Chapter 1 of Class 12 Chemistry introduces you to the fascinating world of solid state chemistry. Solids are all around us — from the salt in your kitchen to the silicon in your phone. Understanding how atoms, ions, and molecules arrange themselves in solids is key to understanding their properties. This chapter carries 5-7 marks in CBSE Board exams and is essential for competitive exams like JEE and NEET.
Key Concepts
Classification of Solids
Solids are classified based on the arrangement of their constituent particles:
| Property | Crystalline Solids | Amorphous Solids |
|---|---|---|
| Shape | Definite geometric shape | Irregular shape |
| Melting Point | Sharp melting point | Melt over a range |
| Cleavage | Clean cleavage along planes | Irregular cleavage |
| Heat of Fusion | Definite | Not definite |
| Anisotropy | Anisotropic | Isotropic |
| Nature | True solids | Pseudo solids / supercooled liquids |
| Examples | NaCl, Diamond, Quartz | Glass, Rubber, Plastics |
Types of Crystalline Solids
| Type | Constituent Particles | Bonding | Properties | Examples |
|---|---|---|---|---|
| Ionic | Cations & Anions | Electrostatic (Coulombic) | Hard, brittle, high m.p., conduct in molten/solution | NaCl, KNO₃, ZnS |
| Molecular | Molecules | van der Waals / H-bonding | Soft, low m.p., poor conductors | Ice, I₂, CO₂ |
| Covalent (Network) | Atoms | Covalent bonds | Very hard, very high m.p., poor conductors (except graphite) | Diamond, SiC, SiO₂ |
| Metallic | Metal atoms (+ e⁻ sea) | Metallic bonding | Lustrous, malleable, ductile, good conductors | Fe, Cu, Au, Ag |
Crystal Lattice and Unit Cell
A crystal lattice is a 3D arrangement of points representing the positions of constituent particles. The smallest repeating unit of this lattice is called the unit cell.
Seven Crystal Systems
There are 7 crystal systems and 14 Bravais lattices. The most important ones for your exam:
| Crystal System | Axes | Angles | Example |
|---|---|---|---|
| Cubic | a = b = c | α = β = γ = 90° | NaCl, Diamond |
| Tetragonal | a = b ≠ c | α = β = γ = 90° | SnO₂, TiO₂ |
| Orthorhombic | a ≠ b ≠ c | α = β = γ = 90° | BaSO₄, KNO₃ |
| Hexagonal | a = b ≠ c | α = β = 90°, γ = 120° | Graphite, ZnO |
Number of Atoms in a Unit Cell
This is one of the most important topics for numericals:
Corner atom: shared by 8 unit cells → contributes 1/8
Edge atom: shared by 4 unit cells → contributes 1/4
Face-centred atom: shared by 2 unit cells → contributes 1/2
Body-centred atom: belongs entirely to 1 unit cell → contributes 1
| Unit Cell Type | Corners | Body | Face | Total Atoms |
|---|---|---|---|---|
| Simple Cubic (SCC) | 8 × 1/8 = 1 | 0 | 0 | 1 |
| Body-Centred Cubic (BCC) | 8 × 1/8 = 1 | 1 | 0 | 2 |
| Face-Centred Cubic (FCC) | 8 × 1/8 = 1 | 0 | 6 × 1/2 = 3 | 4 |
Packing Efficiency
Simple Cubic: 52.4% (coordination number = 6)
BCC: 68% (coordination number = 8)
FCC / HCP: 74% (coordination number = 12) — most efficient!
Relationship Between Edge Length and Atomic Radius
BCC: a = 4r/√3
FCC: a = 2√2 · r
Density of Unit Cell
Where: Z = atoms per unit cell, M = molar mass, a = edge length, Nₐ = Avogadro’s number
Voids (Interstitial Sites)
In close-packed structures, there are spaces between atoms called voids:
| Void Type | Coordination Number | Number of Voids (for N atoms) | Location |
|---|---|---|---|
| Tetrahedral | 4 | 2N | Body diagonal, at 1/4 distance |
| Octahedral | 6 | N | Body centre + edge centres |
Defects in Solids (Imperfections)
Point Defects in Ionic Solids
| Defect | What Happens | Effect on Density | Example |
|---|---|---|---|
| Schottky Defect | Equal number of cations & anions missing | Density decreases | NaCl, KCl, AgBr |
| Frenkel Defect | Smaller ion displaced to interstitial site | Density unchanged | AgCl, AgBr, ZnS |
Non-stoichiometric Defects
Metal Excess Defect:
- F-centres: Anion vacancies trapped with electrons → colour (NaCl = yellow, KCl = violet, LiCl = pink)
- Extra cation in interstitial site (e.g., ZnO turns yellow on heating)
Metal Deficiency Defect: Cation missing, charge balanced by adjacent cation having higher charge (e.g., FeO often found as Fe₀.₉₃O to Fe₀.₉₆O)
Electrical Properties — Band Theory
| Type | Band Gap | Conductivity | Temp Effect | Examples |
|---|---|---|---|---|
| Conductors (Metals) | No gap / overlapping bands | 10² – 10⁸ S/m | Decreases with temp | Cu, Ag, Al |
| Semiconductors | Small gap (< 3 eV) | 10⁻⁶ – 10⁴ S/m | Increases with temp | Si, Ge, GaAs |
| Insulators | Large gap (> 3 eV) | 10⁻²⁰ – 10⁻¹⁰ S/m | — | Diamond, wood |
Types of Semiconductors
- n-type: Doped with Group 15 element (P, As) → extra electron → negative charge carrier
- p-type: Doped with Group 13 element (B, Al, Ga) → electron hole → positive charge carrier
Magnetic Properties
| Type | Behaviour in Magnetic Field | Examples |
|---|---|---|
| Diamagnetic | Weakly repelled, all electrons paired | NaCl, C₆H₆, TiO₂ |
| Paramagnetic | Weakly attracted, unpaired electrons | O₂, Cu²⁺, Fe³⁺ |
| Ferromagnetic | Strongly attracted, domains align | Fe, Co, Ni, CrO₂ |
| Antiferromagnetic | Domains align anti-parallel, cancel out | MnO, MnO₂ |
| Ferrimagnetic | Domains anti-parallel but unequal, net moment | Fe₃O₄, ferrites |
Important Definitions
| Term | Definition |
|---|---|
| Crystal Lattice | Regular 3D arrangement of constituent particles in a crystalline solid |
| Unit Cell | Smallest repeating portion of a crystal lattice |
| Coordination Number | Number of nearest neighbours surrounding a particle |
| Packing Efficiency | Percentage of total space filled by constituent particles |
| Interstitial Void | Empty space between close-packed atoms |
| Schottky Defect | Point defect where equal cation-anion pairs are missing from lattice |
| Frenkel Defect | Point defect where smaller ion is displaced to an interstitial site |
| F-centre | Anion vacancy with trapped electron, responsible for colour |
| Doping | Adding impurity atoms to a semiconductor to modify conductivity |
Solved Examples — NCERT Based
Example 1: Calculating Density of Unit Cell
Q: Silver crystallises in FCC structure. If the edge length of the unit cell is 408.6 pm, calculate the density of silver. (Molar mass of Ag = 108 g/mol)
Solution:
For FCC: Z = 4
a = 408.6 pm = 408.6 × 10⁻¹⁰ cm
ρ = (Z × M) / (a³ × Nₐ)
ρ = (4 × 108) / ((408.6 × 10⁻¹⁰)³ × 6.022 × 10²³)
ρ = 432 / (6.83 × 10⁻²³ × 6.022 × 10²³)
ρ = 432 / 41.13 = 10.5 g/cm³
Example 2: Calculating Number of Atoms in Unit Cell
Q: A compound forms HCP structure. What is the number of atoms per unit cell? Also find the number of tetrahedral and octahedral voids if the structure contains 8 atoms.
Solution:
HCP has same packing as FCC → Z = 4 atoms per unit cell (but for the full HCP unit cell consideration, it’s 6).
For N = 8 atoms:
Tetrahedral voids = 2N = 2 × 8 = 16
Octahedral voids = N = 8
Example 3: Edge Length from Atomic Radius
Q: Iron has BCC structure with atomic radius 124 pm. Find the edge length of its unit cell.
Solution:
For BCC: a = 4r/√3
a = (4 × 124) / 1.732
a = 496 / 1.732 = 286.4 pm
Example 4: Identifying Defect Type
Q: In a sample of NaCl, some Na⁺ ions are missing from their lattice sites. The same number of Cl⁻ ions are also missing. What type of defect is this? What will be the effect on density?
Solution:
Since equal numbers of cations (Na⁺) and anions (Cl⁻) are missing → this is a Schottky defect.
Effect: Number of ions decreases but volume remains same → density decreases.
Important Questions for Board Exams
1 Mark Questions
- What is the coordination number of atoms in BCC structure?
- What type of defect is shown by NaCl?
- What is the packing efficiency of FCC structure?
- Name one substance that shows both Schottky and Frenkel defects.
- What type of semiconductor is formed when Si is doped with Boron?
2 Mark Questions
- Distinguish between tetrahedral and octahedral voids.
- What are F-centres? How do they impart colour to crystals?
- Differentiate between n-type and p-type semiconductors.
- Why does Frenkel defect not change the density of a solid?
3 Mark Questions
- Explain Schottky and Frenkel defects with examples. How do they affect density?
- Derive the relationship between edge length and atomic radius for FCC unit cell.
- Calculate the packing efficiency of BCC structure.
- Classify the following as ionic, molecular, covalent or metallic solids: SiC, Ice, NaCl, Iron, Diamond.
5 Mark Questions
- What is a unit cell? Describe the three types of cubic unit cells. Calculate the number of atoms in each.
- Explain band theory. Distinguish between conductors, semiconductors and insulators on its basis. What are n-type and p-type semiconductors?
Quick Revision Points
- Crystalline solids: definite shape, sharp m.p., anisotropic | Amorphous: irregular, range of m.p., isotropic
- 4 types of crystalline solids: Ionic, Molecular, Covalent (network), Metallic
- 7 crystal systems, 14 Bravais lattices
- SCC: Z=1, CN=6, PE=52.4% | BCC: Z=2, CN=8, PE=68% | FCC: Z=4, CN=12, PE=74%
- Density formula: ρ = ZM / a³Nₐ
- Tetrahedral voids = 2N, Octahedral voids = N
- Schottky: both ions missing → density decreases | Frenkel: ion displaced → density same
- AgBr shows both Schottky and Frenkel defects
- F-centres: anion vacancy + trapped electron → colour
- n-type: Group 15 dopant (extra e⁻) | p-type: Group 13 dopant (hole)
- Ferro > Ferri > Para > Dia (in terms of magnetic response)
Chapter Navigation
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Practice What You Learned
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