Molecular Crystals grade 10

Learning Outcomes

  • 5.5: Describe simple molecular crystals and giant molecular structures.

  • 5.6: Distinguish between ionic and molecular solids.

Ionic Crystals

  • Definition: An ionic crystal is composed of an ionic lattice where strong electrostatic forces of attraction (ionic bonds) link cations and anions in a regular three-dimensional arrangement.

  • Representation: Ionic crystals are represented by empirical formulae or formula units.

  • Examples:

    • Sodium chloride (NaCl) is formed by Na⁺ ions and Cl⁻ ions.

    • All other ionic compounds follow this structure.

Simple Molecular Structures

  • Majority of covalent substances form simple molecular structures which are typically liquids or gases.

  • Composition: Discrete molecules are held together by dipole attractions and/or van-der-Waals forces.

Simple Molecular Crystals

  • Defined as composed of a molecular lattice where small molecules are held together by weak intermolecular forces.

  • Examples:

    • Iodine (I₂)

    • Sulfur (S₈)

    • Ice (H₂O)

    • Dry ice (CO₂)

Physical Properties of Simple Molecular Compounds

  • Melting Points: Low melting points due to weak intermolecular forces, requiring less energy to overcome.

  • Softness: Soft in texture; not much energy is needed to overcome weak intermolecular forces when scratched.

  • Conductivity: Non-conductors of heat and electricity due to absence of free electrons or ions.

  • Solubility:

    • Soluble in organic solvents (non-polar) because interactions with solvent molecules are stronger than intermolecular forces between the molecules.

    • Insoluble in water as water molecules cannot form sufficiently strong bonds to separate the molecules from one another.

Giant Molecular Structures

  • Composed of three-dimensional networks of covalent bonds.

  • Substances like carbon and silicon dioxide exhibit giant repeating structures existing as crystalline solids.

  • Allotropes: Elements that possess more than one structure, leading to different physical properties while retaining the same chemical properties.

    • Carbon Allotropes:

      • Diamond

      • Graphite

Structure of Diamond

  • Each carbon atom is linked to four others by strong covalent bonds.

  • The arrangement forms a tetrahedral structure extending throughout the diamond, creating a vast lattice of millions of atoms.

Structure of Graphite

  • Each carbon atom is bonded to three others in a planar layout by strong covalent bonds.

  • Carbon atoms create six-member hexagonal rings forming flat layers.

  • Weak van-der-Waals forces exist between the layers.

Physical Properties of Diamond and Graphite

  • Diamond:

    • High melting point due to strong covalent bonds.

    • Hardest natural material on Earth.

    • Transparent and sparkling appearance.

    • Non-conductor of electricity; lacks mobile electrons.

    • Insoluble in water and organic solvents due to strong covalent bonds.

  • Graphite:

    • High melting point, similar to diamond.

    • Soft, flaky, and brittle due to weak van-der-Waals forces.

    • Good lubricant as layers slide over one another easily.

    • Good conductor of electricity owing to the presence of mobile electrons.

    • Insoluble in both water and organic solvents.

Quick Check Questions

  1. Complete the table comparing properties of Sodium chloride, Diamond, and Graphite:

    • Melting Point: Very high for all.

    • Hardness: Varies (very high for diamond, soft for graphite).

    • Lubricating Power: Poor for sodium chloride and diamond, good for graphite.

    • Electrical Conductivity: Good in solution for sodium chloride, poor for diamond, very good for graphite.

  2. Identify and explain the structures of various solids based on given characteristics:

    • Example X: High melting point, does not conduct electricity, insoluble — likely a giant molecular structure.

    • Example Y: Low melting point, does not conduct electricity, soluble in organic solvent — likely a simple molecular structure.