Chapter 12: Solids and Modern Materials – Classification, Structure, and Properties

Classification and Structures of Solids

Main Types of Solids

Solids are classified based on the nature of the forces holding their constituent particles together. Understanding these types is essential for predicting their properties and applications.

• Metallic solids: Consist of metal atoms held together by a "sea" of collectively shared valence electrons. This electron mobility gives rise to properties such as electrical conductivity and malleability. Example: Copper (Cu), Iron (Fe).

• Ionic solids: Composed of cations and anions held together by strong electrostatic (Coulombic) attractions. These solids typically have high melting points and are brittle. Example: Sodium chloride (NaCl).

• Covalent-network solids: Atoms are connected by an extensive network of covalent bonds, resulting in very hard materials with high melting points. Example: Diamond (C), Silicon carbide (SiC).

• Molecular solids: Made up of discrete molecules held together by intermolecular forces such as London dispersion, dipole-dipole, or hydrogen bonding. These solids are generally soft and have low melting points. Example: Ice (H2O), Sucrose (C12H22O11).

Other Types of Solids

Polymers and Nanomaterials

Beyond the main categories, polymers and nanomaterials represent important classes of solids with unique properties.

• Polymers: Large molecules composed of repeating units (monomers) connected by covalent bonds. The chains may be linked by weak forces, giving rise to diverse physical properties. Example: Polyethylene (PE), DNA.

• Nanomaterials: Crystalline compounds with crystal sizes in the range of 1–100 nm. Their small size leads to properties distinct from bulk materials, such as altered optical, electrical, or mechanical behavior. Example: Quantum dots, carbon nanotubes.

Organization of Solids

Crystalline vs. Amorphous Solids

The arrangement of atoms or molecules in a solid determines whether it is crystalline or amorphous.

• Crystalline solids: Possess a regular, repeating pattern of atoms, ions, or molecules. This order leads to well-defined shapes and characteristic physical properties. Example: Table salt (NaCl), quartz (SiO2).

• Amorphous solids: Lack long-range order in their atomic arrangement. They do not have a distinct geometric shape and often behave more like supercooled liquids. Example: Glass, many plastics.

Most chemists focus on crystalline solids due to their predictable structure and properties.

Summary Table: Types of Solids

Type of Solid	Constituent Particles	Forces Holding Particles	Examples	Key Properties
Metallic	Metal atoms	Metallic bonding (electron sea)	Cu, Fe, Au	Conductive, malleable, ductile
Ionic	Cations & anions	Electrostatic attraction	NaCl, KBr	High melting point, brittle
Covalent-network	Atoms	Covalent bonds	Diamond, SiO2	Very hard, high melting point
Molecular	Molecules	Intermolecular forces	Ice, CO2	Soft, low melting point
Polymer	Long-chain molecules	Covalent bonds (within chains), weak forces (between chains)	PE, PVC	Flexible, variable properties
Nanomaterial	Atoms/molecules (1–100 nm)	Varies	Quantum dots, nanoparticles	Size-dependent properties

Additional info: The above notes expand on the brief points in the slides, providing definitions, examples, and a comparative table for clarity.