10.5 The Solid State of Matter

Crystalline solids come in various types, each with unique properties. Ionic solids have high melting points and conduct electricity when molten. Molecular solids have low melting points due to weak forces. Metallic solids conduct electricity well and are malleable. Covalent network solids are hard and brittle.

The structure of crystalline solids is defined by unit cells, which determine symmetry and properties. Different unit cell types exist, like simple cubic and face-centered cubic. Crystal systems categorize symmetry, while analysis techniques like X-ray crystallography reveal atomic structures. Crystal defects can impact material properties significantly.

Crystalline Solids

Types of crystalline solids

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• Ionic solids consist of positively and negatively charged ions held together by strong electrostatic forces, have high melting points due to strong ionic bonds, conduct electricity when molten or dissolved in water but not as solids (NaCl, MgO, CaF2)
• Molecular solids composed of molecules held together by weak intermolecular forces (van der Waals forces, hydrogen bonds), generally have low melting points due to weak intermolecular forces, poor electrical conductors (ice, sugar, iodine)
• Metallic solids consist of metal cations surrounded by a sea of delocalized electrons, have high melting points due to strong metallic bonds, excellent electrical and thermal conductors, malleable and ductile due to the ability of metal ions to slide past one another without breaking bonds (copper, aluminum, gold)
• Covalent network solids composed of atoms covalently bonded in a three-dimensional network, have high melting points due to strong covalent bonds throughout the structure, poor electrical conductors (with the exception of graphite), hard and brittle due to the inability of atoms to slide past one another without breaking bonds (diamond, silicon, quartz)
  • Some elements can exist in multiple forms with different crystal structures, known as allotropes (e.g., diamond and graphite for carbon)

Structure of crystalline solids

• Unit cell the smallest repeating unit that makes up a crystal structure, determines the overall symmetry and properties of the crystalline solid
• Packing efficiency the percentage of space occupied by atoms or ions in a unit cell, higher packing efficiency generally results in higher density and stability
• Types of unit cells include:
  1. Simple cubic (SC): atoms at each corner of the cube
  2. Body-centered cubic (BCC): atoms at each corner and one in the center of the cube
  3. Face-centered cubic (FCC): atoms at each corner and the center of each face of the cube
  4. Hexagonal close-packed (HCP): atoms arranged in a hexagonal pattern with alternating layers
• Coordination number the number of nearest neighbors an atom or ion has in a crystal structure (SC: 6, BCC: 8, FCC: 12, HCP: 12)
• Crystal systems categorize the symmetry of unit cells, including cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal

Crystal Analysis and Properties

• X-ray crystallography is a powerful technique used to determine the atomic and molecular structure of crystals
• Lattice energy is the energy required to separate the ions in an ionic solid, influencing properties such as melting point and solubility
• Some substances can exist in multiple crystal structures under different conditions, a phenomenon known as polymorphism

Crystal Defects

Crystal defects and impacts

• Point defects include vacancies (missing atoms or ions in the crystal lattice), interstitials (extra atoms or ions occupying spaces between regular lattice positions), and substitutional defects (impurity atoms or ions replacing regular atoms or ions in the lattice), which can alter electrical conductivity, mechanical strength, and optical properties
• Line defects (dislocations) include edge dislocations (extra half-plane of atoms inserted into the crystal structure) and screw dislocations (spiral arrangement of atoms around a line), which can make materials more malleable and ductile by allowing planes of atoms to slide past one another more easily
• Planar defects include grain boundaries (interfaces between different crystalline regions with different orientations) and stacking faults (local changes in the stacking sequence of atomic planes), which can affect mechanical strength, electrical conductivity, and corrosion resistance
• Bulk defects include pores (small voids within the material), cracks (larger voids that can propagate under stress), and inclusions (impurities or phases trapped within the material), which can reduce mechanical strength, alter electrical and thermal properties, and provide sites for chemical reactions or corrosion

Amorphous Solids

• Amorphous solids lack long-range order in their atomic structure, unlike crystalline solids
• Examples include glass, plastics, and some ceramics
• Properties of amorphous solids often differ from their crystalline counterparts, such as having less defined melting points and different mechanical behaviors