BOARD EXAM PREP | APRIL 2026
Last Updated: April 2026
Chapter 1 — The Solid State carries 3-5 marks in CBSE Class 12 Chemistry board exam. This chapter features one VSA (1 mark), one SA (2 marks), and one LA (3-5 marks) question in most years. Concepts of unit cell, packing efficiency, and defects are the most frequently tested subtopics.
The Solid State is the opening chapter of NCERT Class 12 Chemistry and one of the most conceptually rich chapters in the entire syllabus. It deals with the microscopic structure of solid materials — how atoms, ions, and molecules are packed together, what types of defects arise, and how structure determines properties. This guide covers every topic from the NCERT textbook with clear explanations, memory aids, and board exam-focused practice.
Chapter Overview — Types of Solids
All solids can be broadly classified into two types based on their internal arrangement:
Crystalline vs Amorphous Solids
| Property | Crystalline Solids | Amorphous Solids |
|---|---|---|
| Arrangement | Long-range ordered arrangement | Short-range order only (disordered) |
| Melting Point | Sharp and definite | No sharp melting point (melts over a range) |
| Geometry | Definite geometric shape | Irregular shape |
| Isotropy | Anisotropic (properties differ by direction) | Isotropic (properties same in all directions) |
| Examples | NaCl, diamond, quartz, ice | Glass, rubber, plastic, tar |
| Nature | True solids | Pseudo-solids / Supercooled liquids |
Types of Crystalline Solids
| Type | Constituent Particles | Binding Force | Properties | Examples |
|---|---|---|---|---|
| Ionic | Cations and Anions | Electrostatic (ionic) forces | Hard, brittle; high MP; conducts electricity in molten/solution state | NaCl, MgO, ZnS, CaF2 |
| Molecular | Molecules | Dispersion/London, dipole-dipole, H-bonding | Soft; low MP; non-conductor; volatile | Ice (H2O), dry ice (CO2), I2, glucose |
| Covalent (Network) | Atoms | Covalent bonds | Very hard; very high MP; usually non-conductor (except graphite) | Diamond, SiO2, SiC, graphite |
| Metallic | Metal cations + free electrons (electron sea) | Metallic bonding | Hard to soft; lustrous; good conductor; malleable, ductile | Fe, Cu, Ag, Mg, Na |
Ionic — Molecular — Covalent — Metallic. Melting point increases in this order: Molecular < Ionic < Metallic < Covalent. Diamond (covalent) has the highest melting point of all crystalline solids.
Crystal Lattices and Unit Cells
A crystal lattice is a three-dimensional arrangement of points representing the positions of constituent particles in a crystal. A unit cell is the smallest repeating unit of a crystal lattice that shows the full symmetry of the lattice structure.
The 14 Bravais Lattices
All crystal structures can be described using one of 14 Bravais lattices organized into 7 crystal systems (Cubic, Tetragonal, Orthorhombic, Hexagonal, Rhombohedral, Monoclinic, Triclinic). For board exams, focus on the cubic system, which has 3 types.
Types of Cubic Unit Cells
| Unit Cell Type | Atoms per Unit Cell | Coordination Number | Packing Efficiency |
|---|---|---|---|
| Simple Cubic (SC) | 1 (8 corners x 1/8) | 6 | 52.4% |
| Body-Centred Cubic (BCC) | 2 (8 x 1/8 + 1 body) | 8 | 68% |
| Face-Centred Cubic (FCC/CCP) | 4 (8 x 1/8 + 6 x 1/2) | 12 | 74% |
Calculation shortcut: SC: 8 corners x (1/8) = 1. BCC: 8 corners x (1/8) + 1 body centre = 2. FCC: 8 corners x (1/8) + 6 face centres x (1/2) = 4.
Close Packing of Atoms
1D Close Packing
Spheres arranged in a row, touching each other — a linear chain. This forms the basis for 2D and 3D close-packed structures.
2D Close Packing
Rows of spheres placed side by side. Can be done in two ways:
- Square close packing: Spheres in row 2 align directly above row 1 — coordination number = 4. Less efficient.
- Hexagonal close packing: Spheres in row 2 fit into depressions of row 1 — coordination number = 6. More efficient.
3D Close Packing — HCP and CCP
When 2D hexagonal layers are stacked on top of each other, two arrangements are possible:
- ABAB… pattern (Hexagonal Close Packing, HCP): Third layer aligns directly over the first layer. Examples: Mg, Zn, Ti. Coordination number = 12.
- ABCABC… pattern (Cubic Close Packing, CCP = FCC): Third layer is placed differently from both A and B layers. Examples: Cu, Ag, Au, Al. Coordination number = 12.
Both HCP and CCP have the same packing efficiency of 74% and coordination number of 12 — they are equally efficient.
Voids in Close-Packed Structures
When spheres are close-packed, empty spaces (voids/holes) are left between them:
- Tetrahedral Voids: Formed when a sphere rests in the depression between 3 spheres. Surrounded by 4 spheres arranged tetrahedrally. Radius ratio = 0.225. Number of tetrahedral voids = 2n (where n = number of atoms).
- Octahedral Voids: Formed between two triangular layers. Surrounded by 6 spheres in octahedral arrangement. Radius ratio = 0.414. Number of octahedral voids = n (where n = number of atoms).
For n atoms in CCP: Tetrahedral voids = 2n, Octahedral voids = n.
So ratio of tetrahedral to octahedral voids = 2:1. This is a frequently asked board and entrance exam fact.
Density of a Unit Cell
The density of a crystalline solid can be calculated from its unit cell parameters using the formula:
rho = (Z x M) / (Na x a^3)
Where:
- rho (p) = density of the solid (g/cm3)
- Z = number of atoms per unit cell
- M = molar mass of the substance (g/mol)
- Na = Avogadro’s number (6.022 x 10^23)
- a = edge length of the unit cell (cm)
Board Exam Application: You may be asked to calculate density given edge length, or to find edge length given density. Practise both types of numerical problems from NCERT exercises.
Point Defects in Crystals
Ideal crystals exist only at 0 K. Real crystals at normal temperatures have imperfections called defects. Point defects are deviations from the ideal arrangement at specific lattice points.
A. Stoichiometric Defects
These defects do not disturb the stoichiometry (ratio of cations to anions) of the compound.
| Defect | Description | Compounds | Effect on Density |
|---|---|---|---|
| Schottky Defect | Equal number of cation and anion vacancies | NaCl, KCl, KBr, AgBr | Density decreases |
| Frenkel Defect | Smaller ion (cation) leaves lattice site and occupies interstitial site | ZnS, AgCl, AgBr, AgI | Density unchanged |
B. Non-Stoichiometric Defects
These defects change the stoichiometric ratio of the compound:
- Metal Excess Defect (due to anionic vacancies): Anion vacancies occupied by extra electrons. These electrons (F-centres or colour centres) give characteristic colours. Example: Non-stoichiometric NaCl shows yellow colour; KCl shows lilac/violet colour.
- Metal Excess Defect (due to extra cations): Extra cations at interstitial sites with electrons to maintain electrical neutrality. Example: ZnO heated — excess Zn2+ ions and electrons make it yellow when hot.
- Metal Deficiency Defect: Fewer cations than ideal — some metal sites have higher oxidation state cations to maintain neutrality. Example: FeO (actually Fe0.95O — iron deficiency).
C. Impurity Defects
Foreign ions introduced into the crystal lattice. Example: SrCl2 added to NaCl — Sr2+ occupies Na+ sites, with one cation vacancy per Sr2+ to maintain electrical neutrality. This is used to create ionic conductors.
Electrical Properties of Solids
Based on electrical conductivity, solids are classified as:
- Conductors (Metals): Conductivity 10^4 to 10^7 ohm-1 m-1. Free electrons are the charge carriers. Band gap = zero (valence band overlaps with conduction band).
- Insulators: Conductivity very low (~10^-20 ohm-1 m-1). Very large band gap (5 eV or more). Example: Diamond.
- Semiconductors: Intermediate conductivity (~10^-6 to 10^4 ohm-1 m-1). Small band gap (~1-3 eV). Conductivity increases with temperature.
n-type and p-type Semiconductors
| Type | Dopant | Charge Carrier | Example Dopants |
|---|---|---|---|
| n-type | Group 15 element (pentavalent) added to Si/Ge | Electrons (negative charge carriers) | Phosphorus (P), Arsenic (As), Antimony (Sb) |
| p-type | Group 13 element (trivalent) added to Si/Ge | Holes (positive charge carriers) | Boron (B), Aluminium (Al), Gallium (Ga) |
Magnetic Properties of Solids
- Diamagnetic: All electrons paired; weakly repelled by magnetic field. Examples: NaCl, TiO2, benzene.
- Paramagnetic: Unpaired electrons present; weakly attracted to magnetic field; lose magnetism when field removed. Examples: O2, Cu2+, Fe3+.
- Ferromagnetic: Unpaired electrons in magnetic domains aligned in same direction; strongly attracted; retain magnetism. Examples: Fe, Co, Ni, Gd, CrO2.
- Ferrimagnetic: Magnetic moments of domains partially cancelled (unequal anti-parallel alignment). Weakly magnetic. Examples: Fe3O4 (magnetite), MgFe2O4.
- Antiferromagnetic: Equal and opposite magnetic moments in adjacent domains — net magnetism = zero. Example: MnO, Cr2O3.
Board Exam Important Questions (5-Mark Type)
- Explain Schottky and Frenkel defects with examples. How do they affect the density of the crystal?
- An element with density 2.7 g/cm3 and molar mass 27 g/mol forms FCC lattice. Calculate the edge length of the unit cell. (Na = 6.022 x 10^23)
- What are n-type and p-type semiconductors? How is each formed by doping? Give examples of dopants used.
- Explain the different types of voids in close-packed structures. What is the ratio of tetrahedral to octahedral voids in a CCP structure?
- Compare HCP and CCP packing. Why do both have the same packing efficiency despite having different stacking patterns?
MCQ Practice — The Solid State
Quiz data empty after normalization.
Frequently Asked Questions — The Solid State
What is the difference between Schottky defect and Frenkel defect?
Schottky defect involves equal numbers of cation and anion vacancies in the crystal. Both ions leave their lattice positions, which decreases the density of the crystal. It occurs in ionic crystals where both ions have similar sizes (e.g., NaCl, KCl). Frenkel defect involves a smaller ion (usually cation) leaving its lattice site and occupying an interstitial site. Density remains unchanged. It occurs where ions have different sizes (e.g., ZnS, AgCl).
How many atoms are present in a BCC unit cell?
A Body-Centred Cubic (BCC) unit cell contains 2 atoms. This is calculated as: 8 corner atoms x (1/8 contribution each) + 1 body centre atom x (1 full contribution) = 1 + 1 = 2 atoms per unit cell. Examples of metals with BCC structure include iron (at room temperature), chromium, tungsten, and sodium.
What are F-centres in crystals?
F-centres (Farbe centres, from German for “colour”) are anionic vacancies in a crystal that are occupied by free electrons. They are formed by metal excess defects. When an alkali halide crystal is heated in the presence of alkali metal vapour, extra metal atoms deposit on the surface and the electrons diffuse into the crystal, occupying anion vacancies. These trapped electrons absorb visible light, giving characteristic colours — NaCl appears yellow, KCl appears lilac/violet.
Why does ZnO become yellow on heating?
ZnO becomes yellow when heated due to metal excess defect. On heating, ZnO loses oxygen and the extra Zn2+ ions formed are accommodated in interstitial sites with electrons to maintain electrical neutrality. These interstitial Zn2+ ions and free electrons absorb blue light, making the substance appear yellow. When cooled, ZnO returns to its original white colour as the excess Zn and electrons are released.
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