Class 12 Chemistry Chapter 1 Notes: The Solid State | Important Questions & PYQs

 Class 12 Chemistry – Chapter 1: The Solid State (NCERT Notes)

1. The Solid State

Solids are substances in which particles are closely packed, have fixed positions, and show strong intermolecular forces.

Why solids are rigid

• Particles vibrate about fixed mean positions

• Very strong interparticle forces

• Hence, shape remains unchanged

Why solids have definite volume

• Negligible interparticle space

• Strong forces resist compression

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2. Types of Solids

2.1 Amorphous Solids

• No long-range order

• Isotropic (same properties in all directions)

• No sharp melting point

• Examples: glass, rubber, plastics

2.2 Crystalline Solids

• Long-range orderly arrangement

• Anisotropic

• Sharp melting point

• Examples: NaCl, diamond, copper

Glass as a super-cooled liquid

• Shows slow flow over long time

• Hence considered an amorphous solid with liquid-like behaviour

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3. Classification Based on Intermolecular Forces

Type	Characteristics	Examples
Ionic	Hard, brittle, high mp	Na₂SO₄, ZnS
Metallic	Malleable, ductile, conductor	Cu, Rb
Molecular	Weak forces, soft	Benzene, Ar
Covalent (Network)	Very hard, high mp	Diamond, SiC

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4. Electrical & Magnetic Properties

Electrical Conductivity

• Metallic solids → Conduct in solid state

• Ionic solids → Conduct only in molten or aqueous state

Magnetic Behaviour

• Ferromagnetic → Strong, permanent magnet (Fe, Co)

• Paramagnetic → Weak attraction, temporary (O₂)

• Ferrimagnetic → Partial cancellation (Fe₃O₄)

• Antiferromagnetic → Complete cancellation (MnO)

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5. Crystal Lattice & Unit Cell

Lattice Point

• Represents position of atom / ion / molecule

Unit Cell Parameters

• Edges: a, b, c

• Angles: α, β, γ

Types of Unit Cells

• Simple cubic

• Body-centred cubic (bcc)

• Face-centred cubic (fcc / ccp)

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6. Number of Atoms per Unit Cell

Position	Contribution
Corner	1/8
Face centre	1/2
Body centre	1

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7. Close Packed Structures

hcp (Hexagonal close packed)

• Layer sequence: ABAB

• Packing efficiency: 74%

ccp / fcc (Cubic close packed)

• Layer sequence: ABCABC

• Packing efficiency: 74%

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8. Voids in Crystals

• Octahedral voids = Number of atoms

• Tetrahedral voids = 2 × Number of atoms

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9. Packing Efficiency

Structure	Packing Efficiency
Simple cubic	52.4%
bcc	68%
hcp / fcc	74% (maximum)

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10. Imperfections (Defects) in Solids

Stoichiometric Defects

• Schottky defect → Equal cation & anion vacancies → Density decreases

• Frenkel defect → Cation displaced → Density unchanged

Non-stoichiometric Defects

• Metal excess / deficiency

• Creates colour (F-centres)

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11. Semiconductors

n-Type Semiconductor

• Doped with Group 15 elements (P, As)

• Extra electrons → Negative charge carriers

p-Type Semiconductor

• Doped with Group 13 elements (B, Al)

• Holes act as charge carriers

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12. Band Theory

Material	Energy Gap
Conductor	No gap
Semiconductor	Small gap
Insulator	Large gap

• Conductivity of semiconductors increases with temperature

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13. Important Exam Points

• Ferromagnetic materials are best for permanent magnets

• Vacancy defects reduce density

• fcc unit cell contains 4 atoms

• Coordination number:

• ccp = 12

• bcc = 8

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. Why are solids rigid?
   Because particles vibrate about fixed mean positions due to very strong intermolecular forces.

2. Why do solids have a definite volume?
   Due to negligible interparticle space and strong forces resisting compression.

3. What is an amorphous solid?
   A solid with no long-range order and no sharp melting point.

4. Give two examples of crystalline solids.
   Sodium chloride (NaCl) and diamond.

5. Why is glass called a super-cooled liquid?
   Because it shows slow flow over a long period of time.

6. Which solids conduct electricity in solid state?
   Metallic solids.

7. What is a unit cell?
   The smallest repeating unit that represents the crystal lattice.

8. How many atoms are present in an fcc unit cell?
   4 atoms.

9. What are octahedral voids?
   Voids equal in number to the atoms present in the crystal.

10. Which defect reduces density of a solid?
    Schottky defect.

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📝 Long Answer Questions

1. Explain the types of solids based on their structure.

Solids are classified into two main types:

(a) Amorphous Solids

• No regular long-range arrangement of particles

• Isotropic in nature

• Do not have a sharp melting point

• Examples: glass, rubber, plastics

(b) Crystalline Solids

• Regular and repeating long-range arrangement

• Anisotropic in nature

• Sharp melting point

• Examples: NaCl, diamond, copper

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2. Describe different types of crystalline solids based on intermolecular forces.

Crystalline solids are classified as:

• Ionic solids: Hard, brittle, high melting point (Na₂SO₄, ZnS)

• Metallic solids: Malleable, ductile, good conductors (Cu, Rb)

• Molecular solids: Weak forces, soft (benzene, argon)

• Covalent solids: Very hard, high melting point (diamond, SiC)

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3. Explain defects in solids.

Defects in solids are of two types:

(a) Stoichiometric defects

• Schottky defect: Equal number of cation and anion vacancies, density decreases

• Frenkel defect: Cation displaced from lattice site, density unchanged

(b) Non-stoichiometric defects

• Due to metal excess or deficiency

• Leads to formation of colour centres (F-centres)

📝 PYQs (Previous Years’ Questions) – Board Exam Oriented

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🔹 Very Short Answer / 1 Mark PYQs

1. Why are solids incompressible?

2. Name one amorphous solid.

3. What is the coordination number of atoms in ccp structure?

4. How many atoms are present in an fcc unit cell?

5. What type of defect does not change density of a crystal?

6. Name the defect responsible for colour in ionic solids.

7. Which solid conducts electricity in solid state?

8. What is the packing efficiency of bcc structure?

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🔹 Short Answer / 2 Marks PYQs

1. Differentiate between amorphous and crystalline solids.

2. Why is glass considered a super-cooled liquid?

3. Write any two differences between ionic and metallic solids.

4. What is Frenkel defect? Explain with one example.

5. Define unit cell and lattice point.

6. Why do ionic solids conduct electricity in molten state but not in solid state?

7. Calculate number of atoms present in a bcc unit cell.

8. What are tetrahedral voids? How many are present in a crystal?

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🔹 Short Answer / 3 Marks PYQs

1. Explain Schottky and Frenkel defects with suitable examples.

2. Classify crystalline solids on the basis of intermolecular forces.

3. Describe electrical conductivity in solids.

4. Explain anisotropy in crystalline solids.

5. Write magnetic properties of solids (any three types).

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🔹 Long Answer / 5 Marks PYQs

1. (a) What are crystalline solids?
   (b) Explain different types of crystalline solids with examples.

2. Explain close packed structures in solids. Compare hcp and ccp structures with respect to:

   • Layer arrangement

   • Packing efficiency

   • Coordination number

3. (a) What are imperfections in solids?
   (b) Explain stoichiometric defects in detail.

4. Explain band theory of solids and differentiate between:

   • Conductors

   • Semiconductors

   • Insulators

5. (a) What are n-type and p-type semiconductors?
   (b) How are they prepared by doping?

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❓ FAQs (Concept Clarity)

1. Why are crystalline solids anisotropic?
   Because their properties depend on direction due to regular arrangement of particles.

2. Why do ionic solids conduct electricity only in molten state?
   Because ions are free to move only when molten or dissolved.

3. What is packing efficiency of fcc and hcp structures?
   74%, which is the maximum possible.

4. How many tetrahedral voids are present in a crystal?
   Twice the number of atoms present.

5. Why does conductivity of semiconductors increase with temperature?
   Because more electrons move to conduction band at higher temperature.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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