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Physical Chemistry - The Solid State 📖
SHORT QUESTION ANSWER (2 MARKS) - 200 Questions
Q1. Differentiate between crystalline and amorphous solids.
Answer:
| Crystalline Solids | Amorphous Solids |
|---|---|
| Have definite geometrical shape | No definite geometrical shape |
| Long-range order present | Short-range order only |
| Sharp melting point | Melt over a range of temperatures |
| Anisotropic | Isotropic |
| Example: NaCl, Diamond | Example: Glass, Rubber |
Q2. What is meant by coordination number? What is the coordination number in (i) hcp structure (ii) bcc structure?
Answer: Coordination number is the number of nearest neighboring particles surrounding a particle in the crystal lattice.
- (i) hcp (Hexagonal Close Packing): Coordination number = 12
- (ii) bcc (Body-Centered Cubic): Coordination number = 8
Q3. Explain the difference between Schottky and Frenkel defects.
Answer:
| Schottky Defect | Frenkel Defect |
|---|---|
| Equal number of cations and anions missing | Only cation displaced to interstitial site |
| Density decreases | Density remains same |
| Found in NaCl, KCl | Found in AgBr, ZnS |
| Decreases coordination number | No change in coordination number |
Q4. Calculate the number of atoms per unit cell in (i) Simple cubic (ii) BCC (iii) FCC.
Answer: (i) Simple Cubic:
- 8 corner atoms × (1/8) = 1 atom
(ii) BCC:
- 8 corners × (1/8) + 1 body center = 2 atoms
(iii) FCC:
- 8 corners × (1/8) + 6 faces × (1/2) = 4 atoms
Q5. What are F-centers? How do they give color to crystals?
Answer: F-centers are anionic vacancies occupied by unpaired electrons.
Formation: When NaCl is heated in Na vapor, Cl⁻ ions leave creating vacancies which trap electrons.
Color: Trapped electrons absorb visible light and get excited. The absorbed wavelength gives color to the crystal. Example: NaCl with F-centers appears yellow.
Q6. Why are solids rigid?
Answer: Solids are rigid because:
- Constituent particles (atoms/ions/molecules) are held by strong interparticle forces
- Particles have fixed positions and can only oscillate about their mean positions
- This gives solids a definite shape and volume
Q7. Why do crystalline solids have sharp melting points?
Answer: Crystalline solids have regular arrangement of particles with uniform intermolecular forces throughout the crystal. When heated, all bonds break simultaneously at a specific temperature, giving a sharp melting point.
Q8. Why are amorphous solids isotropic?
Answer: Amorphous solids have irregular arrangement of particles with no long-range order. Due to random distribution, properties like refractive index, electrical conductivity are same in all directions, making them isotropic.
Q9. Explain why glass is considered a supercooled liquid.
Answer: Glass is an amorphous solid that:
- Has irregular structure like liquids
- Flows very slowly over time (viscous liquid)
- Old window panes are thicker at bottom due to slow flow
- Does not have sharp melting point
Q10. What is meant by the term 'lattice point'?
Answer: Lattice point is a position in space where a constituent particle (atom/ion/molecule) is located in a crystal lattice. Each lattice point represents identical environment in the crystal structure.
Q11. Define Bravais lattices. How many Bravais lattices are possible?
Answer: Bravais lattice is a distinct arrangement of lattice points in three-dimensional space such that each point has identical surroundings.
There are 14 Bravais lattices possible, distributed among 7 crystal systems.
Q12. What is the relationship between edge length (a) and radius (r) in simple cubic unit cell?
Answer: In simple cubic unit cell:
- Atoms touch along the edge
- Edge length a = 2r
Where r = radius of atom
Q13. What is the relationship between edge length (a) and radius (r) in BCC unit cell?
Answer: In BCC unit cell:
- Atoms touch along body diagonal
- Body diagonal = 4r
- Body diagonal = √3a
Therefore: √3a = 4r or a = 4r/√3
Q14. What is the relationship between edge length (a) and radius (r) in FCC unit cell?
Answer: In FCC unit cell:
- Atoms touch along face diagonal
- Face diagonal = 4r
- Face diagonal = √2a
Therefore: √2a = 4r or a = 2√2r
Q15. Calculate packing efficiency of simple cubic unit cell.
Answer:
- Volume of 1 atom = (4/3)πr³
- Volume of unit cell = a³ = (2r)³ = 8r³
- Packing efficiency = (Volume of 1 atom / Volume of unit cell) × 100
- = [(4/3)πr³ / 8r³] × 100
- = 52.4%
Q16. What is the packing efficiency of BCC structure?
Answer:
- Number of atoms = 2
- Volume of atoms = 2 × (4/3)πr³
- a = 4r/√3, so a³ = 64r³/(3√3)
- Packing efficiency = [2 × (4/3)πr³ × 3√3/64r³] × 100
- = 68%
Q17. What is the packing efficiency of FCC and HCP structures?
Answer: Both FCC and HCP have:
- Packing efficiency = 74%
- Coordination number = 12
- Most efficient packing arrangements
- 4 atoms per unit cell (FCC)
Q18. Explain tetrahedral void with diagram.
Answer: Tetrahedral void is formed when a sphere (atom) is placed over a triangular void formed by three touching spheres.
Characteristics:
- Surrounded by 4 spheres
- Number of tetrahedral voids = 2n (where n = number of atoms)
- Smaller than octahedral void
Q19. Explain octahedral void.
Answer: Octahedral void is formed when triangular voids of two layers are aligned such that their vertices point in opposite directions.
Characteristics:
- Surrounded by 6 spheres
- Number of octahedral voids = n (where n = number of atoms)
- Larger than tetrahedral void
Q20. How many tetrahedral and octahedral voids are present per atom in a close-packed structure?
Answer: In close-packed structures:
- Tetrahedral voids = 2 per atom (2n total)
- Octahedral voids = 1 per atom (n total)
Where n = number of atoms in the packing
Q21. What is the difference between hexagonal close packing and cubic close packing?
Answer:
| HCP | CCP (FCC) |
|---|---|
| Layer sequence: ABAB... | Layer sequence: ABCABC... |
| Hexagonal unit cell | Cubic unit cell |
| Coordination number = 12 | Coordination number = 12 |
| Packing efficiency = 74% | Packing efficiency = 74% |
Q22. Why is the packing efficiency of FCC and HCP the same?
Answer: Both FCC and HCP have:
- Same coordination number (12)
- Same arrangement of nearest neighbors
- Only differ in stacking sequence (ABCABC vs ABAB)
- Same fraction of space occupied
- Packing efficiency = 74%
Q23. What are interstitial sites? Name two types.
Answer: Interstitial sites are voids (empty spaces) between closely packed atoms in a crystal lattice.
Two types:
- Tetrahedral void - surrounded by 4 atoms
- Octahedral void - surrounded by 6 atoms
These sites can accommodate smaller atoms/ions.
Q24. Calculate the number of octahedral voids in FCC unit cell.
Answer: In FCC unit cell:
- Number of atoms = 4
- Octahedral voids per atom = 1
- Total octahedral voids = 4
Also: 1 at body center + 12 at edge centers × (1/4) = 1 + 3 = 4
Q25. Calculate the number of tetrahedral voids in FCC unit cell.
Answer: In FCC unit cell:
- Number of atoms = 4
- Tetrahedral voids per atom = 2
- Total tetrahedral voids = 8
These voids are located inside the unit cell.
Q26. What type of stoichiometric defect is shown by NaCl? Explain.
Answer: NaCl shows Schottky defect.
Reason:
- Na⁺ and Cl⁻ have similar size
- Equal number of cations and anions missing
- Maintains electrical neutrality
- Density decreases slightly
Q27. What type of stoichiometric defect is shown by AgBr? Explain.
Answer: AgBr shows Frenkel defect.
Reason:
- Large size difference between Ag⁺ (small) and Br⁻ (large)
- Ag⁺ ion displaced to interstitial site
- Density remains unchanged
- Common in compounds with large anions
Q28. What is a non-stoichiometric defect? Give example.
Answer: Non-stoichiometric defect occurs when the ratio of cations to anions differs from the ideal stoichiometric composition.
Types:
- Metal excess defect
- Metal deficiency defect
Example: Fe₀.₉₅O (metal deficiency), ZnO (metal excess)
Q29. Explain metal excess defect due to anionic vacancies.
Answer: Formation:
- Heating crystal in metal vapor
- Anions leave creating vacancies
- Electrons occupy these vacancies (F-centers)
Example: NaCl heated in Na vapor
- Cl⁻ ions diffuse out
- Electrons trapped in vacancies
- Crystal appears yellow
Q30. Explain metal excess defect due to extra cations at interstitial sites.
Answer: Formation:
- Metal atoms go to interstitial positions
- Lose electrons becoming cations
- Electrons remain in interstitial sites
Example: ZnO on heating
- Zn atoms occupy interstitial sites
- Zn → Zn²⁺ + 2e⁻
- Crystal appears yellow, becomes white on cooling
Q31. Explain metal deficiency defect with example.
Answer: Metal deficiency defect occurs when:
- Cations are missing from lattice
- Charge balance maintained by higher oxidation state
Example: FeO (Fe₀.₉₅O)
- Some Fe²⁺ missing
- Replaced by Fe³⁺ ions
- 3 Fe²⁺ = 2 Fe³⁺ + vacancy (charge balance)
- Formula: Fe₀.₉₅O
Q32. Why does FeO exist as Fe₀.₉₅O?
Answer: FeO shows metal deficiency defect:
- Some Fe²⁺ ions missing from lattice
- To maintain neutrality, some Fe²⁺ oxidized to Fe³⁺
- 2 Fe³⁺ replace 3 Fe²⁺ (same charge)
- Results in non-stoichiometric formula Fe₀.₉₅O
Q33. How do metal excess defects affect the properties of crystals?
Answer: Effects:
- Color - F-centers absorb light, impart color
- Electrical conductivity - Free electrons increase conductivity
- Paramagnetic - Unpaired electrons make crystal paramagnetic
- Density - Slight decrease in metal excess via vacancies
Q34. Why does Schottky defect lower the density of crystals?
Answer: In Schottky defect:
- Equal number of cations and anions missing
- Number of particles in unit cell decreases
- Volume of unit cell remains approximately same
- Since Density = Mass/Volume
- Density decreases
Q35. Why does Frenkel defect not change the density of crystals?
Answer: In Frenkel defect:
- Ion displaced from lattice site to interstitial site
- No particle is lost from the crystal
- Total mass and volume remain same
- Therefore, density unchanged
Q36. What are semiconductors? Give examples.
Answer: Semiconductors are solids with electrical conductivity intermediate between conductors and insulators.
Conductivity range: 10⁻⁶ to 10⁴ S/m
Examples:
- Si, Ge (elemental)
- GaAs, CdS (compound)
Property: Conductivity increases with temperature
Q37. Differentiate between n-type and p-type semiconductors.
Answer:
| n-type | p-type |
|---|---|
| Doped with group 15 element | Doped with group 13 element |
| Extra electrons (negative charge carriers) | Electron holes (positive charge carriers) |
| Example: Si doped with P, As | Example: Si doped with B, Al, Ga |
| Electrons in conduction band | Holes in valence band |
Q38. How is n-type semiconductor formed? Explain with example.
Answer: Formation:
- Silicon (group 14) doped with Phosphorus (group 15)
- P has 5 valence electrons, Si has 4
- 4 electrons form bonds, 1 electron free
- Free electron increases conductivity
Charge carrier: Electrons (negative)
Q39. How is p-type semiconductor formed? Explain with example.
Answer: Formation:
- Silicon (group 14) doped with Boron (group 13)
- B has 3 valence electrons, Si has 4
- One bond remains incomplete creating electron hole
- Holes act as positive charge carriers
Charge carrier: Holes (positive)
Q40. What are 12-16 compounds? Give examples and their properties.
Answer: 12-16 compounds are formed between:
- Group 12 elements (Zn, Cd, Hg)
- Group 16 elements (O, S, Se, Te)
Examples: ZnS, CdS, CdSe, HgTe
Properties:
- Show photoconductivity
- Used in solar cells
- Semiconducting nature
Q41. What are 13-15 compounds? Give examples and uses.
Answer: 13-15 compounds are formed between:
- Group 13 elements (B, Al, Ga, In)
- Group 15 elements (N, P, As, Sb)
Examples: InSb, AlP, GaAs
Uses:
- Light-emitting diodes (LEDs)
- Laser diodes
- Solar cells
Q42. What are ferromagnetic substances? Give examples.
Answer: Ferromagnetic substances are strongly attracted by magnetic field and can be permanently magnetized.
Characteristics:
- All magnetic moments aligned in same direction
- Remain magnetic even after removing external field
Examples: Fe, Co, Ni, CrO₂
Q43. What are paramagnetic substances?
Answer: Paramagnetic substances are weakly attracted by magnetic field.
Characteristics:
- Have unpaired electrons
- Magnetic moments randomly oriented
- Align in presence of magnetic field
- Lose magnetism when field removed
Examples: O₂, Cu²⁺, Fe³⁺, Cr³⁺
Q44. What are diamagnetic substances?
Answer: Diamagnetic substances are weakly repelled by magnetic field.
Characteristics:
- Have no unpaired electrons
- All electrons paired
- Weakly repelled by magnet
Examples: H₂O, NaCl, C₆H₆, TiO₂
Q45. What are antiferromagnetic substances? Give example.
Answer: Antiferromagnetic substances have magnetic moments of equal magnitude aligned in opposite directions.
Characteristics:
- Net magnetic moment = zero
- Cancel each other
Examples: MnO, MnO₂, Mn₂O₃, FeO
Q46. What are ferrimagnetic substances? Give examples.
Answer: Ferrimagnetic substances have magnetic moments of unequal magnitude aligned in opposite directions.
Characteristics:
- Net magnetic moment ≠ zero
- Weaker than ferromagnetic
- Used in magnetic storage
Examples: Fe₃O₄ (magnetite), ferrites (MFe₂O₄)
Q47. Distinguish between paramagnetism and ferromagnetism.
Answer:
| Paramagnetism | Ferromagnetism |
|---|---|
| Weakly attracted | Strongly attracted |
| Random alignment | Parallel alignment |
| Loses magnetism when field removed | Permanent magnetization |
| Examples: O₂, Cu²⁺ | Examples: Fe, Co, Ni |
Q48. What is meant by doping in semiconductors?
Answer: Doping is the process of adding controlled amount of impurity (dopant) to a pure semiconductor to modify its electrical conductivity.
Purpose:
- Increase conductivity
- Create n-type or p-type semiconductors
- Control electrical properties
Dopants: Group 13 or 15 elements for Si/Ge
Q49. Why do solids have definite volume?
Answer: Solids have definite volume because:
- Constituent particles held by strong intermolecular forces
- Particles occupy fixed positions
- Very small intermolecular spaces
- Particles cannot move freely
- Incompressible nature
Q50. What is the contribution of a corner atom to a unit cell?
Answer: A corner atom is shared by 8 adjacent unit cells.
Contribution to one unit cell = 1/8
In a cubic unit cell with 8 corners: Total contribution = 8 × (1/8) = 1 atom
Q51. What is the contribution of a face-centered atom to a unit cell?
Answer: A face-centered atom is shared by 2 adjacent unit cells.
Contribution to one unit cell = 1/2
In FCC with 6 face atoms: Total contribution = 6 × (1/2) = 3 atoms
Q52. What is the contribution of an edge-centered atom to a unit cell?
Answer: An edge-centered atom is shared by 4 adjacent unit cells.
Contribution to one unit cell = 1/4
Total atoms from edges = 12 edges × (1/4) = 3 atoms
Q53. What is the contribution of a body-centered atom to a unit cell?
Answer: A body-centered atom lies completely inside the unit cell.
Contribution to one unit cell = 1 (complete atom)
Only present in BCC structure.
Q54. Calculate the density of a cubic crystal in terms of edge length.
Answer: Formula: Density (ρ) = (Z × M) / (a³ × Nₐ)
Where:
- Z = number of atoms per unit cell
- M = molar mass
- a = edge length
- Nₐ = Avogadro's number (6.022 × 10²³)
Q55. What is the difference between crystal lattice and unit cell?
Answer:
| Crystal Lattice | Unit Cell |
|---|---|
| 3D arrangement of lattice points | Smallest repeating unit |
| Entire crystal structure | Building block of lattice |
| Infinite extension | Finite size |
| Represents whole crystal | Generates whole lattice when repeated |
Q56. Why is the number of tetrahedral voids double the number of octahedral voids?
Answer: In close-packed structures:
- Octahedral voids = n (equal to number of atoms)
- Tetrahedral voids = 2n
Reason:
- One octahedral void per atom
- Two tetrahedral voids per atom
- Geometry and packing arrangement determine this ratio
Q57. What is meant by 'impurity defect'? Give example.
Answer: Impurity defect occurs when foreign atoms occupy lattice positions or interstitial sites.
Example:
- NaCl doped with SrCl₂
- Sr²⁺ replaces Na⁺ at lattice site
- Creates cation vacancy for charge balance
- 1 Sr²⁺ replaces 2 Na⁺, creating 1 vacancy
Q58. How does electrical conductivity of semiconductors vary with temperature?
Answer: For semiconductors:
- Conductivity increases with temperature
- More electrons jump to conduction band
- Energy gap is small (≈1 eV)
Reason: At higher temperature, thermal energy helps electrons overcome band gap, increasing charge carriers.
Q59. Why are solids incompressible?
Answer: Solids are incompressible because:
- Constituent particles are very closely packed
- Almost no intermolecular space
- Strong interparticle forces resist compression
- Particles cannot come closer
- Volume remains constant under pressure
Q60. What is the two-dimensional coordination number of a square close-packed layer?
Answer: In a square close-packed layer (2D):
- Each sphere touches 4 neighboring spheres
- Coordination number = 4
This is different from 3D close packing where coordination number is 12.
Q61. What is the two-dimensional coordination number of a hexagonal close-packed layer?
Answer: In a hexagonal close-packed layer (2D):
- Each sphere touches 6 neighboring spheres
- Coordination number = 6
This layer forms the basis for 3D HCP and CCP structures.
Q62. Why do ionic solids conduct electricity in molten state?
Answer: Ionic solids conduct electricity in molten state because:
- Ions become mobile on melting
- Fixed lattice structure breaks down
- Ions can move freely carrying charge
- Cations move to cathode, anions to anode
In solid state, ions are fixed and cannot move.
Q63. What are piezoelectric crystals? Give examples.
Answer: Piezoelectric crystals develop electric potential when subjected to mechanical stress.
Mechanism: Mechanical pressure → Electric polarization
Examples:
- Quartz (SiO₂)
- Rochelle salt
- Titanates
Uses: Microphones, ultrasonic generators, pressure sensors
Q64. What are pyroelectric crystals?
Answer: Pyroelectric crystals develop electric potential when heated or cooled.
Mechanism: Temperature change → Electric polarization
Example: Tourmaline
Uses: Infrared detectors, thermal imaging
Q65. Calculate the void space percentage in simple cubic structure.
Answer:
- Packing efficiency = 52.4%
- Void space = 100 - 52.4 = 47.6%
Nearly half the volume is empty space in simple cubic structure.
Q66. Calculate the void space percentage in BCC structure.
Answer:
- Packing efficiency = 68%
- Void space = 100 - 68 = 32%
BCC has less void space than simple cubic.
Q67. Calculate the void space percentage in FCC/HCP structure.
Answer:
- Packing efficiency = 74%
- Void space = 100 - 74 = 26%
FCC and HCP are the most efficiently packed structures with minimum void space.
Q68. Why is glass considered a pseudo-solid?
Answer: Glass is called pseudo-solid or supercooled liquid because:
- Has amorphous structure like liquids
- No long-range order
- Flows slowly over long periods
- Old glass panes thicker at bottom
- Shows properties of both solid and liquid
Q69. What are molecular solids? Give examples.
Answer: Molecular solids are held together by weak van der Waals forces or hydrogen bonds.
Characteristics:
- Soft, low melting point
- Poor conductors
- Volatile
Examples:
- I₂, CO₂ (van der Waals)
- Ice, sugar (hydrogen bonded)
Q70. What are ionic solids? Give characteristics.
Answer: Ionic solids are composed of cations and anions held by strong electrostatic forces.
Characteristics:
- Hard but brittle
- High melting and boiling points
- Conduct electricity in molten/aqueous state
- Soluble in polar solvents
Examples: NaCl, MgO, CaF₂
Q71. What are covalent/network solids? Give examples.
Answer: Covalent solids have atoms bonded by strong covalent bonds throughout the structure.
Characteristics:
- Very hard
- Extremely high melting point
- Poor conductors (except graphite)
- Insoluble
Examples: Diamond, SiC, SiO₂, graphite
Q72. What are metallic solids? Give characteristics.
Answer: Metallic solids consist of metal cations in a sea of delocalized electrons.
Characteristics:
- Good conductors of heat and electricity
- Malleable and ductile
- Metallic luster
- Moderate to high melting point
Examples: Cu, Fe, Al, Au
Q73. Why is diamond hard while graphite is soft?
Answer: Diamond:
- 3D network of strong covalent bonds
- Each C bonded to 4 others (sp³)
- Very hard
Graphite:
- Layered structure with weak van der Waals between layers
- Layers slide over each other easily
- Soft and slippery
Q74. Why does graphite conduct electricity but diamond does not?
Answer: Graphite:
- Has free delocalized electrons (sp² hybridization)
- 4th electron free to move
- Conducts electricity
Diamond:
- All electrons in covalent bonds (sp³)
- No free electrons
- Does not conduct electricity
Q75. What is polymorphism? Give example.
Answer: Polymorphism is the ability of a solid to exist in more than one crystalline form.
Example:
- Carbon: Diamond, graphite, fullerene
- CaCO₃: Calcite, aragonite
- SiO₂: Quartz, cristobalite, tridymite
Properties differ due to different arrangements.
Q76. What is isomorphism? Give example.
Answer: Isomorphism is the phenomenon where different compounds crystallize in the same crystal structure.
Characteristics:
- Similar chemical formula
- Similar ionic radii
- Can form mixed crystals
Example: NaCl and KCl (both FCC structure)
Q77. Define 'band gap' in semiconductors.
Answer: Band gap is the energy difference between the valence band (highest occupied) and conduction band (lowest unoccupied).
For semiconductors:
- Band gap ≈ 0.1 to 3 eV
- Small enough for thermal excitation
- Determines electrical properties
Q78. What is the band gap in conductors, semiconductors, and insulators?
Answer:
| Type | Band Gap |
|---|---|
| Conductors | No band gap (overlapping bands) |
| Semiconductors | Small (0.1 - 3 eV) |
| Insulators | Large (> 3 eV) |
Q79. Why are ionic solids hard but brittle?
Answer: Hard:
- Strong electrostatic forces between ions
- Difficult to separate ions
Brittle:
- Applying force shifts layers
- Like charges come adjacent
- Repulsion causes cleavage
- Crystal breaks along planes
Q80. Why does NaCl have 6:6 coordination but CsCl has 8:8 coordination?
Answer: Coordination depends on radius ratio (r⁺/r⁻):
NaCl:
- r⁺/r⁻ = 0.55
- Octahedral arrangement
- 6:6 coordination
CsCl:
- r⁺/r⁻ = 0.93
- Cubic arrangement
- 8:8 coordination
Larger Cs⁺ can accommodate 8 neighbors.
Q81. What is the structure of NaCl? Describe briefly.
Answer: NaCl structure (Rock salt):
- FCC lattice of Cl⁻ ions
- Na⁺ ions in octahedral voids
- Coordination number: 6:6
- Each Na⁺ surrounded by 6 Cl⁻ and vice versa
- Formula units per unit cell = 4
Q82. What is the structure of CsCl? Describe briefly.
Answer: CsCl structure:
- Simple cubic lattice of Cl⁻ ions
- Cs⁺ at body center
- Coordination number: 8:8
- Each ion surrounded by 8 opposite ions
- Formula units per unit cell = 1
Q83. What is the structure of ZnS (zinc blende)? Describe.
Answer: ZnS (Zinc blende) structure:
- FCC lattice of S²⁻ ions
- Zn²⁺ in alternate tetrahedral voids (half)
- Coordination number: 4:4
- Tetrahedral arrangement
- Formula units per unit cell = 4
Q84. What is the structure of CaF₂ (fluorite)? Describe.
Answer: CaF₂ (Fluorite) structure:
- FCC lattice of Ca²⁺ ions
- F⁻ ions in all tetrahedral voids
- Coordination number: 8:4 (Ca:F)
- Each Ca²⁺ surrounded by 8 F⁻
- Formula units per unit cell = 4
Q85. What is antifluorite structure? Give example.
Answer: Antifluorite structure is reverse of fluorite structure:
- FCC lattice of anions
- Cations in all tetrahedral voids
- Coordination number: 4:8
Example: Na₂O, K₂O, Li₂O
Na⁺ in tetrahedral voids, O²⁻ in FCC arrangement.
Q86. How many formula units are present in NaCl unit cell?
Answer: NaCl unit cell:
- Na⁺: 12 edges × (1/4) + 1 body center = 4
- Cl⁻: 8 corners × (1/8) + 6 faces × (1/2) = 4
- Formula units (NaCl) = 4
Or: FCC has 4 atoms, so 4 formula units.
Q87. How many formula units are present in CsCl unit cell?
Answer: CsCl unit cell:
- Cs⁺: 1 body center = 1
- Cl⁻: 8 corners × (1/8) = 1
- Formula units (CsCl) = 1
Simple cubic based structure.
Q88. How many Zn²⁺ and S²⁻ ions are there in ZnS unit cell?
Answer: ZnS unit cell:
- S²⁻ (FCC): 8 corners × (1/8) + 6 faces × (1/2) = 4
- Zn²⁺ (alternate tetrahedral voids): 4
- Formula units (ZnS) = 4
Q89. Why do metals conduct electricity?
Answer: Metals conduct electricity because:
- Have free delocalized electrons (sea of electrons)
- Electrons can move throughout the structure
- Apply potential → electrons flow
- Good electrical conductivity
- No band gap
Q90. Why are metals malleable and ductile?
Answer: Metals are malleable and ductile because:
- Non-directional metallic bonding
- Layers of atoms can slide over each other
- Electron sea maintains bonding even after displacement
- No repulsion like in ionic solids
- Can be hammered/drawn without breaking
Q91. What is meant by 'close packing' in crystals?
Answer: Close packing is the arrangement of spheres (atoms/ions) to occupy maximum space with minimum voids.
Types:
- Hexagonal close packing (HCP)
- Cubic close packing (CCP/FCC)
Packing efficiency: 74% (highest possible for spheres)
Q92. What is the difference between 2D square close packing and hexagonal close packing?
Answer:
| Square Close Packing | Hexagonal Close Packing |
|---|---|
| Coordination number = 4 | Coordination number = 6 |
| Less efficient | More efficient |
| Forms square pattern | Forms hexagonal pattern |
| Spheres align in rows | Spheres in triangular voids |
Q93. Describe ABAB... type of packing.
Answer: ABAB... packing (HCP):
- First layer (A): Hexagonal arrangement
- Second layer (B): Spheres in half the tetrahedral voids of A
- Third layer: Exactly above first layer (A again)
- Repetition: ABAB...
- Structure: Hexagonal close packing
- Coordination number: 12
Q94. Describe ABCABC... type of packing.
Answer: ABCABC... packing (CCP/FCC):
- First layer (A): Hexagonal arrangement
- Second layer (B): In half the voids of A
- Third layer (C): In voids different from A and B
- Fourth layer: Repeats A
- Repetition: ABCABC...
- Structure: Face-centered cubic
- Coordination number: 12
Q95. What is the radius ratio rule? State its significance.
Answer: Radius ratio rule relates the ratio of cation to anion radius (r⁺/r⁻) with the coordination number and structure.
| r⁺/r⁻ | Coordination | Structure |
|---|---|---|
| 0.225-0.414 | 4 | Tetrahedral |
| 0.414-0.732 | 6 | Octahedral |
| 0.732-1.0 | 8 | Cubic |
Significance: Predicts crystal structure type.
Q96. Why does the electrical conductivity of semiconductors increase on doping?
Answer: On doping:
- n-type: Extra electrons from dopant → more charge carriers
- p-type: Electron holes created → more charge carriers
- Increase in charge carriers = increased conductivity
- Pure semiconductor has very few free carriers
Q97. What happens to ferromagnetic substance above Curie temperature?
Answer: Above Curie temperature:
- Thermal energy disrupts alignment
- Magnetic domains become randomly oriented
- Ferromagnetic → Paramagnetic
- Loses permanent magnetization
Example: Fe (Curie temp = 1043 K)
Q98. What is the effect of temperature on magnetic properties?
Answer: Paramagnetic/Diamagnetic:
- Diamagnetism: Temperature independent
- Paramagnetism: Decreases with increasing temperature
Ferromagnetic:
- Below Curie temp: Ferromagnetic
- Above Curie temp: Paramagnetic
Reason: Thermal agitation disrupts alignment.
Q99. Why is FeO non-stoichiometric?
Answer: FeO is non-stoichiometric because:
- Shows metal deficiency defect
- Some Fe²⁺ positions vacant
- Compensated by Fe³⁺ for charge balance
- Actual formula: Fe₀.₉₅O to Fe₀.₉₆O
- Never exactly FeO
Q100. Calculate the number of atoms in a body-centered cubic unit cell and a face-centered cubic unit cell.
Answer: BCC:
- 8 corners × (1/8) = 1
- 1 body center = 1
- Total = 2 atoms
FCC:
- 8 corners × (1/8) = 1
- 6 faces × (1/2) = 3
- Total = 4 atoms
Q101. What is meant by 'limiting radius ratio'?
Answer: Limiting radius ratio is the minimum value of r⁺/r⁻ required to maintain contact between cation and anion for a given coordination number.
Examples:
- Tetrahedral (CN=4): r⁺/r⁻ = 0.225
- Octahedral (CN=6): r⁺/r⁻ = 0.414
- Cubic (CN=8): r⁺/r⁻ = 0.732
Q102. Why are crystalline solids anisotropic?
Answer: Crystalline solids are anisotropic because:
- Ordered arrangement in different directions
- Different number of particles along different axes
- Properties like refractive index, conductivity vary with direction
- Exception: Cubic crystals (isotropic due to symmetry)
Q103. What are the characteristics of molecular solids?
Answer: Molecular solids:
- Weak forces - van der Waals or H-bonds
- Soft - easily deformed
- Low melting point - weak intermolecular forces
- Poor conductors - no free electrons/ions
- Volatile - easily vaporized
Examples: I₂, ice, dry ice
Q104. What are the characteristics of ionic solids?
Answer: Ionic solids:
- Hard but brittle - strong electrostatic forces
- High melting point - strong ionic bonds
- Conduct in molten state - mobile ions
- Soluble in polar solvents - ion-dipole interaction
- Non-conductor in solid state - fixed ions
Q105. What are the characteristics of covalent solids?
Answer: Covalent/Network solids:
- Very hard - strong covalent bonds
- Extremely high melting point - need to break covalent bonds
- Poor conductors - no free electrons (except graphite)
- Insoluble - strong 3D network
- Chemically inert
Examples: Diamond, SiC
Q106. What are the characteristics of metallic solids?
Answer: Metallic solids:
- Good conductors - free electrons
- Malleable and ductile - non-directional bonding
- Metallic luster - electron reflection
- Variable hardness - depends on structure
- Variable melting point - depends on bonding strength
Q107. Why do ionic compounds conduct electricity in solution but not in solid state?
Answer: In solid state:
- Ions fixed in lattice positions
- Cannot move
- No conduction
In solution/molten state:
- Ions become mobile
- Free to move and carry charge
- Conduct electricity
Q108. What type of solids are electrical conductors, semiconductors, and insulators?
Answer: Conductors:
- Metallic solids
- Example: Cu, Ag, Al
Semiconductors:
- Covalent network (doped)
- Example: Si, Ge, GaAs
Insulators:
- Ionic, molecular, covalent
- Example: Rubber, diamond, NaCl (solid)
Q109. Calculate the formula of a compound if atoms A form FCC and atoms B occupy all octahedral voids.
Answer: In FCC:
- Number of A atoms = 4
- Number of octahedral voids = 4
- All voids occupied by B
A : B = 4 : 4 = 1 : 1
Formula: AB Example: NaCl
Q110. Calculate the formula if atoms A form FCC and atoms B occupy all tetrahedral voids.
Answer: In FCC:
- Number of A atoms = 4
- Number of tetrahedral voids = 8
- All voids occupied by B
A : B = 4 : 8 = 1 : 2
Formula: AB₂ Example: CaF₂
Q111. Why does ZnS exist in two forms - zinc blende and wurtzite?
Answer: ZnS shows polymorphism with two structures:
Zinc blende:
- S²⁻ in FCC arrangement
- Zn²⁺ in alternate tetrahedral voids
- Cubic structure
Wurtzite:
- S²⁻ in HCP arrangement
- Zn²⁺ in alternate tetrahedral voids
- Hexagonal structure
Both have 4:4 coordination.
Q112. What is the difference between fluorite and antifluorite structure?
Answer:
| Fluorite (CaF₂) | Antifluorite (Na₂O) |
|---|---|
| Ca²⁺ in FCC | O²⁻ in FCC |
| F⁻ in all tetrahedral voids | Na⁺ in all tetrahedral voids |
| Coordination: 8:4 | Coordination: 4:8 |
| Cation to anion ratio 1:2 | Cation to anion ratio 2:1 |
Q113. Why are metals lustrous?
Answer: Metals show luster because:
- Free electrons in the structure
- Electrons absorb light energy
- Get excited to higher energy levels
- Re-emit light when returning
- Reflection of light → metallic luster
Q114. What is the coordination number of Cs⁺ and Cl⁻ in CsCl crystal?
Answer: In CsCl structure:
- Cs⁺ coordination number = 8 (surrounded by 8 Cl⁻)
- Cl⁻ coordination number = 8 (surrounded by 8 Cs⁺)
- 8:8 coordination
Q115. What is the coordination number of Na⁺ and Cl⁻ in NaCl crystal?
Answer: In NaCl structure:
- Na⁺ coordination number = 6 (surrounded by 6 Cl⁻)
- Cl⁻ coordination number = 6 (surrounded by 6 Na⁺)
- 6:6 coordination
Q116. What is the coordination number of Zn²⁺ and S²⁻ in ZnS crystal?
Answer: In ZnS structure:
- Zn²⁺ coordination number = 4 (tetrahedral)
- S²⁻ coordination number = 4 (tetrahedral)
- 4:4 coordination
Q117. What is the coordination number of Ca²⁺ and F⁻ in CaF₂ crystal?
Answer: In CaF₂ structure:
- Ca²⁺ coordination number = 8 (surrounded by 8 F⁻)
- F⁻ coordination number = 4 (tetrahedral)
- 8:4 coordination
Q118. Why does silicon show semiconductor property while carbon (diamond) is an insulator?
Answer: Diamond (C):
- Large band gap (≈ 5.4 eV)
- Electrons cannot jump to conduction band
- Insulator
Silicon:
- Smaller band gap (≈ 1.1 eV)
- Thermal energy sufficient for electron excitation
- Semiconductor
Q119. How does crystal field theory explain color in transition metal compounds?
Answer: Color in crystals:
- d-d transitions in transition metals
- Crystal field splits d-orbitals
- Electrons absorb specific wavelength
- Jump to higher d-orbital
- Complementary color observed
Not directly related to solid state defects but to electronic structure.
Q120. What happens when AgCl is doped with CdCl₂?
Answer: When AgCl is doped with CdCl₂:
- Cd²⁺ replaces Ag⁺ at lattice site
- For charge balance, one Ag⁺ vacancy created
- Impurity defect
- 1 Cd²⁺ = 2 Ag⁺, so one position vacant
Result: Cation vacancy defect
Q121. What is the effect of Schottky defect on density?
Answer: Schottky defect:
- Removes equal cations and anions
- Mass decreases
- Volume remains approximately constant
- Density = Mass/Volume
- Density decreases
Q122. What is the effect of Frenkel defect on dielectric constant?
Answer: Frenkel defect:
- Creates ionic displacement
- Increases local electric dipole moments
- Dielectric constant increases
- Due to increased polarizability
Q123. Why do ionic solids with Schottky defects conduct electricity?
Answer: Schottky defects create:
- Ion vacancies in the lattice
- Neighboring ions can jump into vacancies
- Ionic mobility increases
- Enhanced ionic conductivity
Especially at high temperatures.
Q124. What is the total number of atoms in one mole of a FCC crystal?
Answer: One mole contains Nₐ unit cells (6.022 × 10²³)
Each FCC unit cell has 4 atoms
Total atoms = 4 × Nₐ = 4 × 6.022 × 10²³
Actually, 1 mole = Nₐ atoms regardless of structure!
Q125. If the edge length of a cube is 'a', what is the body diagonal length?
Answer: Body diagonal connects two opposite corners through the body.
Using 3D Pythagorean theorem: Body diagonal = √3 × a = a√3
Used in BCC calculations.
Q126. If the edge length of a cube is 'a', what is the face diagonal length?
Answer: Face diagonal connects two opposite corners of a face.
Using 2D Pythagorean theorem: Face diagonal = √2 × a = a√2
Used in FCC calculations.
Q127. Why are vacancy defects more common in close-packed structures?
Answer: In close-packed structures:
- High packing efficiency (74%)
- Less void space
- Difficult to accommodate extra atoms
- Vacancies energetically favorable
- Easier than interstitial defects
Q128. What is meant by point defect?
Answer: Point defect is a deviation from perfect crystalline structure at a single lattice point.
Types:
- Vacancy - missing atom
- Interstitial - extra atom in void
- Substitutional - wrong atom at position
Affects local properties only.
Q129. What is meant by line defect?
Answer: Line defect (dislocation) is a defect extending along a line in the crystal.
Types:
- Edge dislocation - extra half-plane of atoms
- Screw dislocation - spiral arrangement
Effect: Makes metals ductile, affects mechanical strength.
Q130. Why does conductivity of silicon increase on doping with phosphorus?
Answer: When Si (group 14) is doped with P (group 15):
- P has 5 valence electrons, Si has 4
- 4 electrons form bonds
- 1 electron free to move
- Increases charge carriers
- n-type semiconductor formed
- Conductivity increases
Q131. Why does conductivity of germanium increase on doping with gallium?
Answer: When Ge (group 14) is doped with Ga (group 13):
- Ga has 3 valence electrons, Ge has 4
- One bond incomplete
- Creates electron hole (positive)
- Holes act as charge carriers
- p-type semiconductor formed
- Conductivity increases
Q132. What are the Miller indices? State their use.
Answer: Miller indices are a set of three integers (h k l) that designate crystal planes.
Method:
- Find intercepts on axes
- Take reciprocals
- Clear fractions
- Enclose in parentheses: (h k l)
Use: Identify and describe crystal planes and directions.
Q133. What is the significance of coordination number?
Answer: Coordination number indicates:
- Packing efficiency - higher CN = denser packing
- Type of void - tetrahedral (4), octahedral (6), cubic (8)
- Crystal structure - helps identify lattice type
- Stability - higher CN generally more stable
- Physical properties - affects hardness, melting point
Q134. Why do face-centered cubic crystals have higher packing efficiency than body-centered cubic?
Answer: FCC:
- Atoms touch along face diagonal
- More efficient arrangement
- Packing efficiency = 74%
- Coordination number = 12
BCC:
- Atoms touch along body diagonal
- Less efficient
- Packing efficiency = 68%
- Coordination number = 8
Q135. What causes the yellow color of NaCl crystals when heated in sodium vapor?
Answer: When NaCl heated in Na vapor:
- Cl⁻ ions diffuse out creating vacancies
- Na atoms deposit on surface
- Release electrons: Na → Na⁺ + e⁻
- Electrons trapped in Cl⁻ vacancies (F-centers)
- F-centers absorb blue-violet light
- Appears yellow (complementary color)
Q136. What is the difference between intrinsic and extrinsic semiconductors?
Answer:
| Intrinsic | Extrinsic |
|---|---|
| Pure semiconductor | Doped semiconductor |
| Low conductivity | Higher conductivity |
| Equal electrons and holes | Unequal charge carriers |
| Example: Pure Si, Ge | Example: Si doped with P or B |
Q137. How many unit cells are present in a cubic-shaped ideal crystal of NaCl of mass 1.00 g?
Answer: Given:
- Mass = 1.00 g
- Molar mass of NaCl = 58.5 g/mol
- Formula units per unit cell (Z) = 4
Moles = 1.00/58.5 Formula units = (1.00/58.5) × Nₐ Unit cells = (1.00/58.5) × Nₐ / 4 = 1.00 × 6.022 × 10²³ / (58.5 × 4) ≈ 2.57 × 10²¹ unit cells
Q138. Why is caesium chloride more stable than sodium chloride structure for large cations?
Answer: For large cations (large r⁺/r⁻):
- CsCl structure has CN = 8
- Can accommodate larger cation
- Maximum ion contact
- More stable
NaCl structure:
- CN = 6
- Suitable for smaller r⁺/r⁻ ratio
- Large cation would have poor contact
Q139. What is meant by 'nearest neighbor distance' in crystals?
Answer: Nearest neighbor distance is the minimum distance between the centers of two adjacent atoms/ions in the crystal lattice.
For metals in FCC:
- Distance = a/√2 (face diagonal/2)
Determines:
- Atomic/ionic radius
- Bond length
- Packing arrangement
Q140. Why are tetrahedral voids smaller than octahedral voids?
Answer: Tetrahedral void:
- Surrounded by 4 spheres
- Smaller space
- Radius ratio = 0.225
Octahedral void:
- Surrounded by 6 spheres
- Larger space
- Radius ratio = 0.414
More surrounding spheres → larger void space.
Q141. What is the maximum number of layers in close packing before repetition?
Answer: For HCP: Maximum 2 layers (ABAB...)
For CCP (FCC): Maximum 3 layers (ABCABC...)
Beyond this, the sequence repeats. These are the only two ways to achieve 74% packing efficiency.
Q142. Why is the melting point of ionic crystals high?
Answer: Ionic crystals have high melting point because:
- Strong electrostatic forces between oppositely charged ions
- Large lattice energy
- Requires high energy to break ionic bonds
- Separate ions from lattice
Example: NaCl melts at 801°C
Q143. What are ferrites? Give their uses.
Answer: Ferrites are ferrimagnetic compounds with formula MFe₂O₄ where M = Fe, Ni, Co, Cu, Mg, Zn.
Structure: Inverse spinel
Uses:
- Magnetic storage devices
- Transformer cores
- Microwave devices
- Computer memory
Q144. What is the effect of pressure on melting point of solids?
Answer: General effect:
- Increased pressure → increases melting point
- Forces molecules closer
- Stabilizes solid state
Exception:
- Ice → water (density increases)
- Pressure decreases melting point of ice
Q145. Why do metals have high thermal conductivity?
Answer: Metals have high thermal conductivity because:
- Free electrons present
- Electrons gain kinetic energy from heat
- Move rapidly through the structure
- Transfer energy quickly
- Efficient heat conduction
Q146. What is the relation between atomic radius (r) and edge length (a) for different cubic structures?
Answer: Simple cubic: a = 2r
BCC: a = 4r/√3
FCC: a = 2√2r OR a = 4r/√2
These relations derived from where atoms touch in each structure.
Q147. In a compound, atoms A form ccp lattice and atoms B occupy 1/3rd of tetrahedral voids. What is the formula?
Answer: In CCP (FCC):
- Number of A atoms = 4
- Number of tetrahedral voids = 8
- B atoms occupy 1/3 of voids = 8/3
A : B = 4 : 8/3 = 4 : 8/3 = 12 : 8 = 3 : 2
Formula: A₃B₂
Q148. What are metamaterials?
Answer: Metamaterials are artificially engineered materials with properties not found in nature.
Properties:
- Negative refractive index
- Electromagnetic cloaking
- Super lenses
Applications: Optical devices, telecommunications, sensors
Q149. Why does the density of crystal decrease due to Schottky defect?
Answer: In Schottky defect:
- Atoms/ions removed from lattice
- Mass of unit cell decreases
- Volume remains approximately same
- Since ρ = m/V
- When m decreases, ρ decreases
Q150. Why does the density of crystal remain unchanged in Frenkel defect?
Answer: In Frenkel defect:
- Ion displaced (not removed)
- No loss of mass
- Volume unchanged
- Since ρ = m/V
- Both m and V constant
- ρ remains same
Q151. Calculate the number of octahedral voids in a sample containing 1 mole of atoms in hcp arrangement.
Answer: In close packing (hcp or ccp):
- Number of octahedral voids = number of atoms
For 1 mole of atoms:
- Number of octahedral voids = Nₐ = 6.022 × 10²³
Q152. What percentage of the lattice sites are vacant in the Schottky defects of a crystal if density decreases by 1%?
Answer: Decrease in density = percentage of vacant sites
If density decreases by 1%:
- Approximately 1% of sites are vacant
Since density ∝ number of particles per unit volume.
Q153. Why are solids with Frenkel defects better conductors than pure crystals?
Answer: Frenkel defect creates:
- Interstitial ions (displaced ions)
- Ions can move more easily
- Enhanced ionic conductivity
- Especially at elevated temperatures
Interstitial ions are more mobile than lattice ions.
Q154. What are quasi-crystals?
Answer: Quasi-crystals are ordered structures that lack periodic translational symmetry.
Properties:
- Have 5-fold rotational symmetry
- Non-repeating patterns
- Discovered by Dan Shechtman (2011 Nobel Prize)
Example: Al-Mn alloys
Q155. In a solid AB having NaCl structure, A atoms occupy the corners of the cubic unit cell. What is the formula if all face-centered atoms are missing from one face?
Answer: Normal NaCl FCC:
- A at corners = 8 × 1/8 = 1
- B at faces = 6 × 1/2 = 3
- Formula: AB₃
One face missing:
- A = 1
- B = 5 × 1/2 = 2.5
- Formula: A₂B₅
Q156. What are the seven crystal systems?
Answer: The 7 crystal systems based on unit cell parameters:
- Cubic - a=b=c, α=β=γ=90°
- Tetragonal - a=b≠c, α=β=γ=90°
- Orthorhombic - a≠b≠c, α=β=γ=90°
- Monoclinic - a≠b≠c, α=γ=90°≠β
- Triclinic - a≠b≠c, α≠β≠γ
- Hexagonal - a=b≠c, α=β=90°, γ=120°
- Rhombohedral - a=b=c, α=β=γ≠90°
Q157. What is the value of Avogadro's number? How is it used in solid state calculations?
Answer: Avogadro's number (Nₐ) = 6.022 × 10²³
Used in:
- Calculating density: ρ = (Z × M)/(a³ × Nₐ)
- Finding number of unit cells per mole
- Converting between microscopic and macroscopic quantities
- Determining number of atoms/ions in crystals
Q158. Why are metal oxides generally ionic while non-metal oxides are covalent?
Answer: Metal oxides (ionic):
- Large electronegativity difference
- Complete electron transfer
- Form ionic bonds
- Example: Na₂O, MgO
Non-metal oxides (covalent):
- Small electronegativity difference
- Electron sharing
- Form covalent bonds
- Example: CO₂, SO₂
Q159. Calculate the void volume percentage in a metal with BCC structure.
Answer: For BCC:
- Packing efficiency = 68%
- Void volume = 100 - 68 = 32%
Nearly one-third of the volume is empty space.
Q160. What is the contribution of an atom that lies completely inside the unit cell?
Answer: An atom lying completely inside the unit cell:
- Not shared with any other unit cell
- Contribution = 1 (complete atom)
Example: Body-centered atom in BCC
Q161. Why are glasses called amorphous solids?
Answer: Glasses are called amorphous because:
- No long-range order in structure
- Random arrangement of SiO₄ tetrahedra
- No definite geometrical shape
- Melt over temperature range
- Isotropic properties
Similar to liquid structure but rigid.
Q162. In a hypothetical solid C₃N₄, nitrogen atoms are at the corners of the cubic unit cell. What is the formula if C atoms are present at body-center and face-centers?
Answer: N atoms (corners): 8 × 1/8 = 1
C atoms:
- Body center = 1
- Face centers = 6 × 1/2 = 3
- Total C = 4
C : N = 4 : 1
Formula: C₄N (not C₃N₄ as given in problem)
Q163. What is thermal expansion? How does it relate to crystal structure?
Answer: Thermal expansion is the increase in dimensions of a solid on heating.
Relation to crystal structure:
- Anisotropic in non-cubic crystals (different expansion in different directions)
- Isotropic in cubic crystals
- Depends on bond strength and type
- Metals generally have higher expansion than ceramics
Q164. What is X-ray diffraction? State its use in solid state.
Answer: X-ray diffraction is the scattering of X-rays by crystal lattice planes.
Uses:
- Determine crystal structure
- Calculate unit cell dimensions
- Identify phase composition
- Determine crystallite size
Bragg's Law: nλ = 2d sinθ
Q165. What is Bragg's equation? Explain the terms.
Answer: Bragg's equation: nλ = 2d sinθ
Where:
- n = order of diffraction (1, 2, 3...)
- λ = wavelength of X-rays
- d = distance between crystal planes
- θ = angle of incidence
Used to calculate interplanar distances in crystals.
Q166. Why do ionic solids dissolve in polar solvents?
Answer: Ionic solids dissolve in polar solvents because:
- Ion-dipole interactions form between ions and polar molecules
- Energy released > lattice energy
- Ions get hydrated/solvated
- Stabilized by solvent molecules
Example: NaCl dissolves in water
Q167. Why don't ionic solids dissolve in non-polar solvents?
Answer: Ionic solids don't dissolve in non-polar solvents because:
- No ion-dipole interaction
- Cannot overcome strong lattice energy
- Non-polar molecules cannot stabilize ions
- "Like dissolves like" principle
Example: NaCl doesn't dissolve in benzene
Q168. What happens to the band gap of semiconductors at higher temperature?
Answer: At higher temperature:
- Lattice vibrations increase
- Band gap slightly decreases
- More electrons can jump to conduction band
- Conductivity increases
Effect is small but measurable.
Q169. What is the difference between crystallization and solidification?
Answer: Crystallization:
- Formation of ordered crystalline structure
- Slow, controlled cooling
- Results in crystalline solid
Solidification:
- General term for liquid → solid transition
- Can form crystalline OR amorphous solid
- Depends on cooling rate
All crystallization is solidification, but not vice versa.
Q170. What are liquid crystals?
Answer: Liquid crystals are substances that show properties intermediate between liquids and crystals.
Properties:
- Flow like liquids
- Ordered molecular arrangement
- Anisotropic optical properties
Uses: LCD displays, thermometers
Q171. Calculate the density of a crystal if Z=4, M=60 g/mol, a=400 pm, Nₐ=6.022×10²³.
Answer: Formula: ρ = (Z × M)/(a³ × Nₐ)
Given:
- Z = 4, M = 60 g/mol
- a = 400 pm = 400 × 10⁻¹⁰ cm = 4 × 10⁻⁸ cm
- a³ = 64 × 10⁻²⁴ cm³
ρ = (4 × 60)/(64 × 10⁻²⁴ × 6.022 × 10²³) = 240/(64 × 0.6022) = 240/38.54 ≈ 6.23 g/cm³
Q172. Why are covalent crystals very hard?
Answer: Covalent crystals are very hard because:
- Strong covalent bonds throughout 3D network
- All bonds must be broken for deformation
- High bond energy
- Directional bonding prevents slip
Example: Diamond (hardest natural substance)
Q173. What is meant by primitive and non-primitive unit cells?
Answer: Primitive unit cell:
- Particles only at corners
- 1 particle per unit cell
- Example: Simple cubic
Non-primitive unit cell:
- Particles at corners plus other positions
- More than 1 particle per unit cell
- Example: BCC (2), FCC (4)
Q174. What are the types of unit cells in cubic system?
Answer: Three types of cubic unit cells:
-
Simple cubic (Primitive)
- Atoms only at corners
- Z = 1
-
Body-centered cubic (BCC)
- Corners + body center
- Z = 2
-
Face-centered cubic (FCC)
- Corners + face centers
- Z = 4
Q175. Why does ZnO appear yellow on heating?
Answer: When ZnO heated:
- Loses oxygen: ZnO → Zn + ½O₂
- Zn atoms go to interstitial sites
- Electrons trapped at these sites
- Forms metal excess defect
- Absorbs blue-violet light
- Appears yellow
- Becomes white on cooling (oxygen returns)
Q176. What are n-type materials? How are they formed?
Answer: n-type materials are semiconductors with electrons as majority charge carriers.
Formation:
- Dope Si/Ge with group 15 element (P, As, Sb)
- Dopant has extra valence electron
- Extra electron becomes free
- Negative charge carriers
Example: Si doped with P
Q177. What are p-type materials? How are they formed?
Answer: p-type materials are semiconductors with holes as majority charge carriers.
Formation:
- Dope Si/Ge with group 13 element (B, Al, Ga)
- Dopant has one less valence electron
- Creates electron deficiency (hole)
- Positive charge carriers
Example: Si doped with B
Q178. What is the effect of Schottky defect on electrical conductivity?
Answer: Schottky defect increases electrical conductivity because:
- Creates ion vacancies
- Neighboring ions can jump into vacancies
- Ionic mobility increases
- Conductivity increases
- Effect pronounced at high temperature
Q179. Why are most metals ductile?
Answer: Metals are ductile because:
- Non-directional metallic bonding
- Sea of electrons surrounds cations
- Layers can slide without breaking bonds
- Electron cloud maintains bonding
- Can be drawn into wires
Exception: Some metals like Zn are brittle
Q180. What is the effect of metal deficiency defect on crystal properties?
Answer: Metal deficiency defect causes:
- Composition - Non-stoichiometric formula
- Electrical conductivity - Increases (holes move)
- Color - May change
- Magnetic properties - Can alter
- Density - Slightly decreases
Example: FeO (Fe₀.₉₅O)
Q181. Calculate the percentage efficiency of packing in case of a metal crystal for simple cubic structure.
Answer: Simple cubic packing efficiency:
Volume occupied = (4/3)πr³ Volume of unit cell = a³ = (2r)³ = 8r³
Efficiency = [(4/3)πr³ / 8r³] × 100 = [(4π/3) / 8] × 100 = (π/6) × 100 = 52.4%
Q182. Why is boron doped silicon a p-type semiconductor?
Answer: When Si doped with B:
- Si has 4 valence electrons
- B has 3 valence electrons
- One bond incomplete (electron deficiency)
- Creates hole (positive charge carrier)
- Holes can move and conduct
- p-type semiconductor
Q183. What are the conditions favoring Schottky defect?
Answer: Conditions for Schottky defect:
- Similar size of cations and anions
- High coordination number
- Highly ionic compounds
- High lattice energy
- Small difference in charge
Example: NaCl, KCl, AgBr
Q184. What are the conditions favoring Frenkel defect?
Answer: Conditions for Frenkel defect:
- Large size difference between ions
- Low coordination number
- Small cation, large anion
- Available interstitial sites
Example: AgCl, AgBr, ZnS
Q185. How many types of close packing are possible in crystals?
Answer: Two types of close packing:
-
Hexagonal Close Packing (HCP)
- Layer sequence: ABAB...
- Hexagonal unit cell
-
Cubic Close Packing (CCP/FCC)
- Layer sequence: ABCABC...
- Cubic unit cell
Both have 74% packing efficiency and CN = 12
Q186. What is meant by interstitial compound?
Answer: Interstitial compounds are formed when small atoms (H, B, C, N) occupy interstitial voids in metal lattices.
Properties:
- High melting point
- Very hard
- Retain metallic conductivity
Examples: TiC, TiH₂, VH₀.₅₆
Q187. Why are ionic solids soluble in water?
Answer: Ionic solids dissolve in water because:
- Strong ion-dipole interactions with water
- Hydration energy released > lattice energy
- Ions get hydrated by water molecules
- Enthalpy change favorable
- Entropy increases (disorder)
Q188. How many nearest neighbours does each atom have in a body-centered cubic structure?
Answer: In BCC structure:
- Each atom has 8 nearest neighbors
- Body center atom surrounded by 8 corner atoms
- Each corner atom surrounded by 8 body center atoms (from adjacent cells)
- Coordination number = 8
Q189. How many nearest neighbours does each atom have in hexagonal close packing?
Answer: In HCP structure:
- 6 neighbors in same layer
- 3 neighbors in layer above
- 3 neighbors in layer below
- Total = 12 nearest neighbors
- Coordination number = 12
Q190. What is the relationship between radius of void and radius of sphere?
Answer: For tetrahedral void:
- r(void)/r(sphere) = 0.225
For octahedral void:
- r(void)/r(sphere) = 0.414
For cubic void:
- r(void)/r(sphere) = 0.732
These are limiting radius ratios.
Q191. How many atoms can be assigned to a given unit cell in case of simple cubic lattice?
Answer: Simple cubic lattice:
- 8 corner atoms × (1/8 contribution) = 1
- Number of atoms = 1
Only corners are occupied, no other positions.
Q192. What type of substances exhibit antiferromagnetism?
Answer: Antiferromagnetic substances:
- Have unpaired electrons
- Magnetic moments equal and opposite
- Net magnetic moment = zero
- Weak magnetic properties
Examples: MnO, Cr₂O₃, FeO, MnO₂
Q193. What is meant by 'doping' in semiconductors? Why is it done?
Answer: Doping = adding controlled impurity to pure semiconductor
Purpose:
- Increase conductivity significantly
- Create n-type or p-type material
- Control electrical properties
- Make useful electronic devices
Dopants: Group 13 or 15 elements
Q194. Why do solids have lower energy than liquids and gases?
Answer: Solids have lower energy because:
- Particles closely packed
- Strong intermolecular forces
- Particles in lowest potential energy state
- Maximum attractive interactions
- Stable configuration
Energy order: Solid < Liquid < Gas
Q195. What happens to semiconductor conductivity at absolute zero temperature?
Answer: At absolute zero (0 K):
- No thermal energy available
- Electrons cannot jump to conduction band
- Valence band completely filled
- Conduction band empty
- Acts as perfect insulator
- Zero conductivity
Q196. What are mixed oxides? Give example.
Answer: Mixed oxides are compounds containing two or more different metals with oxygen.
Examples:
- Fe₃O₄ = FeO + Fe₂O₃ (mixed oxide of Fe²⁺ and Fe³⁺)
- Mn₃O₄ = MnO + Mn₂O₃
- Pb₃O₄ = 2PbO + PbO₂
Show mixed oxidation states.
Q197. What is meant by anion excess defect?
Answer: Anion excess defect (rare) occurs when:
- Extra anions occupy interstitial sites
- Trapped electrons maintain neutrality
- Example: Theoretical concept
More common is metal excess defect which appears similar but involves cation chemistry.
Q198. Why are F-centers paramagnetic?
Answer: F-centers are paramagnetic because:
- Contain unpaired electrons
- Electrons trapped in anionic vacancies
- Magnetic moment due to unpaired electron
- Attracted by magnetic field
- Show paramagnetic behavior
Q199. What is the effect of temperature on Frenkel defect?
Answer: With increasing temperature:
- Number of Frenkel defects increases
- More thermal energy available
- Helps ions overcome energy barrier
- More ions displaced to interstitial sites
- Conductivity increases
Q200. Why do crystalline solids have characteristic heat of fusion?
Answer: Crystalline solids have characteristic heat of fusion because:
- All bonds identical (same strength)
- Regular structure throughout
- All bonds break at same temperature
- Specific energy needed
- Sharp melting point → definite heat of fusion
Amorphous solids lack this property.
