What are the important topics in the solid-state chapter for the 12th CBSE boards?

Solid State chemistry forms an integral part of Physical Chemistry in the syllabus of Class 12th CBSE. The importance of Solid state can be understood by the fact that in the board examination, Physical chemistry alone accounts for 23 out of the maximum 70 marks in the question paper. With the growing importance of material science, solid-state chemistry becomes vital to expose students to the frontier areas of research in the domain of science. Solid-state chemistry is also referred to as materials chemistry. It is the learning of the creation, arrangement, and properties of solid phase materials, especially for non-molecular solids. Therefore, solid states have a strong similarity with solid-state physics, mineralogy, crystallography, ceramics, metallurgy, thermodynamics, materials science and electronics with an emphasis on the synthesis of new materials and its characterization. Solids can be classified as crystalline or amorphous on foundation of the nature of order present in the preparation of their constituent particles.

As per the CBSE syllabus, a total of eight hours should be devoted to covering this important section of physical chemistry. However, it has often been observed that a student needs to devote longer time to study solid-state chemistry and must develop the habit of writing notes on their own. Let us have a good look at the syllabus for solid chemistry in order to get the perfect idea about it. The syllabus for solid chemistry can be broadly broken down into the following sections,

• Classification of Solid - Types of solids, categorization of solid based on various binding forces such as molecular, ionic, covalent bonds and elementary idea of metallic solids, amorphous and crystalline solids
• Lattice structures- Unit cell, its definition, lattices, Unit cell representation in two dimensional and three-dimensional lattices, Computation of density of a unit cell, packing of unit cells in solids, packing efficiency and calculation, voids, computation of number of atoms per unit cell in a cubic unit cell,
• Defects in Crystal - point defects, line defects
• Electrical and Magnetic properties of solids

Now, let us define what a solid is. A solid is defined as a state of matter where the fundamental particles are very closely packed. In the case of solid matter, the building blocks or constituent particles can be atoms, volume, ions, etc. Every solid has a certain shape, a mass as well as volume owning to this dense packing of its constituent particles. The properties that are characteristic of a solid can be listed as follows,

• Due to the small intermolecular distance between the particles, solids have a very strong intermolecular force.
• Solids cannot be compressed and posses a rigid shape.
• Due to the dense packing of its constituents, solids are characterized by a fixed shape, size as well as volume
• The constituent particles of a solid have a immovable position due to strong intermolecular force and thus, cannot move around freely, unless acted upon by external force.
• The constituent particles of a solid can only move about its mean position.

Normally there are two types of solids, based on the order of arrangement of constituent particles of the solid. These are divided as either as Crystalline Solids and Amorphous Solids. Crystalline Solids are solids that have a fixed shape due to possessing a long order arrangement of the constituent’s particles. On the other hand, Amorphous Solids are called as super cool liquids due to the fact that they possess a short-order arrangement of its constituent particles. Both crystalline solid and amorphous solid exhibit different properties and thus also have dissimilar chemical and physical properties.

Few important points of dissimilarity between the two can be listed as follows,

• Crystalline solids are also termed as true solids while Amorphous solids are called super cool liquids as they do not have a fixed shape.
• Crystalline solids have a fixed arrangement while amorphous solids do not.
• Crystalline solids possess a longer arrangement of particles while its shorter for amorphous
• Crystalline solids have a fixed melting point whereas amorphous solids exhibit a wide range of melting points.
• Crystalline solids have a fixed heat of fusion while amorphous solids do not have a fixed heat of fusion.

The structure determination of an solid can be done by using an X-Ray. This is so because a beam of x ray when incident upon a crystal made up of regular atoms or ions will tend to diffract after striking it. Bragg’s equation is used to determine this. Now let us understand the concept of a unit cell. A unit cell is the tiniest geometrical constituent of the crystal lattice, which is used as a repeated unit to create an entire crystal. This can be classified into four types as follows,

• Simple or primitive Unit cell– constituents are present at the corners only.
• Face centered unit cell- Each of the six faces of the lattice has particles at the corners and in the center.
• Body centered unit cell- Constituents are present at the corners and at the center of the unit cell.
• End centered unit cell– Constituents present at corners and in the middle of opposite faces.

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