Chemistry Class 12 Chapter 1 – The Solid State

Considered the easiest one amongst the MPC Subjects, the class 12th chemistry syllabus entails a variety of interesting chapters like Solid State, Electrochemistry, Surface Chemistry etc. Solid State is an essential as well as a tricky chapter as it introduces us to a whole new world of Masses and their various properties. If you are a class 12 student who is interested in acing the concepts mentioned in this chapter then you must go through the following blog on the solid-state study notes!

What are the Solids State?

A state of matter in which the constituent particles are very closely packed together is called a Solid. In a solid substance, the constituent particles can be atoms, volume, ions, etc. Because of strong and short interactions between the particles of solid objects, they have a certain shape, mass and volume.

Properties of a Solid State

Like every other substance, solids have some properties which they exhibit all the time and they are-

• The intermolecular force is very strong in them because of the short intramolecular distance 
• They are incompressible and rigid in shape 
• Solids have fixed shapes, sizes and volume 
• The particles of a solid object have a fixed particular position and do not move freely
• Only motion along the mean position is possible for the constituent particles of solids

Types of Solid 

Depending upon the order of arrangement of the particles inside solids, they are divided into two categories as Crystalline and Amorphous Solid. Crystalline Solids have a long-order arrangement of the constituents and thus have a characteristic shape. On the other hand, Amorphous Solids are known as super cool liquids because of their short-order arrangement of the constituent particles. 

Now that you know so much about the solid states,
it is time to also learn about Class 12 Organic Chemistry!

Difference Between Crystalline and Amorphous Solids

Both Crystalline and Amorphous solids are very different and hence, have varied chemical and physical properties. Let us go through the below-mentioned table and understand the difference between them. 

Crystalline Solids	Amorphous Solids
They are called true solids.	They are known as supercooled liquids or pseudo solids.
They consist of a regular and definite arrangement of particles.	They do not have a regular arrangement of the particles.
They have a long-order arrangement of the constituents.	They have a short-order arrangement of the constituents.
Crystalline solids have sharp melting points.	Amorphous solids can be easily melted over a range of temperatures
They are anisotropic and hence, have different physical properties in various directions.	These are isotropic and have and have similar physical properties in all directions.
They have clean and smooth edges when cut.	On cutting, these have irregular and rough edges.
They have definite heat of fusion.	They do not have definite heat of fusion.

Types of Crystalline Solids

Crystalline solids are further divided into four groups based on the characteristics of chemical or intermolecular forces. It’s them,

• Molecular Solids
• Ionic solids
• Metallic solids
• Covalent solids

Structure Determination by X-ray Diffraction (Bragg’s Equation)

The X-rays are diffracted when a beam of X-rays strikes a crystal plane made up of regularly ordered atoms or ions. The difference in distance travelled by the two rays (path difference) must be equal to an integral number of Wavelengths, nλ for constructive if the waves are in phase after reflection.

Thus, path difference = WY + YZ = XY sin θ + XY sin θ = 2 XY sin θ = 2d sin θ

∴ nλ = 2d sin θ

This equation is called Bragg’s equation, where,

• n = 1. 2, 3… (diffraction order)
• θ = angle at which interference occurs
• λ = wavelength of X·rays incident on the crystal
• d = distance between atomic planes

Unit Cell

Let us now proceed with this blog on The Solid State Class 12 notes, further with another sub-topic i.e. of unit cells. The unit cell is the smallest geometrical component of the crystal lattice that can be employed as a repeated unit to build up the entire crystal. The 4 types of unit cells are as follows:

• Simple or primitive Unit cell– particles are present at the corners only.
• Face-centred unit cell- Each of the six faces has particles at the corners as well as in the centre.
• Body-centred unit cell- particles are present at the corners as well as at the centre of the unit cell.
• End-centred unit cell– At the corners and in the middle of two opposing faces, particles are present.

Number of Particles Per Unit Cell

There are around 230 crystal forms that have been classified into 14 types of space lattices, known as Bravais Lattices, based on symmetry and seven different crystal systems based on interfacial angles and axes.

Packing Fraction: It is defined as the volume of the unit cell occupied by the spheres divided by the total volume of the unit cell.

Close Packed Structures

The constituent particles in solids are closely packed, leaving the smallest amount of space possible.

Two Dimensional Packing of Constituent Particles

• Square close packing: Space occupied by spheres is 52.4%.
• Hexagonal close packing: Space occupied by spheres is 60.4%.

Three-Dimensional Packing of Constituent Particles

• Hexagonal tight packing is achieved using the ABAB arrangement (hcp).
• Cubic close packing or face-centred CUbIC packing results from the ABCABC arrangement (ccp or fcc).

Note:

• 740/0 space is occupied in both of these configurations.
• In a hop and cup arrangement, the coordination number is 12, while in a bcc arrangement, it is 8.
• In a cubic structure, the close packing of atoms equals fcc > bcc > sc.
• Except for He, all noble gases have a cap structure (hcp structure).

Void/ Space/ Holes

The Void, also known or termed spaces or holes, is the next sub-topic of this blog on The Solid State class 12 notes. The void or space or a hole, or interstitial void is the empty or blank space between the spheres of a unit cell. When particles are packed tightly together to form a CPP or hcp structure, two types of voids emerge:

• Tetrahedral voids are holes or voids at the corner of a tetrahedron surrounded by four spheres. A tetrahedral void has a coordination number of four.
• On a normal tetrahedron, octahedral voids are holes surrounded by six spheres. The octahedral void has a coordination number of 6.

The Density of Unit Cell (d)

The Density of a unit cell can be calculated with the help of the following formula:

Mass of unit cell / Volume of the unit cell

Thus,

d = Z * M / a³ = ZM / a³ * N_A

where,

• d = density of unit cell
• Z = no. of atoms per unit cell
• M = molecular weight
• N_A = Avogadro number
• a = edge length of the unit cell.

Structure of Ionic Crystals

The ionic radius ratios of cation and anion play a critical role in determining the nature of ionic substance crystal structures. In the chapter on The Solid State of class 12 Chemistry, it is given that ionic crystals can be of two types namely:

• AB-type
• A₂B or AB₂

Note: When pressure is applied, the NAC structure (6:6) transforms into the CCI structure (8:8), while the opposite occurs at high temperatures (760 K).

Imperfections in Solids

Imperfections in solids are our next topic in this blog about The Solid State class 12 notes. The atoms, ions, and molecules in a crystalline solid are arranged in a definite repeating pattern, though there may be some faults in the pattern. Rapid cooling or the inclusion of extra particles might cause deviations from the perfect arrangement. Point defects and line defects are the two forms of flaws.

Point Defects

In a crystalline solid, point defects are abnormalities or deviations from the ideal arrangement around a point or an atom. There are three different types of point defects:

• Stoichiometric defect: In this type of point defect, the solid’s electrical neutrality and the ratio of positive and negative ions (Stoichiometric) are not affected. It’s sometimes referred to as intrinsic or thermodynamic defects. They are divided into two categories: Vacancy defects and Interstitial defects are two types of defects.
• Frenkel defect: The smaller ion (cation) travels out of its location in ionic solids and occupies an intermolecular space, which is known as the Frenkel defect. In this scenario, the original position suffers from a vacancy defect, whereas the new position suffers from an interstitial defect.
• Schottky defect: The Schottky defect is a type of vacancy defect that can be observed in ionic solids. However, with ionic compounds, we must balance the electrical neutrality of the complex by removing an equal number of anions and cations. It decreases the substance’s density. In this case, the cations and anions are nearly the same sizes.

Line Defects

The imperfections or deviations from a perfect arrangement in full rows of lattice points are known as line defects. These blemishes are known as crystal defects.

Electrical Properties

Based on their conductivities, solids can be divided into three groups. It’s as follows:

• Conductors
• Insulators
• Semiconductors

Magnetic Properties

Magnetic materials are normally placed in a homogenous magnetic field and then the magnetic field is adjusted to examine their magnetic properties. Magnetic behaviour can be divided into five categories:

• Diamagnetic materials
• Paramagnetic materials
• Antiferromagnetic materials
• Ferromagnetic materials
• Ferrimagnetic materials

We hope by now you are through with the important topics of the solid-state chapter. Now, let’s move on to some important questions in this chapter. 

The Solid State: Important Questions 

Here are some mandatory questions which you must prepare to have a strong grip on the chapter. 

• Why is glass considered to be a supercooled liquid?
• What types of Solids are electrical conductors?
• What is the significance of a ‘Lattice Point’?
• Why do ionic solids always conduct electricity in a molten state and not in a solid state?
• Determine the volume of a compound that is formed through two different elements named A and B. The atoms of A occupy 1/3rd tetrahedral voids and element B forms up. What will be the formula of this compound?
• Explain how vacancies are introduced in an ionic solid whenever the cation of a higher valency is added as an impurity in it
• Explain Paramagnetism with a suitable example.
• Explain the term coordination number.
• Explain why Ionic solids are brittle and hard.
• Distinguish between a semiconductor and a conductor.
• Distinguish between Cubic close-packing and Hexagonal close-packing.
• Determine between ferromagnetic or ferrimagnet which will make the better permanent magnets

Read this very interesting blog on Examples of Chemistry in Everyday Life!

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