Solid State: Definition, Properties, Amorphous & Crystalline Solids

Solids are substances that are characterized by a definite volume, shape, and high density. In the solid-state, the constituting particles are arranged in a variety of manners. Solid-state, in a simple manner, means "no moving parts." Thus solid-state electronic devices are the ones made up of solid components that don’t change their position. Solid is a state of matter where the constituting particles are arranged close to each other. The constituent particles can be either atoms, molecules, or ions. 

Key Terms: Solids, Atoms, Crystalline solids, Amorphous solids, Solid state, Unit cell, Molecules, Ions

Properties of Solids

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• Solids have fixed mass, volume, and shape.
• They are compressible and rigid.
• Intermolecular distances are very short, thus they are stronger.
• Their constituent particles are placed at the same position and can only oscillate in their mean positions.

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Types of Solids - Amorphous and Crystalline Solids

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Solids can be divided into two types as crystalline or amorphous based on the nature of the order that is present in the arrangement of their constituent particles. 

Amorphous solids behave the same as super cool liquids because constituent particles are arranged in short-range order. They are isotropic and have no sharp melting point. 

Main features of amorphous solids:

1. Amorphous solids don’t have a regular shape.
2. They have short-range orders.
3. They gradually soften over arrange of temperature.
4. They are isotropic and their physical characteristics remain the same in all directions.
5. When they are cut with a sharp-edged tool, they are divided into smaller pieces having irregular surfaces.
6. They don’t have any fixed heat of fusion.
7. They are also being referred to as pseudo solids or super cooled liquids as they have flow very slowly.

Some key features of crystalline solids are-

1. Crystalline solids have fixed geometrical shapes.
2. They have a long-range order.
3. They have a sharp melting point.
4. They are anisotropic. Their physical properties represent different values on getting measured with different directions in the same crystal.
5. They have a fixed heat of fusion.
6. Crystalline has true solids.
7. When crystalline solids are cut with a sharp-edged tool, they get divided into two pieces. The newly created surfaces are plain and smooth.

Polymorphic forms or polymorphs

The different crystalline forms of a substance are referred to as polymorphic forms or polymorphs. Some of its examples include graphite and diamond.

Classification of Crystalline Solids

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The classification of crystalline solids is done as per their property. The crystalline property of solid is dependent upon the nature of interactions between the constituent particles, and thus these solids are segmented into four different categories comprising:

• Ionic solids
• Covalent or Network solids
• Molecular solids
• Metallic solids

(1) Ionic Solids

• Constituent Particles are Ions.
• They are hard and brittle.
• They have high melting points.
• In a solid-state, they don’t conduct electricity, while in a molten state they become good conductors
• They have low volatility.
• They are mainly soluble in polar solvents such as water.

(2) Covalent or Network Solids

• Constituent Particles are Atoms
• They are hard but malleable and ductile.
• They have high melting points.
• They are poor in conducting electricity or heat. 
• A few examples include SiO₂, (quartz), SiC, C (diamond), C(graphite)

(3) Molecular Solids

• Constituent Particles are Molecules of a substance.
• They are soft.
• They have low to moderate-high melting points.
• They are poor conductors of electricity or heat

(4) Metallic Solids

• Constituent Particles are metal atoms.
• They are ductile and malleable.
• They have high boiling and melting points.
• They are good conductors of electricity and heat.

Unit Cell

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The smallest atom group that can be used to form the entire lattice is known as a unit cell.

Types of unit cells:

1. Primitive or simple unit cells showcase constituent particles but at the corners only.
2. Centered unit cells are being referred to as those unit cells where more than one constituent particles are present at positions in addition to those present at the corners.

Types of centered unit cells

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• Face-centered unit cell: These cells consist of one constituent particle that is present at the center of each face along with the ones that are present at the corners.
• Body-centered unit cell: These cells consist of one constituent particle that is present at its body center and others are present at the corners.
• End-centered unit cell: They have 1 constituent particle located at the center of any two opposite faces and the extremes. 

Number of particles at different lattice positions:

Face center: In case an atom is present at the center of the face, it is shared by two unit cells. So, half of the atom belongs to the unit cell.

Body center: In case an atom is present at the body center, it is not shared by some other unit cell. So, that kind of atom completely belongs to the same unit cell.

End center: In case an atom is present at the edge center, it is shared by four unit cells. So, in this case, one-fourth of an atom belongs to the unit cell.

• Number of atoms in different unit cells:

• Primitive unit cells have 1 atom.
• Face-centered unit cells have 3 atoms.
• Body-centered unit cells have 2atoms.

Coordination number: The coordination number is referred to as the number of nearest particles.

• Close packed structures

Close packing in two dimensions: It is done by stacking the rows of close-packed spheres in two ways including Square close packing and hexagonal close packing.

Close packing in three dimensions: They can be obtained by loading two-dimensional layers one above each other. It can be obtained in two ways, either by Square close-packed layers and hexagonal-close-packed layers.

Square close packing: The spheres of the second row are placed exactly above the first row. This way the spheres are aligned horizontally and vertically and their arrangement is AAA type. The coordination number is 4. 

Hexagonal close-packing: Thes e spheres are placed above the first one in such a manner that their spheres fit in the depression of the first row.

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Things to Remember

• Solids have a definite volume, shape, and high density.
• There are two types of solids - Crystalline solid and amorphous solid.
• Crystalline solids are isotropic whereas amorphous solids are anisotropic.
• The different crystalline forms of a substance are referred to as polymorphs.
• Crystalline solids are classified into Ionic solids, Covalent or Network solids, Molecular solids and Metallic solids.
• The smallest atom group that can be used to form the entire lattice is known as a unit cell.

Previous Year Questions

• The molecular formula of the crystal is
• A isIf the molar mass of crystal B is 200gmol−1, then that of crystal 
• Which one of the following has a different crystal lattice from those of the rest?​
• The number of aluminium atoms present per unit cell is​
• If its density is 4gcm−34gcm−3, the atomic mass of the element (in gmol−1gmol−1) is (consider NA=6×1023NA=6×1023)​
• Substance which is weakly repelled by a magnetic field is​
• What percentage of voids are left in the lattice?​
• The octahedral voids are occupied by a monovalent metal B. The formula of the oxide is​
• A unit cell of diamond consists of​
• Which of the following does not show electrical conductance?​
• What is packing fraction of 26Fe5626Fe56 ? (At. mass = 55.92066)
• Which type of defect is present in NaCl.​
• Certain crystals produce electric signals on application of pressure. This phenomenon is called:​
• Total volume of atoms present in a face centred cubic unit cell of a metal is (r is atomic radius)​
• CsCl has a crystal structure of
• Which arrangement of electrons leads to ferromagnetism?​
• Which of the following forms molecular solid?​