" The Solid State Chapter 1 Chemistry NCERT Solutions for CBSE Class 12, pdf, notes, Videos, Question bank

NCERT Solutions for Class 12 Chemistry Chapter 1 - The Solid State

The National Council of Education Research and Training (NCERT) (officaial wesite www.ncert.nic.in) sets the curriculum for all schools that follow the Central Board of Secondary Education (CBSE)(officaial wesite cbse.nic.in) across the nation. NCERT solutions for class 12 Chemistry Chapter 1 - The Solid State has been written to help students understand all the content under Chapter 1 - The Solid State in the textbooks prescribed by NCERT and as per the syllabus. The class 12 Chemistry NCERT solutions help students solve the exercises given in the textbooks and get good marks in their board exam.The book is structured in a step-by-step and logical manner, which allows the student to understand the concepts easily.
Class 12 Chemistry NCERT solutions for Chapter 1 The Solid State covers Topics such as General Characteristics of Solid State, Amorphous and Crystalline Solids, Classification of Crystalline Solids, Crystal Lattices and Unit Cells, Number of Atoms in a Unit Cell, Close Packed Structures, Packing Efficiency, Calculations Involving Unit Cell Dimensions, Imperfections in Solids, Electrical Properties and Magnetic Properties.
. The class 12 Chemistry Chapter 1 - The Solid State contains NCERT solutions, Videos, Question bank, pdf, notes and Solutions to the problems in the NCERT textbook are available in PDF format in smart solutions android app "PU Smart learning"..

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Crystal Lattices and Unit Cells


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1.1 General Characteristics of Solid State

The following are the characteristic properties of the solid state:

1. They have definite mass, volume and shape.
2. Intermolecular distances are short.
3. Intermolecular forces are strong.
4. Their constituent particles (atoms, molecules or ions) have fixed positions and can only oscillate about their mean positions.
5. They are incompressible and rigid.

1.2 Amorphous and Crystalline Solids

Solids can be classified as crystalline solid or amorphous solid on the basis of the nature of order present in the arrangement of their constituent particles.

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Crystal Lattices and Unit Cells

crystalline solid

A crystalline solid usually consists of a large number of small crystals, each of them having a definite characteristic geometrical shape. In a crystal, the arrangement of constituent particles (atoms, molecules or ions) is ordered.

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Crystal Lattices and Unit Cells

amorphous solid

An amorphous solid (Greek amorphos = no form) consists of particles of irregular shape. The arrangement of constituent particles (atoms, molecules or ions) in such a solid has only short range order.

1.3 Classification of Crystalline Solids

1.3.1 Molecular Solids

Molecules are the constituent particles of molecular solids. These are further sub divided into the following categories:

1. Non polar Molecular Solids:
2. Polar Molecular Solids:
3. Hydrogen Bonded Molecular Solids:

1.3.2 Ionic Solids

Ions are the constituent particles of ionic solids. Such solids are formed by the three dimensional arrangements of cations and anions bound by strong coulombic (electrostatic) forces. These solids are hard and brittle in nature.

1.3.3 Metallic Solids

Metals are orderly collection of positive ions surrounded by and held together by a sea of free electrons. These electrons are mobile and are evenly spread out throughout the crystal.

Each metal atom contributes one or more electrons towards this sea of mobile electrons. These free and mobile electrons are responsible for high electrical and thermal conductivity of metals. When an electric field is applied, these electrons flow through the network of positive ions.

1.3.4 Covalent or Network Solids

A wide variety of crystalline solids of non-metals result from the formation of covalent bonds between adjacent atoms throughout the crystal. They are also called giant molecules.

Covalent bonds are strong and directional in nature, therefore atoms are held very strongly at their positions. Such solids are very hard and brittle. They have extremely high melting points and may even decompose before melting. They are insulators and do not conduct electricity.Ecample Diamond, silicon carbide.

1.4 Crystal Lattices and Unit Cells

crystal lattice
a regular three dimensional arrangement of points in space is called a crystal lattice.

There are only 14 possible three dimensional lattices. These are called Bravais Lattices. The following are the characteristics of a crystal lattice:

1. Each point in a lattice is called lattice point or lattice site.
2. Each point in a crystal lattice represents one constituent particle which may be an atom, a molecule (group of atoms) or an ion.
3. Lattice points are joined by straight lines to bring out the geometry of the lattice.

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Crystal Lattices and Unit Cells

Unit cell is the smallest portion of a crystal lattice which, when repeated in different directions, generates the entire lattice.

1. A unit cell is characterised by:

1. its dimensions along the three edges, a, b and c. These edges may or may not be mutually perpendicular.

2. angles between the edges, α (between b and c) β (between a and c) and γ (between a and b). Thus, a unit cell is characterised by six parameters, a, b, c, α, β and γ.

Primitive Cubic Unit Cell

1.5.1 Primitive cubic unit cell has atoms only at its corner. Each atom at a corner is shared between eight adjacent unit cells as shown in Figure, four unit cells in the same layer and four unit cells of the upper (or lower) layer. Therefore, only 1/8th of an atom (or molecule or ion) actually belongs to a particular unit cell.

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Crystal Lattices and Unit Cells

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Crystal Lattices and Unit Cells

1.5 Number of Atoms in a Unit Cell

We know that any crystal lattice is made up of a very large number of unit cells and every lattice point is occupied by one constituent particle (atom, molecule or ion). Three types of cubic unit cells and for simplicity assume that the constituent particle is an atom.

1.5.2 Body - Centred Cubic Unit Cell

A body-centred cubic (bcc) unit cell has an atom at each of its corners and also one atom at its body centre. Figure (a) open structure (b) space filling model and (c) the unit cell with portions of atoms actually belonging to it. It can be seen that the atom at the body centre wholly belongs to the unit cell in which it is present. Thus in a body-centered cubic (bcc) unit cell:

(i) 8 corners atoms × 1/8 atom per unit cell = 8 × 1/8 = 1 atom

(ii) 1 body centre atom = 1 × 1 = 1 atom

∴ Total number of atoms per unit cell = 2 atoms

1.5.3 Face - Centred Cubic Unit Cell

A face-centred cubic (fcc) unit cell contains atoms at all the corners and at the centre of all the faces of the cube. It can be seen in the following figure each atom located at the face-centre is shared between two adjacent unit cells and only 1/2 of each atom belongs to a unit cell.

(i) 8 corners atoms × 1/8 atom per unit cell = 8 × 1/8 = 1 atom

(ii) 6 face-centred atoms 6× 1/2 = 3 atom

∴ Total number of atoms per unit cell = 4 atoms

1.6 Close Packed Structures

In solids, the constituent particles are close-packed, leaving the minimum vacant space. Let us consider the constituent particles as identical hard spheres and build up the three dimensional structure in three steps.

(a) Close Packing in One Dimension

There is only one way of arranging spheres in a one dimensional close packed structure, that is to arrange them in a row and touching each other.

In this arrangement, each sphere is in contact with two of its neighbours. The number of nearest neighbours of a particle is called its coordination number.

(b) Close Packing in Two Dimensions

This can be done in two different ways. (i) The second row may be placed in contact with the first one such that the spheres of the second row are exactly above those of the first row.

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Crystal Lattices and Unit Cells

(ii)The second row may be placed above the first one in a staggered manner such that its spheres fit in the depressions of the first row.

(c) Close Packing in Three Dimensions

(i) Three dimensional close packing from two dimensional square close-packed layers:

Three dimensional close packed structure can be generated by placing layers one over the other.

(ii) Three dimensional close packing from two dimensional hexagonal close packed layers:

While placing the second square close-packed layer above the first we follow the same rule that was followed when one row was placed adjacent to the other. The second layer is placed over the first layer such that the spheres of the upper layer are exactly above those of the first layer.

(a) Placing second layer over the first layer

(b) Placing third layer over the second layer

      (i) Covering Tetrahedral Voids:

      (ii) Covering Octahedral Voids:

1.6.1 Formula of a Compound and Number of Voids Filled

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Crystal Lattices and Unit Cells

1.7 Packing Efficiency

Packing efficiency is the percentage of total space filled by the particles.

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Crystal Lattices and Unit Cells

1.7.1 Packing Efficiency in hcp and ccp Structures

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1.8 Calculations Involving Unit Cell Dimensions

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1.9 Imperfections in Solids

Usually a solid consists of an aggregate of large number of small crystals. These small crystals have defects in them. This happens when crystallisation process occurs at fast or moderate rate.

the defects are of two types, namely,point defects and line defects. Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance,

1.9.1 Types of Point Defects

Point defects can be classified into three types :

1. stoichiometric defects

2. impurity defects and

3. non-stoichiometric defects.

1.10 Electrical Properties

Solids can be classified into three types on the basis of their conductivities.

1. Conductors:

2. Insulators :

3. Semiconductors :

1.11 Magnetic Properties

On the basis of their magnetic properties, substances can be classified into five categories:

1. paramagnetic

2. diamagnetic

3. ferromagnetic

4. antiferromagnetic and

5. ferrimagnetic.

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