CBSE Class 12 Chemistry Chapter 1: The Solid State - NCERT Solutions

Amorphous solids are characterized by the absence of a long-range, ordered arrangement of their constituent particles. These particles can be atoms, ions, or molecules, and they are arranged in a disordered or irregular manner. Due to this irregular structure, amorphous solids exhibit the following properties:

• They are isotropic, meaning their physical properties (like electrical resistance or refractive index) are the same in all directions.
• They do not have a sharp melting point; instead, they soften gradually over a range of temperatures.
• They are sometimes referred to as 'pseudo solids' or 'super cooled liquids' because they exhibit some liquid-like properties.
• They do not possess a definite heat of fusion.
• When cut with a sharp tool, they tend to break into pieces with irregular surfaces.

Common examples of amorphous solids include:

• Glass
• Rubber
• Plastic
• Certain polymers

The fundamental difference between glass and quartz lies in the arrangement of their constituent particles:

• Glass: It is an amorphous solid. Its constituent particles (silicon and oxygen atoms in a disordered network) exhibit only short-range order. This means there is some degree of order over short distances, but it does not extend throughout the entire solid.
• Quartz: It is a crystalline solid. Its constituent particles are arranged in a highly ordered, repeating three-dimensional pattern, exhibiting both short-range and long-range order.

Conditions for converting quartz into glass:

Quartz can be converted into glass by a process involving heating and rapid cooling. When quartz is heated to a sufficiently high temperature (above its melting point) and then cooled very quickly, the ordered crystalline structure of quartz does not have enough time to reform. Instead, the particles get 'frozen' in a disordered arrangement, resulting in the formation of glass.

The classification of the given solids based on their bonding and structure is as follows:

• Ionic Solids: These solids are formed by the electrostatic attraction between oppositely charged ions. They typically have high melting points and are hard but brittle.
  • (ii) Ammonium phosphate ((NH₄)₃PO₄) - Contains ammonium cations (NH₄⁺) and phosphate anions (PO₄^3-).
  • (x) LiBr - Contains lithium cations (Li⁺) and bromide anions (Br^-).
• Metallic Solids: These solids consist of metal atoms held together by metallic bonds, characterized by a 'sea' of delocalized electrons. They are good conductors of heat and electricity.
  • (viii) Brass - An alloy, which is a mixture of metals (copper and zinc), exhibiting metallic properties.
  • (ix) Rb (Rubidium) - An alkali metal, forming a metallic lattice.
• Molecular Solids: In these solids, molecules are held together by weaker intermolecular forces (like van der Waals forces or dipole-dipole interactions). They are generally soft and have low melting points.
  • (i) Tetra phosphorus decoxide (P₄O₁₀) - Consists of discrete P₄O₁₀ molecules.
  • (iv) I₂ (Iodine) - Consists of discrete I₂ molecules.
  • (v) P₄ (White Phosphorus) - Consists of discrete P₄ molecules.
• Covalent Network Solids: These solids have atoms joined in a continuous network by covalent bonds. They are typically very hard and have very high melting points.
  • (iii) SiC (Silicon Carbide) - A network of silicon and carbon atoms joined by covalent bonds.
  • (vii) Graphite - A form of carbon with a layered structure where atoms within layers are covalently bonded.
  • (xi) Si (Silicon) - A metalloid forming a network covalent structure similar to diamond.
• Amorphous Solids: These solids lack a regular, long-range internal structure.
  • (vi) Plastic - A general term for amorphous polymeric materials.

To determine the atomic mass of an unknown metal, we can utilize the relationship between its density, unit cell dimensions, and the number of atoms within the unit cell. The fundamental formula connecting these quantities is derived from the definition of density:

Density (d) =

\frac{\text{Mass of the unit cell}}{\text{Volume of the unit cell}}

Let:

• 'a' be the edge length of the cubic unit cell.
• 'd' be the density of the metal.
• 'm' be the atomic mass of the metal (which we want to find).
• 'z' be the number of atoms per unit cell. (This value depends on the type of unit cell: z=1 for simple cubic, z=2 for body-centred cubic (BCC), and z=4 for face-centred cubic (FCC)).

The mass of the unit cell can be expressed as the product of the number of atoms in the unit cell and the mass of a single atom (which is related to the atomic mass). The volume of a cubic unit cell is given by the cube of its edge length.

Mass of the unit cell = z × m

Volume of the unit cell = a3

Substituting these into the density formula, we get: d = \frac{z \times m}{a^3}

To find the atomic mass 'm', we can rearrange this formula: m = \frac{d \times a^3}{z}

Therefore, by measuring or knowing the density (d) of the metal, the edge length (a) of its unit cell (often determined using X-ray diffraction), and identifying the type of unit cell to know 'z', we can calculate the atomic mass (m) of the unknown metal.

Coordination Number: The coordination number of an atom in a crystal lattice is defined as the number of nearest neighbouring atoms (or ions or molecules) that are in direct contact with it. It essentially indicates how many atoms are directly surrounding a central atom.

Coordination Numbers in Specific Structures:

• (a) Cubic Close-Packed (CCP) Structure: In a CCP structure (which is identical to the face-centred cubic, FCC, lattice), each atom is surrounded by 12 nearest neighbours. Imagine an atom in the middle layer; it touches 4 atoms in its own layer, 4 atoms in the layer above, and 4 atoms in the layer below. Thus, the coordination number is 12.
• (b) Body-Centred Cubic (BCC) Structure: In a BCC structure, the central atom is located at the body centre of the cube, and it touches the 8 atoms located at the corners of the cube. The atoms at the corners do not touch each other along the edges or faces. Therefore, the coordination number of an atom in a BCC structure is 8.