Covalent Molecular Properties

Introduction to Covalent Molecular Substances

Definition and Characteristics

• Covalent Molecular Substances: Created through covalent bonds by the sharing of valence electrons between atoms. Examples include water (H₂O), carbon dioxide (CO₂), and benzene (C₆H₆).
• Key Characteristics:
  • Low Melting and Boiling Points:
    • This is due to weak intermolecular forces.
    • Example: Ice (H₂O) melts readily.
  • Electrical Conductivity:
    • Typically do not conduct electricity.
    • Exception: Benzene (C₆H₆) under certain conditions.
  • Non-Polar Structures:
    • Many are non-polar due to uniform electron distribution.
    • Example: Methane (CH₄).

Did You Know?

Comparison with Ionic and Metallic Substances

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Electron Sharing and Bond Characteristics

1. Electron Sharing

• Covalent Bond: The sharing of electron pairs among atoms.
• Visualised by Lewis dot structures:
  • Examples: H₂ and Cl₂

2. Bond Strength and Length

• Types of Covalent Bonds:

  • Single Bond: Sharing one pair of electrons (e.g., Ethane, C₂H₆).
  • Double Bond: Sharing two pairs of electrons (e.g., Ethylene, C₂H₄).
  • Triple Bond: Sharing three pairs of electrons (e.g., Nitrogen, N₂).

• Bond Strength and Stability:

  • Multiple bonds confer greater stability.

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VSEPR Theory and Molecular Shapes

Introduction to VSEPR Theory Application

• VSEPR Theory: Valence Shell Electron Pair Repulsion theory anticipates molecular shapes by minimising repulsion.
• Importance: Fundamental for predicting reactions and properties.

Detailed Molecular Geometry Exploration

• Key Geometries:
  • Linear: Angle ~180°, Example: CO₂
  • Bent (Angular): Angle <120°, Example: H₂O
  • Tetrahedral: Angle 109.5°, Example: CH₄
  • Trigonal Planar: Angle 120°, Example: BF₃
  • Trigonal Bipyramidal: Angles 90° & 120°, Example: PCl₅
  • Octahedral: Angle 90°, Example: SF₆

Impact of Lone Pairs and Multiple Bonds

• Lone pairs induce stronger repulsion, reducing angles in molecules such as NH₃ and H₂O.

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Physical Properties

Melting and Boiling Points

• Influenced by Intermolecular Forces: Stronger forces lead to higher melting/boiling points.
  • Example: Water > CO₂ due to hydrogen bonds.

Electrical Conductivity

• Generally poor due to absence of free ions or electrons.

Solubility Trends

• 'Like Dissolves Like' Principle:
  • Polar substances dissolve in polar solvents and vice-versa.
• Temperature Effect:
  infoNote

  Higher temperatures typically increase solubility of solids but decrease gas solubility.

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Exam Strategies

Answer Organisation

• Structure responses with an introduction, discussion, and conclusion.

Common Exam Questions

• Predict Properties: Assess melting/boiling points and solubility from structures.
• Structure Identification: Employ VSEPR models to understand geometries.

Worked Example: Predicting Molecular Geometry

For a molecule with formula AB₃, where A is the central atom with no lone pairs:

1. Count total electron pairs around the central atom: 3 bonding pairs, 0 lone pairs
2. Apply VSEPR theory to minimise repulsion
3. The molecular geometry is trigonal planar with bond angles of 120°

This can be seen in molecules like BF₃, where boron forms three single bonds with fluorine atoms, resulting in a flat, triangular arrangement.