Solid State Class 12 Chemistry Notes | Complete Chapter, MCQs, Short & Long Q&A

The solid state is one of the fundamental states of matter. Solids are characterized by definite shape, volume, rigidity, and incompressibility. Unlike gases and liquids, the particles in solids are closely packed due to strong intermolecular forces. This chapter deals with the classification of solids, crystal structures, unit cells, packing efficiency, imperfections, and properties of solids. A Notes clear understanding of the solid state is crucial as it forms the foundation of material science, metallurgy, and many industrial applications.

Solid State Class 12 Chemistry Notes | Complete Chapter, MCQs, Short & Long Q&A

General Characteristics of Solid State

• Solids have a definite shape and volume.

• They are rigid and incompressible.

• Intermolecular forces are very strong.

• Diffusion in solids is negligible.

• Solids have high density compared to liquids and gases.

• They can be crystalline or amorphous.

Classification of Solids

Solids are broadly classified into two categories:

1. Crystalline Solids

• Have long-range order (regular arrangement of particles).

• Sharp melting point.

• Anisotropic (properties differ in different directions).

• Definite heat of fusion.

• Examples: NaCl, Diamond, Quartz.

2. Amorphous Solids

• Have short-range order (irregular arrangement of particles).

• Do not have a sharp melting point; they soften over a range of temperature.

• Isotropic (same properties in all directions).

• Considered pseudo solids or supercooled liquids.

• Examples: Glass, Plastic, Rubber.

Types of Crystalline Solids

Based on intermolecular forces, crystalline solids are of four types:

1. Ionic Solids

• Constituent particles: ions (cations and anions).

• Forces: Strong electrostatic forces of attraction.

• Properties: Hard, brittle, high melting point, conduct electricity in molten or aqueous state.

• Example: NaCl, KCl, MgO.

2. Metallic Solids

• Constituent particles: positive ions in a sea of delocalized electrons.

• Forces: Metallic bond.

• Properties: Good conductors of heat and electricity, malleable, ductile.

• Example: Fe, Cu, Ag.

3. Covalent (Network) Solids

• Constituent particles: atoms held by covalent bonds.

• Properties: Hard, brittle, high melting point, poor conductors of electricity (except graphite).

• Example: Diamond, Quartz, Graphite.

4. Molecular Solids

• Constituent particles: molecules held by van der Waals’ forces, dipole–dipole interactions, or hydrogen bonding.

• Properties: Soft, low melting point, poor conductors of electricity.

• Example: Solid CO₂ (dry ice), Ice, Iodine.

Crystal Lattice and Unit Cell

Crystals have a regular arrangement of particles. This arrangement is described in terms of a lattice.

Crystal Lattice

• A three-dimensional arrangement of points representing the positions of particles in a crystal.

• Each point is called a lattice point.

Unit Cell

• The smallest repeating unit of a crystal lattice which, when repeated in all directions, gives the entire lattice.

• Defined by lattice parameters: edge lengths (a, b, c) and angles (α, β, γ).

Seven Crystal Systems

There are 7 crystal systems, each defined by its edge lengths and angles:

1. Cubic (a = b = c; α = β = γ = 90°)

   • Example: NaCl, Cu.

2. Tetragonal (a = b ≠ c; α = β = γ = 90°)

   • Example: TiO₂.

3. Orthorhombic (a ≠ b ≠ c; α = β = γ = 90°)

   • Example: KNO₃.

4. Hexagonal (a = b ≠ c; α = β = 90°, γ = 120°)

   • Example: ZnO.

5. Rhombohedral (Trigonal) (a = b = c; α = β = γ ≠ 90°)

   • Example: Calcite.

6. Monoclinic (a ≠ b ≠ c; α = γ = 90°, β ≠ 90°)

   • Example: Gypsum.

7. Triclinic (a ≠ b ≠ c; α ≠ β ≠ γ ≠ 90°)

   • Example: K₂Cr₂O₇.

Bravais Lattices

• There are 14 Bravais lattices, which represent all possible arrangements of particles in a 3D lattice.

• These lattices are derived from the 7 crystal systems.

Types of Unit Cells

1. Primitive Unit Cell

• Particles are present only at the corners.

2. Body-Centered Cubic (BCC)

• Particles at corners and one at the center of the cube.

3. Face-Centered Cubic (FCC)

• Particles at corners and centers of all faces.

4. End-Centered Cubic

• Particles at corners and centers of two opposite faces.

Number of Atoms in a Unit Cell

• Simple Cubic (SC): 1 atom per unit cell.

• BCC: 2 atoms per unit cell.

• FCC: 4 atoms per unit cell.

Close Packing in Solids

Atoms in solids pack closely to minimize empty space.

Types of Packing

1. 1D Packing – Linear arrangement.

2. 2D Packing

   • Square packing.

   • Hexagonal packing.

3. 3D Packing

   • Hexagonal Close Packing (HCP).

   • Cubic Close Packing (CCP, same as FCC).

Packing Efficiency

• SC: 52%

• BCC: 68%

• FCC/HCP: 74%

Voids in Solids

• Tetrahedral voids: Formed when a sphere is surrounded by 4 spheres.

• Octahedral voids: Formed when a sphere is surrounded by 6 spheres.

• Relation: Number of tetrahedral voids = 2 × Number of octahedral voids.

Density of a Unit Cell

The density of a crystal is given by:

d=z×Ma3×NAd = \frac{z \times M}{a^3 \times N_A}d=a3×NA​z×M​

Where,

• z = Number of atoms per unit cell.

• M = Molar mass.

• a = Edge length of the unit cell.

• Nₐ = Avogadro number.

Imperfections in Solids

Solids are not perfectly ordered; they contain defects.

1. Point Defects

• Stoichiometric Defects

  • Vacancy defect

  • Interstitial defect

  • Frenkel defect

  • Schottky defect

• Non-Stoichiometric Defects

  • Metal excess defect

  • Metal deficiency defect

• Impurity Defect

  • Due to the presence of foreign atoms.

2. Line Defects

• Related to irregularities in the rows of atoms.

Electrical and Magnetic Properties of Solids

Electrical Properties

• Conductors: Metals with free electrons.

• Insulators: No free electrons.

• Semiconductors: Conductivity depends on temperature and impurities.

Magnetic Properties

• Diamagnetic: Weakly repelled by magnetic field.

• Paramagnetic: Weakly attracted due to unpaired electrons.

• Ferromagnetic: Strongly attracted; permanent magnetism (Fe, Co, Ni).

• Antiferromagnetic: Opposite spins cancel out.

• Ferrimagnetic: Unequal opposite spins → net magnetism.

Applications of Solid State

• Semiconductors in electronics (Si, Ge).

• Superconductors in magnetic levitation.

• Ionic solids in batteries and fuel cells.

• Magnetic materials in data storage.

• Amorphous solids in glass industry.

Objective Questions – Solid State

1. The coordination number of atoms in a body-centered cubic (BCC) structure is:

a) 6
b) 8
c) 12
d) 4

Answer: 8

2. The number of atoms present in a face-centered cubic (FCC) unit cell is:

a) 1
b) 2
c) 3
d) 4

Answer: 4

3. The packing efficiency of a simple cubic lattice is:

a) 52%
b) 68%
c) 74%
d) 60%

Answer: 52%

4. The crystal system of NaCl is:

a) Cubic
b) Hexagonal
c) Tetragonal
d) Orthorhombic

Answer: Cubic

5. The number of tetrahedral voids in a unit cell is:

a) Equal to number of atoms
b) Twice the number of atoms
c) Half the number of atoms
d) Four times the number of atoms

Answer: Twice

6. Diamond is an example of:

a) Ionic solid
b) Molecular solid
c) Covalent solid
d) Metallic solid

Answer: Covalent

7. In a Schottky defect:

a) Equal number of cations and anions are missing
b) Only cations are missing
c) Only anions are missing
d) Foreign particles occupy interstitial sites

Answer: Equal

8. The formula for density of a unit cell is:

a) Ma3NA\frac{M}{a^3 N_A}a3NA​M​
b) zMa3NA\frac{zM}{a^3 N_A}a3NA​zM​
c) a3NAM\frac{a^3N_A}{M}Ma3NA​​
d) NAzMa3\frac{N_A}{zM a^3}zMa3NA​​

Answer: zMa3NA\frac{zM}{a^3 N_A}a3NA​zM​

9. The coordination number in hexagonal close packing (HCP) is:

a) 6
b) 8
c) 12
d) 4

Answer: 12

10. The edge length ratio of a body-centered cubic (BCC) is:

a) 2r\sqrt{2}r2​r
b) $2r$
c) 4r3\frac{4r}{\sqrt{3}}3​4r​
d) 3a\sqrt{3}a3​a

Answer: 4r3\frac{4r}{\sqrt{3}}3​4r​

11. Graphite is a good conductor of electricity because of:

a) Covalent bonds
b) Free electrons
c) Ionic bonds
d) Metallic bonds

Answer: Free

12. Which one is an amorphous solid?

a) NaCl
b) Quartz
c) Glass
d) Diamond

Answer: Glass

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13. The number of Bravais lattices in three dimensions is:

a) 7
b) 14
c) 21
d) 10

Answer: 14

14. Which defect increases the density of the solid?

a) Schottky defect
b) Frenkel defect
c) Interstitial defect
d) Vacancy defect

Answer: Interstitial

15. The coordination number in simple cubic packing is:

a) 12
b) 6
c) 8
d) 4

Answer: 6

16. Which type of solid is dry ice?

a) Ionic solid
b) Molecular solid
c) Metallic solid
d) Covalent solid

Answer: Molecular

17. The maximum packing efficiency is observed in:

a) Simple cubic
b) BCC
c) FCC/HCP
d) Orthorhombic

Answer: FCC/HCP

18. In which crystal system a = b = c and α = β = γ = 90°?

a) Cubic
b) Tetragonal
c) Orthorhombic
d) Triclinic

Answer: Cubic

19. Which magnetic material shows strong permanent magnetism?

a) Diamagnetic
b) Paramagnetic
c) Ferromagnetic
d) Antiferromagnetic

Answer: Ferromagnetic

20. Which point defect does not change the density of the solid?

a) Schottky defect
b) Frenkel defect
c) Vacancy defect
d) Interstitial defect

Answer: Frenkel

Short Answer Questions – Solid State

1. What is the difference between crystalline and amorphous solids?

Crystalline solids have a long-range order, definite melting point, and anisotropic nature, whereas amorphous solids have a short-range order, do not have a sharp melting point, and are isotropic.

2. Define unit cell.

A unit cell is the smallest repeating structural unit of a crystal lattice that, when repeated in space, produces the whole crystal.

3. What is meant by coordination number?

The coordination number is the number of nearest neighboring particles surrounding a particle in a crystal lattice.

4. What is the packing efficiency of FCC?

The packing efficiency of Face-Centered Cubic (FCC) structure is 74%, the highest among cubic lattices.

5. Name one example of covalent solid and one of ionic solid.

Diamond is an example of covalent solid, and sodium chloride (NaCl) is an example of ionic solid.

6. What is a Schottky defect?

It is a type of point defect where equal numbers of cations and anions are missing from their lattice sites, maintaining electrical neutrality.

7. What are Bravais lattices?

Bravais lattices are 14 distinct three-dimensional arrangements of lattice points that represent all possible crystal structures.

8. What is a Frenkel defect?

In this defect, a smaller ion (usually a cation) leaves its normal lattice site and occupies an interstitial position, without affecting the density of the solid.

9. What is meant by anisotropy?

Anisotropy is the property of crystalline solids to show different physical properties in different directions due to their ordered structure.

10. Give one difference between paramagnetic and diamagnetic substances.

Paramagnetic substances are weakly attracted by a magnetic field due to unpaired electrons, while diamagnetic substances are weakly repelled by a magnetic field due to all electrons being paired.

Long Answer Questions – Solid State

1. Explain the types of crystalline solids with examples.

Crystalline solids are classified based on the nature of forces:

• Ionic solids – ions held by electrostatic forces (NaCl, KCl).

• Metallic solids – positive ions in a sea of electrons (Cu, Fe).

• Covalent solids – atoms held by covalent bonds (Diamond, Quartz).

• Molecular solids – molecules held by van der Waals’ forces or H-bonds (Ice, Dry Ice).

2. Describe the different types of cubic unit cells with diagrams.

• Simple Cubic (SC): Atoms at corners only; 1 atom per unit cell.

• Body-Centered Cubic (BCC): Atoms at corners + 1 at center; 2 atoms per unit cell.

• Face-Centered Cubic (FCC): Atoms at corners + centers of all faces; 4 atoms per unit cell.
  Each type differs in coordination number, packing efficiency, and density.

3. What are the different types of packing in solids?

• 2D Packing: Square close packing and Hexagonal close packing.

• 3D Packing: Formed by stacking 2D layers.

  • Hexagonal Close Packing (HCP): ABAB type arrangement.

  • Cubic Close Packing (CCP): ABCABC arrangement (equivalent to FCC).
    Packing efficiency: SC (52%), BCC (68%), FCC/HCP (74%).

4. Explain point defects in ionic solids with examples.

Point defects are irregularities at one or two lattice points:

• Stoichiometric Defects: Vacancy defect, interstitial defect, Frenkel defect, Schottky defect.

• Non-stoichiometric Defects: Metal excess defect (color centers), metal deficiency defect.

• Impurity Defect: Due to the presence of foreign ions (e.g., NaCl doped with SrCl₂).

5. Explain the different types of magnetic properties in solids.

• Diamagnetic: Weakly repelled, all electrons paired (Zn, NaCl).

• Paramagnetic: Weakly attracted, unpaired electrons (O₂, Cu²⁺).

• Ferromagnetic: Strong attraction, permanent magnetism (Fe, Co, Ni).

• Antiferromagnetic: Opposite spins cancel out (MnO).

• Ferrimagnetic: Unequal opposite spins give weak magnetism (Fe₃O₄).

FAQs – Solid State

1. Why is glass considered an amorphous solid?

Glass does not have a regular long-range order and does not show a sharp melting point. Therefore, it is classified as an amorphous solid.

2. Which has a higher packing efficiency – BCC or FCC?

FCC has higher packing efficiency (74%) compared to BCC (68%).

3. Why is graphite a good conductor while diamond is not?

Graphite has delocalized electrons that conduct electricity, whereas diamond has a rigid covalent network with no free electrons, making it an insulator.

4. What is the difference between tetrahedral and octahedral voids?

Tetrahedral voids are surrounded by 4 spheres, while octahedral voids are surrounded by 6 spheres. Number of tetrahedral voids = 2 × Number of octahedral voids.

5. How many atoms are present in BCC and FCC unit cells?

A BCC unit cell has 2 atoms, while an FCC unit cell has 4 atoms.

Conclusion

The solid state is a vital topic that explains the structural and physical properties of solids. From the classification of solids to the study of crystal systems, unit cells, packing efficiency, and defects, this chapter forms the base of understanding material properties. It has wide applications in electronics, metallurgy, energy storage, and modern technology.

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