Chapter: Solid State
1. General Characteristics of Solid State
- Solids: Solids are the states of matter that have definite shape, volume, and rigidity.
- Properties of solids:
- Definite mass, volume, and shape.
- Incompressible and rigid.
- High density compared to liquids and gases.
- Particles in solids have fixed positions and can only oscillate around their mean positions.
2. Classification of Solids
Solids can be classified into two categories based on the arrangement of constituent particles:
(i) Crystalline Solids:
- These have a regular arrangement of constituent particles.
- Long-range order and sharp melting points.
- Examples: Sodium chloride (NaCl), Quartz.
Properties:
- Anisotropy: Properties like refractive index, conductivity, etc., differ in different directions.
- Definite Heat of Fusion: They have a sharp melting point and a definite heat of fusion.
- Order in Structure: Exhibit long-range order in structure.
(ii) Amorphous Solids:
- These have irregular or random arrangements of constituent particles.
- Short-range order and no sharp melting points.
- Examples: Glass, rubber, and plastics.
Properties:
- Isotropy: Properties are the same in all directions.
- No Definite Melting Point: Soften over a range of temperatures.
- Considered supercooled liquids due to the ability to flow, though very slowly.
3. Crystal Lattices and Unit Cells
- Crystal Lattice: A three-dimensional arrangement of atoms, ions, or molecules in space.
- Unit Cell: The smallest repeating unit in space lattice which when repeated in different directions results in a crystal lattice.
Types of Unit Cells:
- Primitive Unit Cell: Constituent particles are present only at the corners of the unit cell.
- Centered Unit Cells:
- Body-centered Unit Cell (BCC): Particles are present at the corners and one at the center of the unit cell.
- Face-centered Unit Cell (FCC): Particles are present at the corners and at the centers of all faces of the unit cell.
- End-centered Unit Cell: Particles are present at the corners and two opposite faces of the unit cell.
4. Number of Atoms in a Unit Cell
- Primitive Cubic Unit Cell: 1 atom/unit cell (Only 1/8th of each corner atom belongs to the unit cell).
- Body-Centered Cubic Unit Cell (BCC): 2 atoms/unit cell (1 from corners and 1 from the center).
- Face-Centered Cubic Unit Cell (FCC): 4 atoms/unit cell (1/8 from corners and 1/2 from faces).
5. Close Packing in Crystals
- Close Packing in One Dimension: Arrangement in a straight line with each sphere in contact with two others. Coordination number = 2.
- Close Packing in Two Dimensions:
- Square Close Packing: Each sphere is in contact with four others. Coordination number = 4.
- Hexagonal Close Packing: Each sphere is in contact with six others. Coordination number = 6.
- Close Packing in Three Dimensions:
- Hexagonal Close Packing (HCP): ABAB pattern; coordination number = 12.
- Cubic Close Packing (CCP or FCC): ABCABC pattern; coordination number = 12.
6. Types of Voids
- Octahedral Voids: Formed by the contact of six spheres. The number of octahedral voids is equal to the number of close-packed particles.
- Tetrahedral Voids: Formed by the contact of four spheres. The number of tetrahedral voids is twice the number of close-packed particles.
7. Packing Efficiency
- Simple Cubic Unit Cell: 52.4% of space is occupied by particles.
- Body-Centered Cubic Unit Cell (BCC): 68% of space is occupied.
- Face-Centered Cubic Unit Cell (FCC): 74% of space is occupied.
8. Types of Crystals
- Ionic Crystals: Made up of ions, held by strong electrostatic forces.
- Examples: NaCl, MgO.
- Covalent Crystals: Made up of atoms, held by covalent bonds.
- Examples: Diamond, graphite.
- Metallic Crystals: Made up of metal cations in a sea of delocalized electrons.
- Examples: Iron, copper.
- Molecular Crystals: Held by intermolecular forces (Van der Waals forces).
- Examples: Ice, solid CO₂.
9. Imperfections in Solids (Defects)
(i) Point Defects: These occur when deviation from ideal arrangement is around a point/atom.
- Vacancy Defect: Atoms are missing from their regular positions.
- Interstitial Defect: Extra atoms occupy interstitial sites.
- Frenkel Defect: A cation leaves its lattice site and occupies an interstitial site.
- Schottky Defect: Equal numbers of cations and anions are missing from their lattice sites.
(ii) Impurity Defects: Occur when foreign atoms are present in the crystal.
(iii) Electrical Properties of Solids:
- Conductor: Solids with delocalized electrons (metals).
- Insulators: Solids where electrons are tightly bound to atoms (non-metals).
- Semiconductors: Solids that have conductivity between conductors and insulators (Si, Ge).
10. Magnetic Properties
- Diamagnetic Substances: Weakly repelled by a magnetic field (e.g., NaCl, Benzene).
- Paramagnetic Substances: Weakly attracted by a magnetic field (e.g., O₂, Cu²⁺).
- Ferromagnetic Substances: Strongly attracted by a magnetic field (e.g., Fe, Co, Ni).
- Antiferromagnetic Substances: Have zero net magnetic moment due to opposite alignment of magnetic moments (e.g., MnO).
- Ferrimagnetic Substances: Exhibit weak net magnetic moment due to partial cancellation of magnetic moments (e.g., Fe₃O₄).