Course Topics

Types of Solids

Name Details
Crystalline Solids
  • Molecules, ions or atoms arranged in specific three-dimensional pattern
  • Examples: NaCl, KCl, sucrose, I₂, ice
  • Have definite geometry and sharp melting points
Amorphous Solids
(Pseudo solids)
  • Molecules, ions or atoms do not have regular order of arrangement
  • Examples: rubber, glue, glass
  • No definite geometry and melt over a range of temperatures
Memory Tip

Crystalline = Ordered like a crystal, Amorphous = Amoeba-like (no shape)

Properties of Crystalline Solids

Property Description
Geometric shape Have definite geometry and shape
Melting Point Sharp melting point
Cleavage planes When broken, they cleave along definite planes called cleavage planes
Anisotropy Properties depend upon direction of crystal (refractive index, thermal expansion, electrical conductivity, etc.)
Symmetry Repetition of faces, angles and edges when rotated by 360° along its axis
Habit of a crystal The shape in which a crystal usually grows
CRITICAL CONCEPT: Crystalline solids exhibit three important phenomena:
  1. Isomorphism: Two different compounds exist in same crystalline form (e.g., NaNO₃ and KNO₃)
  2. Polymorphism: One compound exists in more than one crystalline form (e.g., AgNO₃ exists in rhombohedral and orthorhombic forms)
  3. Allotropy: An element exists in more than one crystalline form (e.g., Carbon as graphite, diamond, Bucky balls)
Memory Tip

Remember: ISOmorphism = Same form for different compounds, POLYmorphism = Many forms for same compound, ALLOtropy = Allotropic elements!

Crystal Lattice & Unit Cell

Crystal Lattice: “An array of points representing the arrangement of particles (atoms, ions or molecules) in three dimensional spaces is called crystal lattice”.

Unit Cell: “The smallest unit of volume of a crystal, which shows all the properties of its pattern, is called a unit cell”.

Cell Dimensions (Crystallographic elements): Unit cell has six parameters:

  • Three unit cell lengths: a, b, c
  • Three angles: α (between b and c), β (between a and c), γ (between a and b)
CRITICAL CONCEPT: Contribution of ions at different positions in a unit cell:
Position in Cube Contribution to Unit Cell
Center of cube 1
Face of cube 1/2
Edge of cube 1/4
Corner of cube 1/8
Memory Tip

Unit cell is like a building block – the whole crystal is made by repeating this block in 3D space!

Ionic Solids

Definition: Solids in which ions are held together by electrostatic forces / ionic bonds.

Na⁺

Properties of Ionic Solids:

  • Three-dimensional arrangement of ions
  • High melting and boiling points, non-volatile
  • Structure depends on radius ratio of cations and anions
  • Non-conductors in solid state but conductors in molten state or solution
  • Soluble in polar solvents, insoluble in non-polar solvents
CRITICAL CONCEPT – NaCl Structure:
  • Independent molecules of NaCl do not exist in vapour phase or solid state
  • NaCl has giant ionic structure
  • Coordination number of NaCl is 6 (each Na⁺ surrounded by 6 Cl⁻ and vice versa)
  • There are 4 formula units (4NaCl) per unit cell
Memory Tip

NaCl is a 6-6 coordinated structure – like a 3D chessboard where each piece has 6 neighbors!

Radius Ratio & Crystal Shape

The structure and shape of an ionic solid depends upon the radius ratio (r⁺/r⁻) of cations and anions.

No. Shape of Ionic Solid Limiting Radius Ratio (r⁺/r⁻) Examples
1. Body centered cubic 0.732 and above CsCl (0.93)
2. Octahedral/Cubic 0.414 to 0.732 NaCl (0.522)
3. Tetrahedral 0.22 to 0.414 ZnS (0.40)
4. Triangular 0.15 to 0.22 –
Radius Ratio = r⁺ / r⁻

Factors affecting shape of ionic solid:

  1. Electrostatic forces of attraction: Strong forces create well-defined geometric shapes
  2. Radius ratio: Determines coordination number and geometry
  3. Poor conductivity in solid state: Ions fixed at positions, but conduct when molten or dissolved
Memory Tip

Radius ratio rule: Big cation (r⁺/r⁻ > 0.732) = Cubic, Medium cation (0.414-0.732) = Octahedral, Small cation (0.22-0.414) = Tetrahedral!

Lattice Energy

Definition 1: “The amount of energy released when gaseous ions of opposite charges combine to give one mole of a crystalline ionic compound.”

Definition 2: “The amount of energy required to break one mole crystal lattice into its gaseous ions.”

M⁺(g) + X⁻(g) → MX(s) + Lattice Energy
Ionic Compound Lattice Energy (kJ/mol)
LiCl -833
NaF -895
NaCl -787
KCl -690
NaBr -728
KBr -665
NaI -690
CRITICAL CONCEPT – Factors affecting lattice energy:

Lattice energy ∝ Charge density ∝ Charge/Size

  1. Size of ion: Lattice energy decreases with increase in size of ions (cations or anions)
  2. Charge of ion: Lattice energy increases with increasing ionic charge
Memory Tip

Lattice energy: More charge and smaller size = Stronger attraction = Higher lattice energy!

Molecular Solids

Definition: Solids in which polar or non-polar molecules are held together by Van der Waal’s forces (London forces) or Dipole-dipole forces (including Hydrogen bonding).

Properties of Molecular Solids:

  • Soft and easily compressible
  • Mostly volatile with low melting and boiling points
  • Bad conductors of electricity
  • Examples: I₂, CO₂ (dry ice), sucrose, ice
CRITICAL CONCEPT – Iodine Structure:
  • Iodine is a molecular solid with I₂ molecules
  • In solid state, molecules align in layer lattice
  • I-I bond distance: 271.5 pm (solid) vs 266.6 pm (gas)
  • Face centered cubic structure
  • Sublime solid (direct solid to gas transition)
Memory Tip

Molecular solids = Molecules held by weak forces = Soft, low melting point, poor conductors!

Comparison of All Solids

Type of Solid Structural Units Forces Typical Properties Examples
Metallic Cations + delocalized electrons Metallic bonds
  • Hardness varies (soft to very hard)
  • Melting points varied (low to very high)
  • Lustrous, ductile, malleable
  • Very good conductors
 Na, Mg, Al, Fe, Cu, Ag, W
Ionic Cations and anions Electrostatic attractions
  • Hard, moderate to high melting points
  • Nonconductors in solid state
  • Conductors in molten state
 NaCl, NaNO₃, MgO
Molecular Molecules London, dipole-dipole or H-bonds
  • Soft, low melting points
  • Non-conductors
  • Sublime easily in many cases
 Noble gases, CH₄, CO₂, I₂, H₂O
Covalent Network Atoms Covalent bonds
  • Very hard, very high melting points
  • Non-conductors (except graphite)
 C(diamond), SiC, SiO₂
Memory Tip

Solid types: Metallic (electrons sea), Ionic (ions), Molecular (molecules), Covalent (atoms network)!

Formula Units Calculation in NaCl

Step-by-step calculation of formula units per unit cell in NaCl:

  1. Cl⁻ ions at corners:
    • Unit cells sharing one Cl⁻ ion at one corner = 8
    • Share of one Cl⁻ ion at one corner = 1/8
    • Total share at eight corners = 8 × 1/8 = 1 Cl⁻
  2. Cl⁻ ions at faces:
    • Share of one Cl⁻ ion at one face = 1/2
    • Total share at six faces = 6 × 1/2 = 3 Cl⁻
  3. Total Cl⁻ ions in unit cell: 1 + 3 = 4 Cl⁻
  4. Similarly, Na⁺ ions in unit cell: 4 Na⁺
  5. Total formula units: 4 NaCl
NaCl unit cell contains 4 formula units
CRITICAL CONCEPT – Coordination Number:

Coordination number: The number of ions of same kind that surround an oppositely charged ion.

  • In NaCl, each Na⁺ surrounded by 6 Cl⁻ ions
  • Each Cl⁻ surrounded by 6 Na⁺ ions
  • So, coordination number of NaCl is 6
Memory Tip

NaCl unit cell: 4 NaCl, Coordination number: 6-6, Structure: Face-centered cubic!

Transition Temperature

Definition: The temperature at which two crystalline forms of the same substance can co-exist in equilibrium with each other.

  • Above and below this temperature, only one form exists
  • Example: Grey tin (cubic) ⇌ White tin (tetragonal) at 13.2°C
  • Also called inversion temperature

Grey Tin (α-tin)

  • Cubic structure
  • Diamond-like structure
  • Semiconductor
  • Stable below 13.2°C
  • Less dense (5.75 g/cm³)

White Tin (β-tin)

  • Tetragonal structure
  • Metallic character
  • Conductor
  • Stable above 13.2°C
  • More dense (7.31 g/cm³)
CRITICAL CONCEPT: Transition temperature is specific to polymorphic forms of a substance. At this temperature, both forms are equally stable and can coexist in equilibrium.
Memory Tip

Transition temperature = Temperature where two crystal forms can live together peacefully!

Applications & Importance

Practical Applications of Solid State Chemistry:

  • Semiconductors: Silicon, germanium crystals for electronics
  • Superconductors: Certain solids at low temperatures
  • Catalysts: Solid catalysts in industrial processes
  • Construction Materials: Cement, ceramics, glass
  • Pharmaceuticals: Different polymorphs have different bioavailability
  • Jewelry: Diamonds, rubies, sapphires as gemstones
  • Energy Storage: Lithium-ion battery electrodes
  • Optoelectronics: LEDs, lasers, solar cells
CRITICAL CONCEPT: The properties of solids (electrical, thermal, mechanical, optical) are determined by their crystal structure and bonding. Understanding solid state is crucial for materials science and technology development.
Memory Tip

From smartphones (semiconductors) to buildings (cement) to medicine (drug crystals) – solids are everywhere in technology!