Chemistry Notes - Crystalline Solids, Metals, Alloys, and Polymers

Chemistry Notes by Engr. Romel Adrian G. Romero

Crystalline Solids

  • Definition: A crystalline solid is a rigid body with particles arranged in definite and repeating three-dimensional patterns, forming a crystal lattice that extends in all directions.
    • Characteristics include:
    • Well-defined edges and faces
    • Can diffract x-rays
    • Sharp melting points
Characteristics of Crystalline Solids
  • Regular structure with definite geometrical shape.
  • Exhibits sharp freezing points due to constant distance between atoms/molecules.
  • Has a definite and fixed heat of fusion owing to regular crystal lattice.
  • Referred to as True Solids because they don't flow like pseudo solids.
  • Cutting a crystalline solid results in a flat and smooth surface.
  • Nature of crystalline solids is anisotropic; properties vary in different directions.
  • Exhibits both long-range and short-range order.
Classes of Crystalline Solids
  • Described by the types of particles and chemical bonds involved:
    1. Ionic
    • Composed of positive and negative ions.
    • High melting points and brittleness.
    • Poor conductors in solid state.
    • Example: Table salt (NaCl).
    1. Molecular
    • Made of atoms/molecules held together by intermolecular forces (e.g., hydrogen bonds).
    • Low melting points and flexibility.
    • Poor conductors.
    • Example: Sucrose.
    1. Covalent (also called atomic)
    • Atoms connected by covalent bonds.
    • Very hard with high melting points, poor conductors.
    • Examples: Diamond, graphite.
    1. Metallic
    • Composed of metal atoms with metallic bonds.
    • High melting points, can be malleable or hard, good conductors.
    • Examples: Copper, Gold.

Crystal Structure

  • Composed of a unit cell, which is a set of atoms repeatedly arranged in three dimensions on a lattice.
  • Based on three fundamental laws:
    • Law of Constancy of Interfacial Angles: Faces of a crystal intersect at a constant angle.
    • Law of Rationality of Indices: Unit distances along coordinate axes can create integer or fractional ratios for crystal plane intercepts.
    • Law of Symmetry: All crystals of the same substance exhibit the same symmetry elements (plane, line, center symmetry).
Unit Cells and Crystal Systems
  • Crystals categorized into seven systems based on symmetry:
    1. Simple Cubic
    2. Body-Centered Cubic
    3. Face-Centered Cubic
    4. Tetragonal
    5. Orthorhombic
    6. Monoclinic
    7. Triclinic
    8. Hexagonal

Metals

  • Predominantly make up known elements and are characterized by properties such as:
    • Luster
    • Malleability
    • Ductility
    • High melting and boiling points
    • High electrical and heat conductivity
  • Found mostly in ores but some, like gold and copper, exist naturally.
Uses of Metals
  • Construction materials (e.g., buildings, ships).
  • Electrical components (wires, gadgets).
  • Jewelry and decorative arts (silver, gold).

Alloys

  • Mixtures or metallic solid solutions composed of two or more elements, exhibiting different properties from their components.
  • Types of Alloys:
    1. Substitution Alloys: Atoms of the original metal are replaced (e.g., brass).
    2. Interstitial Alloys: Smaller atoms occupy interstices among metal atoms.
    • Fusible Alloys: Have melting points below that of tin, used for safety devices, etc.
Important Alloys and Their Composition/Uses
  • Babbitt Metal: Tin, antimony, copper – low friction in bearings.
  • Bronze: Copper, tin – coins and medals.
  • Sterling Silver: Silver, copper – jewelry.

Polymers

  • Made of long chains of repeating molecules (monomers).
  • Types of Polymerization:
    1. Addition Polymerization: Coupling of monomers.
    2. Condensation Polymerization: Joining molecules while releasing a small molecule (e.g., water).
  • Useful Properties:
    • High strength-to-weight ratios.
    • Toughness.
    • Resistance to corrosion.
    • Variety in color and transparency.
Classifications of Polymers
  • Thermoplastics: Soften when heated, used in packaging, insulation.
  • Thermosets: Solidify irreversibly when heated, used in durable products like automotive parts.
  • Elastomers: Stretchy, return to shape, used in rubber products.
  • Natural vs Synthetic Polymers: Natural occur in nature; synthetic are petroleum-based products.
Summary of Synthetic Polymers

Examples include:

  • Polyethylene,
  • PVC,
  • Nylon,
  • Polystyrene.
    Researchers often add plasticizers to enhance flexibility.