Study Notes on Formal Charges and Resonance Structures

Assigning Formal Charges

• Definition of Formal Charge:
  • Formal charge is a theoretical charge assigned to an atom in a molecule to help determine the most stable structure.
  • It is calculated using the formula:
    $\text{Formal Charge} = \text{Group Number} - \text{(Nonbonding Electrons)} - \frac{1}{2}(\text{Shared Electrons})$
  • Group number approximates the number of valence electrons for main-group elements, but can vary on different periodic tables.
• Example - Oxygen:
  • Oxygen has six valence electrons.
  • Calculation:
  • Non-bonding electrons: 2
  • Shared electrons from bonds: 3 (one bond for each bond)
  • Bond calculation for typical stable states leads to:
    $\text{Formal Charge} = 6 - 2 - 3 = +1$
• Hydronium Ion (H₃O⁺):
  • The overall charge of the hydronium ion is +1.
  • In this case, the formal charges must add up to match the overall charge of the molecule.

Formal Charges in Assignments

• Cation Example - Nitrogen:
  • Nitrogen typically has five valence electrons.
  • Calculation:
    • Non-bonding electrons = 2
  • Shared electrons from triple bond: 3
  • Overall charge:
    $\text{Formal Charge} = 5 - 2 - 3 = 0$
• Cation Example - Oxygen:
  • Oxygen with three bonds and two lone pairs
  • Calculation yields 0 formal charge:
    $\text{Formal Charge} = 6 - 2 - 3 = +1$
• Lone Pairs and Formal Charge Assignment:
  • Hydrogen with one bond: never has a formal charge.
  • Calculation:
    $1 - 1 = 0$
• Practice Problem:
  • Assign formal charges to various atoms in a structure without considering hydrogen.

Common Bonding Patterns

• Neutral atoms have specific bonding patterns:
  • Carbon: typically forms 4 bonds.
  • Nitrogen: typically forms 3 bonds.
  • Oxygen: typically forms 2 bonds.
  • Halogens: typically form 1 bond.
• Unstable Intermediates:
  • Discusses the behavior of reactive intermediates like cations and anions in chemical reactions.
  • Carbocations: positively charged carbon species; unstable intermediates.
  • Carbanions: negatively charged carbon with a lone pair; also unstable.
• Electronegativity and Charge Carries:
  • Negatively charged atoms can hold a charge due to their electronegativity.

Resonance Structures

• Definition of Resonance:
  • Resonance structures are different ways to represent the same molecule. All atoms should aim for an octet, and must maximize bonding.
• Example - Carbonate Ion (CO₃²⁻):
  • Multiple resonance structures with equivalent formal charges are drawn.
  • Two oxygens have negative formal charges while carbon remains neutral.
  • Distinction between resonance structures:
  • Various configurations represent the same hybrid structure, showing delocalization of electrons.

Evaluating Resonance Structures

• Criteria for Stable Structures:
  • Octet rule is respected in all structures.
  • Prefer structures where electronegative atoms bear negative charges.
  • Minimization of charge separation leads to increased stability.
• Common Mistakes:
  • Drawing structures without all atoms achieving octets.
  • Assigning negative charges to less electronegative atoms.

Curved Arrow Notation in Resonance

• Type of Arrows:
  • Type 1 Arrow: lone pair to multiple bond.
  • Type 2 Arrow: bonded pair to lone pair.
  • Type 3 Arrow: pairs of electrons move towards the positive charge.
• Use in Organic Chemistry:
  • Curved arrows depict movement of electrons and designate how charges and bonds shift.

Practice Examples

• Demonstrating formal charge calculations with resonance structures.
• Practice drawing structures following types of electron movement using curved arrows.