The Solid State

Step 1: Definition.
Crystalline solids are those solids in which the constituent particles (atoms, ions, or molecules) are arranged in a definite, orderly, and repeating pattern in three-dimensional space.
Step 2: Characteristics.
- They have a definite geometric shape.
- Sharp melting point.
- Long-range order.
- Anisotropic in nature (properties vary in different directions).
Step 3: Examples.
Sodium chloride (NaCl), Diamond, Quartz, and Ice are examples of crystalline solids.
Step 4: Explanation.
In NaCl, the arrangement of Na and Cl ions is highly regular, forming a cubic lattice, which is a characteristic feature of crystalline solids.

Step 1: Definition.
Point defects are irregularities or deviations from the ideal arrangement of atoms or ions at specific lattice points in a crystalline solid.
Step 2: Explanation.
They occur due to the absence, displacement, or presence of extra atoms or ions at lattice sites. These defects affect the physical and electrical properties of the crystal.
Step 3: Types of point defects.
1. Stoichiometric defects – e.g., Schottky and Frenkel defects.
2. Non-stoichiometric defects – due to excess or deficiency of ions.
3. Impurity defects – caused by foreign atoms.
Step 4: Example.
In NaCl crystal, when equal numbers of Na and Cl ions are missing from their lattice sites, it produces a Schottky defect.

Step 1: Recall definition.
The coordination number is the number of nearest neighbours surrounding a particle in a crystal lattice.
Step 2: hcp structure.
In hexagonal close packing, each atom is surrounded by 12 equidistant atoms (6 in the same layer, 3 above, and 3 below).
Step 3: Conclusion.
Thus, the coordination number in hcp is 12.

Step 1: Definition.
A solution of a gas in solid occurs when gas molecules are uniformly distributed throughout a solid medium. Such solutions are often called solid solutions of gases.
Step 2: Mechanism.
In these solutions, the gas particles occupy interstitial or void spaces within the crystal lattice of the solid without disturbing its structure.
Step 3: Example.
- Hydrogen in palladium (H in Pd) — Hydrogen gas gets adsorbed into palladium metal forming a solid solution.
This property is used in hydrogen storage and catalytic reactions.
Step 4: Explanation.
Palladium can absorb up to 900 times its volume of hydrogen due to the formation of a stable interstitial solid solution.

Step 1: Recall the crystal systems.
In crystallography, the seven crystal systems are: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and rhombohedral (trigonal).
Step 2: Eliminate wrong options.
- 8: Incorrect (there are 8 Bravais lattices types in cubic only, not crystal systems).
- 6: Incorrect.
- 4: Incorrect.
Step 3: Conclusion.
Thus, there are 7 crystal systems.

Step 1: Types of solids.
- Metallic solids: Iron, Copper.
- Ionic solids: Sodium chloride.
- Covalent (network) solids: Diamond, quartz.
Step 2: Analysis.
Diamond is a covalent solid because each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a giant covalent lattice.
Step 3: Conclusion.
Thus, the correct answer is (B) Diamond.

Step 1: Geometry of the void.
In close packing, an octahedral void is created at the center of six touching spheres: three from a lower triangular layer and three from the adjacent upper triangular layer, the triangles being staggered.
Step 2: Conclusion.
Hence, an octahedral hole is coordinated by 6 spheres option (A).

Step 1: Definition of Brownian Movement.
Brownian motion, also known as Brownian movement, is the random motion of particles suspended in a fluid (liquid or gas) resulting from their collision with fast atoms or molecules in the gas or liquid. This motion was first observed by botanist Robert Brown in 1827.
Step 2: Explanation.
The suspended particles, when viewed under a microscope, exhibit continuous, erratic motion. This is due to the constant, random bombardment by molecules in the surrounding medium. Brownian motion provides direct evidence for the kinetic theory of matter, which explains the nature of atoms and molecules in constant motion.
Step 3: Characteristics of Brownian Motion.
- It occurs in liquids and gases.
- The motion is unpredictable and random.
- It is a result of the thermal energy of the medium.
- It helps explain diffusion processes.

Step 1: Definition of Network Solids.
Network solids are a class of solids where the atoms are bonded together in a continuous network, extending throughout the material. The atoms are connected by strong covalent bonds, forming a giant structure with no distinct molecules. These solids tend to have high melting points, are generally hard, and are poor conductors of electricity.
Step 2: Example.
A common example of a network solid is diamond, where each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a three-dimensional network. Other examples include quartz (SiO₂) and graphite.

Step 1: Definition of Schottky Defect.
A Schottky defect occurs in ionic crystals when an equal number of cations and anions are missing from the lattice sites, creating vacancies. This type of defect maintains the electrical neutrality of the crystal but leads to a decrease in density because of the missing ions.
Step 2: Example.
In sodium chloride (NaCl), a Schottky defect occurs when one Na⁺ ion and one Cl⁻ ion are removed from their lattice sites, creating two vacancies.

Step 1: Understanding the NaCl crystal structure.
The structure of sodium chloride (NaCl) is face-centred cubic (FCC), where each sodium ion is surrounded by six chloride ions and vice versa, in a 3D arrangement. This arrangement is characteristic of an FCC lattice. Step 2: Analysis of options.
(A) Hexagonal close packing: This is a structure where particles are closely packed in a hexagonal arrangement. NaCl does not follow this structure.
(B) Face centred cubic: Correct — NaCl adopts a face-centred cubic (FCC) lattice, which is one of the most common structures for ionic crystals.
(C) Square planar: This is not the correct structure for NaCl, as square planar arrangements occur in two-dimensional structures.
(D) Body centred cubic: This is not the correct arrangement for NaCl, as it does not form a body-centred cubic structure.
Step 3: Conclusion.
The correct answer is (B) because NaCl has a face-centred cubic crystal structure.

Step 1: Crystalline vs. amorphous.
Crystalline solids have long–range periodic order (regular repeating arrangement of particles). Amorphous solids lack long–range order and are often called “supercooled liquids.”
Step 2: Analyze options.
Diamond and graphite are crystalline allotropes of carbon (well-defined lattices).
Common salt (NaCl) is ionic and crystalline (face-centred cubic lattice).
Glass has no long-range periodic arrangement — it is amorphous.
Step 3: Conclusion.
Therefore, the amorphous solid among the options is glass (D).

Step 1: Understanding diamond structure.
Diamond has a crystal structure where each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a very strong, three-dimensional network. This makes diamond a covalent crystal. Step 2: Analysis of options.
(A) Ionic crystal: Ionic crystals are held together by ionic bonds (electrostatic attraction between oppositely charged ions). This is not the case for diamond.
(B) Covalent crystal: Correct — Diamond is a covalent crystal, where atoms are held together by strong covalent bonds.
(C) Molecular crystal: Molecular crystals consist of molecules held together by intermolecular forces, which is not the case with diamond.
(D) Metallic crystal: Metallic crystals are held together by metallic bonds, which is not the case for diamond.
Step 3: Conclusion.
The correct answer is (B) because diamond is a covalent crystal where atoms are bonded by strong covalent bonds.

Step 1: Understanding Bravais lattices.
Bravais lattices describe the different ways atoms can be arranged in a crystalline solid. There are 7 distinct types of Bravais lattices in three-dimensional space. These lattices are categorized based on their symmetry and are fundamental in crystallography.
Step 2: Explanation of other options.
- 23, 30, and 14 are incorrect as there are only 7 distinct Bravais lattices.
Final Answer:

Step 1: Understanding body-centred cubic (BCC) unit cell.
In a body-centred cubic (BCC) unit cell, atoms are located at the eight corners and at the center of the cell. The space not occupied by the atoms is free space.
Step 2: Calculation of free space.
- The volume of the BCC unit cell is determined by the length of its edge and the volume of the atoms inside it. - The percentage of free space is calculated by comparing the total volume of the unit cell to the volume occupied by the atoms. The percentage of free space in a BCC unit cell is approximately 28%.
Final Answer:

Step 1: Understanding body-centred cubic (BCC) unit cell.
In a body-centred cubic unit cell, there is one atom at each corner of the cube (each shared by 8 neighboring cubes) and one atom in the centre of the cube. Therefore, the total number of atoms in a BCC unit cell is:
Step 2: Explanation of other options.
- The number of atoms in face-centred cubic (FCC) unit cells is 4. - In simple cubic unit cells, the number of atoms is 1. - The number of atoms in a unit cell with 12 atoms would correspond to a different structure.
Final Answer:

Step 1: Understanding crystalline and amorphous solids.
Crystalline solids have a well-ordered structure with atoms arranged in a regular pattern. Amorphous solids, like glass, lack a well-defined structure.
Step 2: Explanation of other options.
- KCl and CsCl are both ionic solids that form a regular crystalline structure. - Rhombic sulphur is a crystalline form of sulfur.
Final Answer: