NEET Study Notes for Solid State: Solid substances can be characterized by volume, rigidity, high density, low compressibility, mechanical strength, definite shape, and hard nature. The constituent particles in solid substances are closely bound to each other. On the basis of these characteristics, solids can be classified into crystalline solids and amorphous solids.

• Crystalline Solids are the particles that constitute crystalline solids are in an arranged geometrical pattern.
• Amorphous Solids are the constituent particles in amorphous solids that are randomly arranged.
• In NEET Chemistry Syllabus Solid State is considered to be of medium importance with at least one question expected in NEET 2022. The other chapters from which around 10-15 questions are expected include Environmental Chemistry, Chemistry in Everyday Life, Polymers, basic concepts, and coordination.

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NEET Study Notes for Solid State: Important Topics

NEET Study Notes on Crystalline and Amorphous Solids

The constituent particles of Crystalline Solids are orderly arranged whereas the constituent particles of Amorphous Solids are not arranged in an orderly manner.

Difference between Crystalline and Amorphous Solids

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What is Crystal Lattice?

A three-dimensional structural arrangement of constituent particles like atoms, ions, and molecules in symmetry, in a crystalline solid, is called a crystal lattice.

• All constituent particles in the crystal lattice are represented by single points known as lattice points.
• In a crystal lattice, all lattice points are connected by a straight line.

Unit Cell

The smallest part of which is the repetitive unit in the crystal is called the Unit Cell. The crystal lattice is a generation of the unit cells in different directions. Unit cells can be broadly classified into Primitive unit cells and centered unit cells. Centered Unit cells can be classified into body-centered, face-centered, and end-centered.

• Primitive cells- the constituent particles exist only in the corner positions.

• Face Centered Cells- The constituent particles exist in the center of the body

• Body-Centered- The constituent particles exist at the corners and the center of the body.

• End- Centered- The constituent particles exist in the center of two opposite faces.

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Calculation of density of unit cell

Density in unit cell can be calculated by using the formula:

g/cm³

where Z is the unit cell constant

M is the molecular weight

N_A is the Avogadro number and a is the edge length of the unit cell

Packing in Solids

A repeating pattern of particles that constitute solid is exhibited in crystalline solids. Constituent particles are arranged in a three-dimensional arrangement. This is known as crystal lattice. In a crystal lattice, the constituent particles are closely bound to each other leaving no space. The packing in different dimensions is tabulated below:

Close Packing in two dimensions

In two dimensions, packing of solids can be of two types

• Square close Packing- In a square close packing, the second row of constituent particles are placed below the first row in a close packing arrangement. With four other spheres, each sphere is in contact. The coordination number in this arrangement is equal to four. When the center of four adjoining spheres is connected, there is the formation of a square This is known as Square Close Packing in two dimensions.

• Hexagonal close-packing- Hexagonal system of packing in solids is more efficient than squares as the particles are arranged in a manner that the space between the first row is filled by the arrangement of particles in the second row. With the other six spheres, each sphere in contact. The coordination number in Hexagonal close packing is 6. When the center of six adjoining spheres is connected, it leads to a hexagonal formation.

Read NEET Study Notes for Reaction Mechanism

Voids

The spaces between constituent particles are known as voids. In the close packing of solids, there are gaps.

• Packing is done in one of the three dimensions- 1D close packing, 2 dimension close packing, and 3 dimension close packing
• Two-dimensional packing- the gaps in two-dimensional square close packing are known as voids whereas the gaps in two-dimensional hexagonal packing are known as triangular voids due to the formation of triangle-like gaps between them.
• In the three-dimensional packing structure of CCP and HCP, 26% of space is vacant These voids are known as interstitial voids. Interstitial voids are of two types:
• Tetrahedral Voids- In a tetrahedron void, when the center of the flour sphere is connected, each sphere is in contact with three spheres of layer one. This leads to the formation of tetrahedrons. The gap that is formed on connecting these spheres forms a tetrahedral void. The number of tetrahedral voids can be determined by calculation of twice the sphere number.
• Octahedral Voids- When the triangular voids of the first layer and the second layer are combined, it leads to the formation of a void that encloses six spheres, these are known as Octahedral voids. For the determination of Octahedral voids, the number of spheres is to be determined.

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What is Packing Efficiency?

The percentage of constituent particles that occupy total space in the unit cell is known as packing efficiency. Hexagonal Packing is considered to be very efficient in packing structure.

The factors that determine packing efficiency are

• the Volume of the unit cell
• The number of atoms
• Volume of atoms

Packing efficiency helps in the determination of the solid structure of the body, Properties of solids like density, isotropy, and consistency are determined. The other characteristics of a solid body can also be determined by understanding the packing efficiency.

Imperfections in Solids

The imperfection in Solids can be of two types:

• Electronic Imperfections- In temperature above 0k, some electrons of pure silicon or germanium are released from their covalent bonds and move freely in the crystals. The bonds that are formed as a result of this electron deficiency are called Holes. Such Electrons and Holes (h) are known as electronic imperfections.
• Atomic Imperfections- the atomic imperfections can be divided into Point, Lattice, Line, and Plane imperfections.

Point imperfections- When the orderly arrangement of constituent particles is disturbed it is known as Point imperfections. The point defects in solids can be further classified into Stoichiometric defect, Frenkel defect, and Schottky defect.

• Stoichiometric Defect- The positive and negative ion ratio, also known as the Stoichiometric ratio gets disturbed without affecting the electric neutrality. It is also known as thermodynamic defects. Stoichiometric Defects can be further classified into- Vacancy and Interstitial defects.

• Schottky Defect- if cations or anions are missing from lattice sites, it causes Schottky defect. The electrical neutrality is maintained. However, the density of crystals decreases. When the coordination number of compounds is high, it becomes frequent, and there is equality in the size of positive and negative ions.

• Frenkel Defect- when ions are dislocated from their original position to the interstitial position, Frenkel Defect occurs. When the coordination number of compounds is low and the difference in positive and negative ions is large, the effect is likely to occur. Examples - ZnS, AgCl, AgBr and AgI.

Non- Stoichiometric Point Defects

Non stoichometric Point defects are displayed in compounds of D elements of the periodic table. Noin stoichiometric defects can be classified into defects of metal excess. The types of metal excess defects are

• Metal Excess Defect due to Anion Vacancy- In absence of anion from the lattice site, a hole is created which is thereon occupied by the electron. Alkali metal halides display this kind of defect where on being heated in an atmosphere vapor, the creation of anion vacancy takes place. These holes that are occupied by electrons are called F- Centers. F- Centers are responsible for: NaCl -yellow color, KCl- violet, HCl- pink color.

• Metal Excess Defect due to Interstitial Cations- An extra positive ion occupies the interstitial site and leads to this defect. ZnO displays this defect wherein, on heating, oxygen is lost reversibly. The extra positive ions occupy interstitial space in order to maintain neutrality. ZnO turns yellow on being heated due to loss of oxygen.

• Metal Deficiency Defect- The missing cations from the lattice site are occupied by another remaining cation to increase valency. Example- Crystal of FeO, the missing cations is made up by the other ions of

Read NEET Study Notes on Basic Concepts of Chemistry

NEET Sample Mcqs for Solid State

Question: What is the crystalline allotrope of carbon?

1. Graphite
2. Charcoal
3. Coal
4. None of the above

Answer: Graphite

Question: On the basis of the arrangement of constituent particles in solids, what can solids be classified into/

1. Metallic or non-metallic
2. Magnetic or non-magnetic
3. Conductor or non-conductor
4. Crystalline or amorphous

Answer: crystalline or amorphous

Question: Which of the following is an amorphous solid?

1. Glass
2. Lead
3. Sodium Chloride
4. None of the above

Answer: Glass

Question: What is a crystalline solid?

1. No definite melting point
2. Transforms from solid to liquid when heated
3. its arrangement gets deformed easily
4. Irregular 3-dimensional arrangements

Answer: transforms from solid to liquid when heated

Question: Which one of the following does not display the Frenkel defect?

1. ZnS
2. AgBr
3. Agl
4. KCl

Answer: KCl

Question: When is Schottky defect observed in a crystal/

1. Crystal density increases
2. Missing and unequal cations and anions in lattice
3. Equal number of cations and anions are missing
4. Ions leave normal position to occupy interstitial position

Answer: Equal number of missing cations and anions in lattice

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