CBSE Chapter 1: Solid State Class 12 Notes
A Solid-state is a type of state of matter, in addition to liquid and gaseous forms. Solids have relatively few empty spaces between atoms, ions, and molecules and very strong intermolecular interactions. They thus have a predetermined volume and shape.
Characteristic Properties of Solids
(i) They have a distinct mass, volume, and shape.
(ii) The distances between molecules are small.
(iii) There are powerful intermolecular forces.
(iv) The atoms, molecules, or ions that make them have fixed positions and are only capable of oscillating about their mean positions.
(v) They are stiff and incompressible.
Types of Solids
- Crystalline solids
- Amorphous solids
Crystalline solids have a consistent structure throughout the entire volume and sharp properties while amorphous solids have an irregular structure over large distances. The table below details the many variations.
Properties of crystalline solids
- They clearly have a geometric shape.
- They follow a long-term plan.
- They melt at a rapid rate.
- They are anisotropic in nature, which means that when measured along with several directions in the same crystal, their physical properties exhibit varied values.
- They exhibit a distinct and recognizable heat of fusion.
- We refer to them as genuine solids.
- They break into two pieces when cut with a sharp-edged tool, and the freshly formed surfaces are plain and smooth.
Properties of amorphous solids
- They are shaped strangely.
- They operate on a limited range.
- Over a variety of temperatures, they soften gradually.
- Since they are isotropic, their physical characteristics are constant in all directions.
- They split into two pieces with asymmetrical surfaces when sliced with a sharp-edged instrument.
- They do not possess distinct fusion heat.
- They are referred to as supercooled liquids or pseudo-solids. This is due to their propensity to flow, albeit extremely slowly.
Classification of Crystalline Solids
- Ionic solids
- Covalent or Network solids
- Molecular solids
- Metallic solids
Characteristics of Crystalline Solids
- Ionic Solids
- Constituent Particles: Ions
- Bonding/Attractive Forces: Coulombic or Electrostatic
- Electrical Conductivity: Insulators are in solid-state but conduct in a molten state and in aqueous solutions
- Physical Nature: Hard but brittle
- Melting Point: High
- Examples: ZnS, MgO, NaCl
- Covalent or Network Solids
- Constituent Particles: Atoms
- Bonding/Attractive Forces: Covalent bonding
- Electrical Conductivity: Conductors in solid-state as well as in molten state
- Physical Nature: Hard but malleable and ductile
- Melting Point: Fairly high
- Examples: (quartz), SiC, C (diamond), C(graphite)
- Molecular Solids
Constituent Particles: Molecules
Bonding/Attractive Forces: Van Der Waals dipole-dipole
Electrical Conductivity: Conductors and insulators
Physical Nature: Very soft
Melting Point: Low melting point
Examples: Iodine, Camphor, Naphthalene
- Metallic Solids
Constituent Particles: Positive ions in a sea of delocalized electrons
Bonding/Attractive Forces: Metallic bonding
Electrical Conductivity: Conductors in solid-state as well as in molten state
Physical Nature: Hard but malleable and ductile
Melting Point: Fairly high
Examples: Fe, Cu, Ag, Mg
Crystal Lattices and Unit Cells
The unit cell, the fundamental component of a crystal, is the smallest repeating unit of the crystal lattice.
Types Of Unit Cell
- Primitive Cubic Unit Cell
- Body-centered Cubic Unit Cell
- Face-centered cubic unit cell
Atoms make up the structure of a crystal. Points make up the lattice of a crystal. A group of axes makes up a crystal system. Alternatively put, the structure is an organized collection of atoms, ions, or molecules.
Characteristics of Crystal Lattice
(a) A lattice’s individual points are referred to as sites or lattice points.
(b) In a crystal lattice, each point corresponds to one constituent particle, which could be an atom, a molecule (a collection of atoms), or an ion.
(c) To highlight the lattice’s geometry, straight lines are used to connect the lattice’s points.
Number of Atoms in a Unit Cell
- Primitive unit cells have 1atom
- Face-centered unit cells have 3 atoms
- Body-centered unit cells have 2atoms
Seven Crystal Systems
- Cubic: 90° ,a=b=c
- Tetragonal: 90° ; a=bc
- Orthorhombic: 90°; ABC
- Monoclinic: 90°,90°; ABC
- Hexagonal: 90°,=120°; a=bc
- Rhombohedral or trigonal: 90°;a=b=c
- Triclinic: 90°; ABC
It is described as the proportion of the unit cell’s volume that the spheres occupy to the total volume of the unit cell.
(i) Primitive cubic unit cell: Atoms touch each other along edges.
Hence, d = a or r = a / 2
(r = radius of atom and a = edge length)
Therefore, PF = 4 / 3 πr3 / (2r)3 = 0.524 or 52.4%
(ii) Face-centered cubic unit cell: Atoms touch each other along the face diagonal.
Hence, d = a / √2
or r = √2a / 4
Therefore; PF = 4 * 4 / 3 πr3 / (4r / √2)r3 = 0.74 or 74%
(iii) Body-centered cubic unit cell: Atoms touch each other along the body diagonal.
Hence, √3a / 2
or r = √3a / 4
Therefore; PF = 2 * 4 / 3 πr3 / (4r / √3)r3 = 0.68 or 68%
It is described as the number of particles in the crystal lattice that are directly close to one another.
[CN is 6 in a simple cubic lattice, 8 in the body-centered lattice, and 12 in a face-centered cubic lattice]
High temperature lowers the CN whereas high pressure raises it.
Close Packing in Crystals
- Two Dimensional Packing of Constituent Particles
(i) Square close packing Space occupied by spheres is 52.4%.
(ii) Hexagonal close-packing Space occupied by spheres is 60.4%. Hence. It is more efficient.
- Three Dimensional Packing of Constituent Particles
(i)The ABAB configuration results in hexagonal close packing (hcp).
(ii) ABCABC arrangement results in face-centered Cubic packing or cubic close packing (ccp or fcc).
- Both of these configurations occupy 740/0 space.
- In a hop and ccp arrangement, the coordination number is 12, while in a bcc arrangement, it is 8.
- Close atom packing in a cubic structure results in fcc > bcc > sc.
- Except for He, all noble gases have a ccp structure (hcp structure).
Void or Space or Holes
- Void, space, hole, or interstitial void are all terms used to describe the empty or blank space between two spheres in a unit cell. Two types of voids are produced when particles are closed-packed, resulting in either a cpp or hcp structure:
- Tetrahedral voids are openings or voids that are present at each corner of a tetrahedron and are encircled by four spheres. A tetrahedral void has a coordination number of four.
- On a typical tetrahedron are holes encircled by six spheres known as octahedral voids. The octahedral void has a coordination number of 6.
The density of Unit Cell (d)
The density of unit ce11 = mass of unit cell/volume of the unit cell
d = Z * M / a3 = ZM / a3 * NA
(The density of the unit cell is the same as the density of the substance.)
where, d = density of unit cell
M = molecular weight
Z = no. of atoms per unit cell
NA = Avogadro number
a = edge length of the unit cell
Imperfections in Solids
Atoms, ions, and molecules are arranged in a definite, repeating pattern in crystalline solids, yet the pattern may contain certain imperfections. If additional particles are present or the cooling process is quick, deviations from the ideal arrangement may happen.
There are two different kinds of faults: line defects and point defects.
- Point defects: In a crystalline solid, point defects are irregularities or deviations from the ideal arrangement around a point or an atom.
- Line Defects: In full rows of lattice points, there may be inconsistencies or deviations from the ideal arrangement known as line defects. These imperfections are referred to as crystal flaws.
Types of Point Defects
Three categories of point defects exist:
- Stoichiometric defect – In this type of point defect, the solid’s electrical neutrality and ion ratio (Stoichiometry) are unaffected. It may also go by the name intrinsic or thermodynamic flaws. They basically fall into two categories: Interstitial defect and Vacancy defect
- Frenkel defect – Typically, in ionic solids, the smaller ion (cation) displaces and takes up space in between molecules. In this instance, the interstitial defect is experienced at the new position while the vacancy defect is formed at the original position.
- Schottky defect– Ionic solids contain this type of vacancy defect. However, in ionic compounds, we must balance the compound’s electrical neutrality such that an equal number of anions and cations will be absent. It lessens the substance’s density. Cations and anions in this are roughly the same sizes.
Based on their conductivities, solids can be divided into three categories. Those are;
The material is typically placed in a constant magnetic field and then the magnetic field is changed in order to analyze the magnetic characteristics of magnetic materials. The five main categories of magnetic behavior are as follows;
(i) Diamagnetic materials
(ii) Paramagnetic materials
(iii) Ferromagnetic materials
(iv) Antiferromagnetic materials
(v) Ferrimagnetic materials
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