Chem chapter 12 study guide

Chapter 12: Crystalline Solids and Modern Materials

12.3 Crystalline Solids: Unit Cells and Basic Structures

Crystalline solids are characterized by highly ordered arrangements of particles, forming repeating patterns known as unit cells. Understanding unit cells is essential for analyzing the structure and properties of solids.

• Definition of Unit Cell: The unit cell is the smallest repeating unit in a crystal lattice that, when stacked in three dimensions, recreates the entire structure of the solid.

• Types of Crystalline Lattice Structures:

  • Simple Cubic (SC): Atoms are located at each corner of the cube.

  • Body-Centered Cubic (BCC): Atoms at each corner and a single atom at the center of the cube.

  • Face-Centered Cubic (FCC): Atoms at each corner and at the center of each face of the cube.

• Coordination Number: The number of nearest neighbor atoms surrounding a given atom in the lattice.

  • Simple cubic: 6

  • Body-centered cubic: 8

  • Face-centered cubic: 12

• Atoms per Unit Cell:

  • Simple cubic: 1 atom per unit cell

  • Body-centered cubic: 2 atoms per unit cell

  • Face-centered cubic: 4 atoms per unit cell

• Packing Efficiency: The fraction of volume in a crystal structure that is occupied by constituent particles.

  • Face-centered cubic has the highest packing efficiency.

  • Simple cubic has the lowest packing efficiency.

• Calculating Unit Cell Dimensions:

  • Use the type of unit cell and the radius of an atom to calculate:

    • Cell edge length

    • Volume of unit cell

    • Density of a material

• Example: For a face-centered cubic cell, the relationship between the edge length ($a$) and atomic radius ($r$) is $a = 2\sqrt{2}r$.

12.4 Crystalline Solids: The Fundamental Types

Crystalline solids are classified based on the nature of their constituent particles and the forces holding them together. This classification helps predict their properties and applications.

• Types of Crystalline Solids:

  • Molecular Solids: Composed of molecules held together by intermolecular forces (e.g., ice, dry ice).

  • Ionic Solids: Composed of ions held together by electrostatic forces (e.g., sodium chloride).

  • Atomic Solids: Composed of atoms held together by covalent bonds (e.g., diamond, graphite).

• Properties and Examples:

  • Molecular solids: Low melting points, soft, poor conductors.

  • Ionic solids: High melting points, hard, brittle, conduct electricity when molten or dissolved.

  • Atomic solids: Variable melting points, hardness depends on bonding (diamond is very hard, graphite is soft).

• Relative Melting Points: Generally, ionic solids have higher melting points than molecular solids, while atomic solids vary depending on the strength of covalent bonding.

• Example: Sodium chloride (ionic solid) has a much higher melting point than ice (molecular solid).

Type of Solid	Constituent Particles	Forces	Melting Point	Examples
Molecular	Molecules	Intermolecular	Low	Ice, CO2
Ionic	Ions	Electrostatic	High	NaCl, KBr
Atomic	Atoms	Covalent	Variable	Diamond, Graphite

Additional info: Academic context and examples have been added to expand on the brief points in the original notes.