CH 12 (2/6)(PG 1-5)

Introduction to Solids

  • Crystalline Solids:

    • Definition: Have well-ordered, definite arrangements of molecules, atoms, or ions.

    • Melting Point: Melts at specific temperatures.

    • Examples: Quartz, glucose, sodium chloride.

  • Amorphous Solids:

    • Definition: Molecules, atoms, or ions do not have orderly arrangements.

    • Melting Point: Have no specific melting point.

    • Examples: Rubber, glass.

Types of Crystalline Solids

  • General Characteristics:

    • Ordered structure with repeating units.

    • Smallest repeating unit in a crystal: Unit Cell.

    • Definition of Unit Cell: Selected to be the smallest unit reflecting the symmetry of the crystal.

    • Arrangement: Atoms are placed in corners or other specific places in the unit cell, with the 3-D stacking of unit cells giving the crystal lattice.

Types of Unit Cells

  • Basic Unit Cells: Seven basic three-dimensional unit cells known.

    • Cubic Unit Cell: Has equal lengths of the edges that meet at 90° angles.

Primitive (Simple) Cubic Unit Cell

  • Definition: Atoms are located at the corners of a cube.

  • Atom Sharing: Each atom is shared by eight unit cells (8 corners of a cube).

    • Contribution per Unit Cell:

    • (8 corners of a cube)(1/8 of each corner atom within a unit cell) = 1 net atom per unit cell

Body Centered Cubic Unit Cell

  • Structure: Atoms at the corners of a cube plus one atom in the center of the body of the cube.

  • Atom Contributions:

    • Contribution per Unit Cell:

    • (8 corners of a cube)(1/8 of each corner atom within a unit cell) + (1 atom at the center of unit cell) = 2 net atoms per unit cell

Face Centered Cubic Unit Cell

  • Structure: Atoms at corners plus one atom in the center of each face of the cube.

  • Atom Contributions:

  • Contribution per Unit Cell:

    • (8 corners of a cube)(1/8 of each corner atom within a unit cell) + (6 atoms at the center of each face)(1/2 of each face atom within a unit cell) = 4 net atoms per unit cell

Summary of Cubic Unit Cells

  • Types and Atoms per Unit Cell:

    • Primitive (simple cubic): 1 atom/unit cell

    • Body Centered: 2 atoms/unit cell

    • Face Centered: 4 atoms/unit cell

Crystal Lattice and Coordination Number

  • Coordination Number: Number of adjacent atoms (the closest ones).

  • Values of Coordination Number per Cubic Unit Cell:

    • Simple cubic: 6

    • Body-centered cubic: 8

    • Face-centered cubic: 12

Packing Efficiency of Spherical Objects

  • Types of Packing:

    • Simple Cubic (SC): 52% packing efficiency.

    • Body Centered Cubic (BCC): 68% packing efficiency.

    • Face Centered Cubic (FCC): 74% packing efficiency.

  • Configurations:

    • Hexagonal Arrangement: ABAB… (3-D hexagonal unit cell configuration).

    • Cubic Close-Packed Structure: ABCABC… (face-centered cubic unit cell configuration).