Untitled Flashcards Set

Chemistry Study Guide: Polarity, Molecular Geometry, and Polyatomic Formulas

Polarity

• Definition: A molecule is polar if it has an uneven distribution of electrons, creating partial positive and negative charges.

• Bond Polarity: Determined by the difference in electronegativity between two atoms.

  • Nonpolar Covalent: Electronegativity difference of 0 to 0.4

  • Polar Covalent: Electronegativity difference of 0.5 to 1.7

  • Ionic: Electronegativity difference greater than 1.7

• Molecular Polarity: Depends on both bond polarity and molecular geometry.

  • Asymmetrical shapes often lead to polar molecules.

  • Symmetrical shapes typically result in nonpolar molecules.

Molecular Geometry (VSEPR Theory)

• Valence Shell Electron Pair Repulsion (VSEPR): Predicts the shape of molecules based on electron pair repulsion.

• Electron Domains: Regions of electron density around the central atom, including bonded atoms and lone pairs.

• Key Shapes and Angles:

  • Linear: 180° bond angle (e.g., BeF₂)

  • Trigonal Planar: 120° bond angle (e.g., H₂CO)

  • Tetrahedral: 109.5° bond angle (e.g., CH₄)

  • Trigonal Pyramidal: <109.5° bond angle due to lone pair repulsion (e.g., NH₃)

  • Bent: <109.5° bond angle (e.g., H₂O)

Lone Pairs and Geometry

• Lone pairs occupy more space than bonding pairs, causing bond angles to shrink.

• Molecules with the same steric number can have different shapes due to lone pairs.

Steric Number and Molecular Geometry

1. Finding Steric Number:

   • Count bonded atoms to the central atom.

   • Count lone pairs on the central atom.

   • Add these together.

2. Shapes Based on Steric Number:

   • Steric Number 2: Linear

   • Steric Number 3: Trigonal Planar

   • Steric Number 4: Tetrahedral, Trigonal Pyramidal, or Bent (depending on lone pairs)

Examples

1. Methane (CH₄)

   • 4 bonded atoms, no lone pairs

   • Steric number = 4

   • Shape: Tetrahedral

2. Ammonia (NH₃)

   • 3 bonded atoms, 1 lone pair

   • Steric number = 4

   • Shape: Trigonal Pyramidal

3. Water (H₂O)

   • 2 bonded atoms, 2 lone pairs

   • Steric number = 4

   • Shape: Bent

Polarity and Molecular Shape Connection

• Polar molecules have asymmetrical shapes or highly electronegative atoms.

• Nonpolar molecules have symmetrical shapes or balanced charge distributions.

Polyatomic Ions

• Definition: Ions composed of two or more covalently bonded atoms.

• Common Polyatomic Ions:

  • Ammonium (NH₄⁺)

  • Nitrate (NO₃⁻)

  • Sulfate (SO₄²⁻)

  • Hydroxide (OH⁻)

  • Carbonate (CO₃²⁻)

• Bonding: Polyatomic ions form ionic bonds with other ions in compounds.

Important Notes from Class Worksheet

• VSEPR Summary: Molecule shape is based on bonding and lone pairs around the central atom.

• Electron Domains:

  • Bonding pairs (single, double, or triple bonds count as one domain)

  • Lone pairs on the central atom

• Lone Pairs:

  • Impact shape by repelling bonded atoms

  • Found only on the central atom in molecular geometry considerations

• Shapes You Must Identify:

  • Linear

  • Bent

  • Trigonal Planar

  • Trigonal Pyramidal

  • Tetrahedral

• Sticky Outies Concept: Bonded atoms and lone pairs "stick out" from the central atom, determining shape.

• How to Find Steric Number:

  • Count bonded atoms to the central atom.

  • Count lone pairs on the central atom.

  • Add these numbers together.

Examples from Worksheet

• Methane (CH₄): Steric number 4, tetrahedral shape

• Ammonia (NH₃): Steric number 4, trigonal pyramidal shape

• Water (H₂O): Steric number 4, bent shape

• Bond Angle Trends: Lone pairs decrease bond angles compared to ideal geometries.

• Why Draw Lewis Structures: Ensures accurate shape prediction and identification of lone pairs.

This comprehensive guide should help you review the core concepts and examples thoroughly. Best of luck on your quiz!