Unit cell

Crystalline and Amorphous Solids

Crystalline Solids

  • Definition: Crystalline solids have a regular, repeating pattern of atoms, ions, or molecules that forms a lattice structure extending in all directions.

  • Characteristics:

    • Distinct geometric shapes

    • Sharp melting points

    • Anisotropic properties

  • Examples: Quartz, calcite, sugar, mica, diamonds, snowflakes, rock, calcium fluoride, silicon dioxide, and alum.

Amorphous Solids

  • Definition: Amorphous solids have an irregular internal atomic structure and lack a well-defined shape.

  • Characteristics:

    • Break into uneven pieces with irregular edges

    • Pitted, jagged breaking pattern

    • No sharp melting points; they soften and melt over a range of temperatures

  • Examples: Glass, gels, plastics, various polymers, wax, thin films, pitch, rubber, and butter.

Contrast Between Amorphous and Crystalline Solids

  • Structure:

    • Amorphous solids lack repeating units and do not have fixed geometrical shapes, making them less rigid.

    • Crystalline solids possess regularly repeating units with perfectly ordered geometric shapes and maintain a high rigidity.

  • Melting Points:

    • Amorphous solids have no definite melting points.

    • Crystalline solids have definite melting points.

  • Properties:

    • Amorphous solids are isotropic, meaning their properties are the same in all directions; crystalline solids are anisotropic, showing different properties in different directions.

  • Internal Order:

    • Amorphous has short-range ordered molecules; crystalline has long-range ordered molecules.

Metallic Structures and Unit Cells

Overview of Metallic Structures

  • Elemental metals are characterized by a crystalline solid structure where metal atoms are closely packed in a repeating pattern.

  • Properties like malleability and ductility are influenced by the regular arrangement of identical atoms.

Unit Cells

  • Definition: A unit cell is the simplest repeating unit in the structure of a solid, consisting of lattice points that represent the locations of atoms or ions.

  • Crystalline Solid Structure: Comprises unit cells repeating in three dimensions, forming the overall lattice of the material.

Types of Unit Cells

Cubic Unit Cells

  • Simple Cubic:

    • Edge Length: a = b = c

    • Angles: α = β = γ = 90°

    • Number of atoms: 1

  • Body-Centered Cubic (BCC):

    • Edge Length: a = b = c

    • Angles: α = β = γ = 90°

    • Number of atoms: 2

  • Face-Centered Cubic (FCC):

    • Edge Length: a = b = c

    • Angles: α = β = γ = 90°

    • Number of atoms: 4

Face-Centered Cubic Structure

Characteristics

  • The FCC structure is a close-packed arrangement of atoms.

  • Atomic Packing Factor (APF) for FCC: 0.74, indicating maximum packing.

  • Coordination Number (CN): For FCC, the coordination number is 12, meaning each atom is surrounded by 12 others.

Calculating APF

  • Volume of the unit cell: V = (Edge length)^3

  • APF is calculated by the formula: APF = (Volume occupied by atoms) / (Volume of the unit cell).

Grain Structure

  • Formation of Grains: Grains form when molten material solidifies. They vary in size and shape; some are large enough to be observed under a microscope.

  • Applications: Certain minerals and synthetic crystals (like silicon and gallium arsenide) are critical in electronics such as integrated circuits and LEDs.

Influence of Grain Size on Properties

  • Impact on Metallurgical Properties:

    • Fine-grained metals exhibit higher hardness and strength compared to coarse-grained ones.

    • Fine grained increases toughness while coarse grained enhances ductility.

    • Grain refinement leads to enhanced overall metallurgical performance.

Chemical Bonds

Ionic Bonds

  • Formed through the transfer of electrons, leading to the creation of cations and anions.

  • Properties:

    • High melting and boiling points due to strong electrostatic forces.

    • Soluble in polar solvents; conduct electricity when molten or dissolved.

    • Characterized by a crystal lattice structure.

Covalent Bonds

  • Formed by sharing electrons between atoms, often leading to flexible bond strengths.

  • Properties:

    • Lower melting and boiling points than ionic compounds.

    • Typically soluble in nonpolar solvents; poor electrical conductivity except in specific cases.

Metallic Bonds

  • Involve a delocalization of valence electrons, contributing to characteristic properties of metals:

    • High thermal and electrical conductivity

    • Malleability and ductility due to the ability of the lattice to deform

    • Distinctive shine due to light reflection by free electrons.