Chemistry - Course Companion - Bylikin, Horner, Grant and Tarcy - Oxford 2023

Structure 2.2: The Covalent Model

Determining Covalent Nature

Covalent bonds lead to a wide variety of substances with different properties. Their classification can be seen in two main categories:

• Covalent Network Structures (e.g., diamond, silicon):

  • These structures are characterized by:

    • High melting and boiling points: Due to extensive bonding which requires significant energy to break.

      • Example: Diamond has a melting point of around 3550 °C due to its strong covalent bonds.

    • Low volatility: They do not readily vaporize.

    • Poor water solubility: Many covalent network substances do not dissolve in water due to their strong covalent bonds.

      • Example: Silicon carbide (SiC) is insoluble in water due to its strong covalent network.

• Molecular Covalent Structures (e.g., water, plastics):

  • These structures exhibit:

    • Low melting and boiling points: Resulting from weaker intermolecular forces compared to covalent networks.

      • Example: Water (H2O) has a boiling point of 100 °C, owing to its hydrogen bonding.

    • Variable solubility and volatility: Dependent on the nature of their intermolecular forces, e.g., hydrogen bonding in water enhances its solubility and boiling point.

      • Example: Ethanol (C2H5OH) is soluble in water due to hydrogen bonding.

Covalent Bonds

A covalent bond forms through the electrostatic attraction between a shared pair of electrons and positively charged nuclei, represented as follows:

• Single Bond: One shared pair of electrons (e.g., H2).

• Double Bond: Two shared pairs of electrons (e.g., O2).

• Triple Bond: Three shared pairs of electrons (e.g., N2).

• Coordination Bond: Both electrons in a shared pair stem from one atom (e.g., in complex ions such as [Cu(NH3)4]2+).

Bond Formation:

Bond Polarity arises from differing electronegativities, leading to partial positive and negative charges in polar covalent bonds.

Shapes and Structures

The Valence Shell Electron Pair Repulsion (VSEPR) Model predicts molecular shapes based on electron domain repulsion around central atoms.

• Common Geometries:

  • Linear: 180° bond angles (e.g., CO2).

  • Trigonal Planar: 120° bond angles (e.g., BF3).

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

  • Trigonal Bipyramidal: 90° and 120° bond angles (e.g., PCl5).

  • Octahedral: 90° bond angles (e.g., SF6).

• Resonance Structures: Occur when a molecule can be represented by multiple valid Lewis structures (e.g., ozone O3 and benzene C6H6), impacting stability and reactivity.

Lewis Formulas

Lewis formulas depict the arrangement of valence electrons among the atoms in a molecule, where shared and unshared electrons are represented using lines or dots.

• Example:

Formal Charge Calculation: A method for determining the most stable Lewis structure by minimizing the formal charges on atoms.

Molecular Geometry

The shape of molecules influences their physical and chemical properties, including reactivity.

• Bond Types:

  • Single Bonds: Sigma bonds (σ) created by head-on orbital overlap.

  • Double Bonds: Comprised of one σ bond and one Pi bond (π).

  • Triple Bonds: Made of one σ bond and two π bonds.

Expanded Octets

Certain elements (e.g., phosphorus, sulfur) can accommodate more than eight electrons, forming expanded octets which support unique molecular geometries like trigonal bipyramidal and octahedral.

• Example: Phosphorus pentachloride (PCl5) has five bonding pairs of electrons leading to a trigonal bipyramidal structure.

Intermolecular Forces Overview

The physical properties of substances are greatly influenced by intermolecular forces, including:

• London Dispersion Forces: Present in all molecules due to temporary dipoles.

• Dipole-Dipole Forces: Occur between polar molecules, affecting boiling points and solubility.

• Hydrogen Bonds: Strong dipole-dipole interactions involving hydrogen bonded to highly electronegative atoms (e.g., F, O, N), significantly raising boiling points and solubilities.- Example: Water molecules exhibit hydrogen bonding, resulting in a high boiling point compared to other group 16 hydrides.

Chromatography

Chromatography is a separation technique based on differences in affinities for mobile vs. stationary phases, essential for analyzing chemical mixtures.

• Techniques:

  • Paper Chromatography: Utilizes absorbent paper as the stationary phase to separate colored substances in a mixture.

  • Thin-Layer Chromatography (TLC): Employs a thin layer of adsorbent material on a flat substrate to identify compounds in complex mixtures.

Bond Order and Stability

Bond order indicates the number of shared electron pairs between atoms. Higher bond orders correlate with stronger bonds and shorter bond lengths.

• Example: Comparison of bond lengths and strengths in C-C bonds across single (1.54 Å), double (1.34 Å), and triple (1.20 Å) configurations.

Key Takeaways

Understanding electron configurations, molecular geometries, and bond types enhances comprehension of chemical behaviors and properties in covalent compounds, vital for success in the IB Chemistry syllabus 2025.