry to represent the bonding between atoms in a molecule or ion. It depicts the arrangement of atoms, the type of bonds (single, double, or triple), and the presence of lone electron pairs.
Lewis structures: Lewis structures are diagrams used to represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They help visualize the arrangement of valence electrons around atoms.
Lone Pair Electrons: Lone pair electrons are pairs of valence electrons that are not involved in covalent bonding between atoms. These unshared electron pairs occupy space around an atom and contribute to the overall shape and geometry of a molecule.
Molecular orbital theory: Molecular orbital theory is a method used to describe the electronic structure of molecules by combining atomic orbitals to form molecular orbitals that are spread over multiple atoms. This theory explains how electrons are shared between atoms, leading to bonding and anti-bonding interactions that dictate the stability and properties of molecules. Understanding this concept is crucial when examining the behavior of electrons in resonance structures, hybridization, and the formation of multiple bonds.
Molecular structure: The molecular structure describes the three-dimensional arrangement of atoms within a molecule. It determines the molecule's shape, bond angles, and overall geometry.
Octet rule: The octet rule states that atoms tend to form bonds in such a way that they achieve eight electrons in their valence shell, gaining a stable electron configuration similar to noble gases. This rule is fundamental for understanding molecular structures and chemical bonding.
Octet Rule: The octet rule is a fundamental concept in chemistry that states that atoms tend to form stable configurations by attaining a full outer electron shell, typically containing eight electrons. This principle helps explain the formation of ionic and covalent bonds, as well as the stability of various chemical compounds.
Pi Bonds: Pi bonds (π-bonds) are a type of covalent bond that forms between atoms when they share a pair of electrons in a side-to-side arrangement, rather than the head-to-head arrangement of sigma (σ) bonds. Pi bonds are crucial in understanding the concepts of formal charges and resonance within chemical structures.
Resonance: Resonance describes the phenomenon where a molecule can be represented by two or more valid Lewis structures. These structures, called resonance forms, contribute to the overall hybrid structure of the molecule.
Resonance forms: Resonance forms are different Lewis structures for the same molecule that depict the delocalization of electrons within the molecule. These forms help to illustrate how electrons can be distributed across various atoms while maintaining the overall connectivity.
Resonance Forms: Resonance forms, in the context of chemistry, are the different possible arrangements of electrons in a molecule that contribute to the overall stability of the compound. These resonance structures represent the delocalization of electrons within a molecule, which is a key concept in understanding formal charges and the bonding in organic and inorganic compounds.
Resonance hybrid: A resonance hybrid is a molecule or ion that is best represented by two or more valid Lewis structures, called resonance structures. The actual structure is a weighted average of these resonance forms, resulting in delocalized electrons.
Resonance Hybrid: A resonance hybrid is a representation of the bonding in a molecule where the actual structure is a combination or average of multiple possible Lewis structures, rather than a single, discrete structure. It arises when multiple valid Lewis structures can be drawn for a molecule, and the true structure is a resonance hybrid of these possibilities.
Valence electrons: Valence electrons are the outermost electrons of an atom and are involved in forming chemical bonds. They determine an element's chemical properties and reactivity.
Valence Electrons: Valence electrons are the outermost electrons in an atom that participate in chemical reactions and bonding. They are the electrons in the highest occupied energy level of an atom and are responsible for an element's chemical properties and behavior.
δ-: The Greek letter delta (δ) is used in chemistry to represent a partial or fractional charge. It is used to indicate the distribution of electron density in a molecule, particularly in the context of formal charges and resonance structures.
δ+: The Greek letter delta (δ) with a superscript plus sign (δ+) represents a partial positive charge on an atom or molecule. This term is particularly relevant in the context of formal charges and resonance structures in chemistry.