Chapter 4, Section 4: Lewis Symbols and Structures

Learning Objectives

• Understand and draw Lewis symbols.
• Comprehend the formation of ions and ionic compounds.
• Grasp the basics of Lewis structures and the octet rule.
• Identify exceptions to the octet rule.

Lewis Symbols

• Lewis symbols depict the valence electron configuration of atoms and monatomic ions.
• Elements & Valence Electrons:
  • Represented by dots around the elemental symbol.
  • Sodium (Na) example: One electron shown as a single dot.
• Ion Formation:
  • Cations: E.g., Na loses an electron to become Na⁺.
  • Anions: E.g., Sulfur starts with 6 electrons, gains 2 to form S²⁻.
• Ionic Compounds:
  • Sodium chloride formation: Na loses an electron transferred to Cl, forming Na⁺ and Cl⁻.

Lewis Structures

• Definition: Drawings that show bonding in molecules and polyatomic ions where electrons are shared.
• Example: Two Cl atoms share electrons forming a diatomic molecule with a single covalent bond shown as a line.
• Octet Rule:
  • Main group atoms form bonds to have 8 valence electrons, mimicking noble gas configuration.
  • Predictive for bond formation in elements like Carbon (4 bonds), Nitrogen (3 bonds), Oxygen (2 bonds), and Fluorine (1 bond).

Examples of Lewis Structures

• Carbon Tetrachloride (CCl₄): Carbon forms four bonds with Chlorine.
• Ammonia (NH₃): Nitrogen forms three bonds with Hydrogen.
• Water (H₂O): Oxygen forms two bonds with Hydrogen.
• Hydrofluoric Acid (HF): Fluorine forms one bond with Hydrogen.

Multiple Bonds

• Double Bonds: Two pairs of electrons shared, e.g., Formaldehyde (CH₂O).
• Triple Bonds: Three pairs of electrons shared, e.g., Carbon Monoxide (CO).

Exceptions to the Octet Rule

• Odd Number of Electrons: Example, Nitric Oxide (NO) with a free radical.
• Electron-Deficient Molecules: Central atom has fewer electrons, e.g., Boron Trifluoride (BF₃) with 6 electrons around Boron.
• Hypervalent Molecules: Central atom has more than 8 electrons, e.g., Phosphorus Pentachloride (PCl₅) with 10 electrons around Phosphorus.
  • Only third row elements and below can exceed the octet due to d orbitals.