Solid State Chemistry

Solid State is a state of matter in which the shape and volume of matter remain unchanged irrespective of the container they are kept in. Solid state materials are of huge importance due to their wide ranging applications in daily lives. This versatility of solid state lead to formation of a separate field under the domain of chemistry named as Solid State chemistry.

In this article, we will lean in detail about solid state, its properties, types. We will also cover complete solid state chemistry to cover all the points related to solid state that would help in last minute revision for our exams.

What is Solid State?

Solid state is a physical state of matter in which particles, such as atoms, ions, or molecules, are closely packed together in a fixed arrangement. In a solid state, particles have strong intermolecular forces holding them in place, resulting in a rigid and well-defined shape. A solid state of matter is characterized by the following properties:

• Fixed Volume and Shape
• High Density
• Limited Molecular Motion
• Resistance to Compression

Let’s learn about these properties in detail.

Properties of Solids

The properties of solids can be broadly classified into following two categories:

• Physical Properties of Solids
• Chemical Properties of Solids

Physical Properties of Solids

Solids are different from liquids and gases in that they have certain physical characteristics. The physical properties of solids are mentioned below:

• Defined Shape and Volume: The shape and volume of a solid are distinct. The particles of a are regularly and systematically packed together to have a defined form and volume.
• Density: Because of the closely spaced particle structure, a solid has a usually higher density than liquids and gases. In case of solid more particles are arranged in less volume which increase the density of the solid
• Rigidity and Incompressibility: Solids exhibit rigidity are resist any compressive force applied on it because of low intermolecular space.
• Melting Point: Solids have high melting point due to large intermolecular force
• Thermal Expansion: Solids expand least when heated because of high intermolecular force
• Electrical Conductivity: Solids can conduct electricity in different amount. Based on this, they are classified as conductors, insulators and semiconductors.

Chemical Properties of Solids

The following are some important solid chemical properties:

• Reactivity: Depending on the chemical linkages and the presence of reactive sites, solids can display varying degrees of reactivity. Many solids are inert under specific circumstances while several react with other compounds easily.
• Combustibility: When some solids come into contact with oxygen and a heat source, they have the ability to ignite and undergo combustion.
• Corrosion: Some solids are susceptible to corrosion when exposed to certain environmental conditions, such as moisture, oxygen, or acidic substances.
• Acidic/Basic: Solids can exhibit acidic or basic properties depending on their chemical composition. Acidic solids can donate protons (H⁺ ions), while basic solids can accept protons.
• Catalytic Activity: Certain solids can act as catalysts, facilitating chemical reactions without being consumed in the process.

Solid State Chemistry

Solid-state chemistry is a branch of chemistry that focuses on the study of solids, their composition, structure, properties, and reactivity. It studies in detail about types of solids, unit cell, lattice structure, formation of solid, defects in solid and several other properties of solid.

Let’s study all the aspects of solid chemistry in detail below, starting with types of solids.

Types of Solids

Primarily there are following two types of solids:

• Crystalline Solid
• Amorphous Solid

Crystalline Solid

A crystalline solid is a type of solid material that has a highly ordered arrangement of atoms, ions, or molecules in a repeating three-dimensional lattice structure. In a crystalline solid, the constituent particles are arranged in a regular and periodic pattern, resulting in well-defined geometric shapes and surfaces.

Properties of Crystalline Solids

The properties of crystalline solids are mentioned below:

• Ordered Structure: Crystalline solids have a highly ordered and repetitive arrangement of particles in a three-dimensional lattice structure
• Long-Range Order: The ordered arrangement of particles in a crystalline solid extends over large distances
• Distinctive X-ray Diffraction Patterns: Different crystalline solid shows different patterns when subjected to X-Ray Diffraction.
• Anisotropy: Crystalline solids are anisotropic which means the magnitude of property in a crystal vary with direction.
• Hardness: The regular arrangement of the elements in these solid forms gives them solid properties of hardness.
• Definite Melting Point: Crystalline solids have a well-defined melting point at which they transition from the solid phase to the liquid phase
• Cleavage and Fracture: Crystalline Solids cleaves into smaller parts with smooth surfaces

Types of Crystalline Solids

Crystalline solids are be further classified into following types:

• Molecular Solids
• Ionic Solids
• Metallic Solids
• Covalent or Network Solids

Molecular Solids

Molecular solids are a type of crystalline solid in which the constituent particles are individual molecules held together by intermolecular forces, such as van der Waals forces, hydrogen bonding, or dipole-dipole interactions. Here are some of the key characteristics and properties of molecular solids:

• Intermolecular Forces: A molecule in a solid state is naturally held together by weak forces of attraction
• Low Melting and Boiling Points: As the intermolecular forces are weak, the molecular solids normally exhibit a low melting and boiling point
• Soft and brittle: Molecular Solids may be relatively soft and fragile.

Molecular solids can be classified into different types based on the nature of the intermolecular forces holding the molecules together. These are mentioned below:

Nonpolar Molecular Solids: These molecular solids consist of nonpolar molecules held together by van der Waals forces, such as London dispersion forces. Examples are solid nitrogen (N₂), solid oxygen (O₂), and solid methane (CH₄).

Polar Molecular Solids: Polar molecular solids contain polar molecules held together by stronger intermolecular forces, such as dipole-dipole interactions and hydrogen bonding. Examples include solid water (ice)

Hydrogen-Bonded Molecular Solids: These molecular solids are characterized by strong hydrogen bonding between molecules. Examples include solid hydrogen fluoride (HF), solid acetic acid (CH₃COOH).

Ionic Solids

Ionic solids are the solids whose constituent particles are ions. These ions are held together by strong electrostatic forces. These solids are created when the cations and the anions that serve as healthy opposite forces arrange themselves in a fixed order. Here are some of the key characteristics and properties of ionic solids:

• Ionic Bonding: Ionic solids have strong electrostatic attraction between oppositely charged ions that holds the solid together.
• High Melting and Boiling Points: Ionic solids have extremely high melting and boiling points.
• Hard and brittle: Ionic Salts are typically hard and brittle, characteristic of lattices with strong ionic bonding forces.
• Poor electrical conductors in a solid state: Ions in ionic solids are immobile. They become the conductors of electricity when they are in molten state or dissolved substance in the liquid. .

Metallic Solids

Metallic solids are a type of solid material composed of metal atoms arranged in a closely packed, three-dimensional lattice structure. Here are some of the key characteristics and properties of metallic solids:

• Metallic Bonding: Metallic bonding happens when metal atoms are sharing their electrons in “sea of electrons” in which these free electrons can move throughout the structure.
• Conductivity: Metallic solids are conducting in nature as the electrons can move freely.
• Malleability and Ductility: Metallic solids are malleable which means they can be transformed into thin sheets on applying force. They are also ductile which means thin wires can be drawn from them.
• Luster: Metals have luster which means they can shine. This property is related to the capacity of electrons for absorbing and re-emitting photons of light.
• High Melting and Boiling Points: Since metallic bonds are strong, metallic solids have high melting and boiling points.

Covalent or Network Solids

Covalent, or network solids, are a kind of crystalline solid in which the particles make up a network that is connected by covalent bonds in all directions. Covalent solids are different from molecular solids. In a molecular solid, molecules are joined to each other by different bonds such as dipole interaction etc., whereas in covalent or network solids, molecules are permanently bound by covalent bonds to its neighbors. The properties of covalent solids are mentioned below:

• Strong covalent bonds: Covalent or network solids are characterized by the presence of strong covalent bonds between atoms.
• High Melting and Boiling Points: Covalent or network solids typically have high melting and boiling points due to the strong covalent bonds holding the atoms together in the lattice.
• Hard and Brittle: Covalent Solids are hard and brittle due to strong covalent bonds
• Poor electrical conductors: Covalent solids are generally poor conductors due to absence of free electrons
• Insolubility in Solvents: Covalent bonds are strong and thus prevent from dissolving the covalent solids in a solvent

Amorphous Solid

Amorphous Solids are the solids which do not have long range order of arrangement of atoms. In these solids, unlike crystalline solids, a particular repetition of arrangement of particles occur for very short distance. The property of amorphous solids are mentioned below:

• No long range order of atoms
• No defined melting and boiling point
• Isotropic in Nature
• Also called supercooled liquids
• They can brittle or flexible
• No smooth cleavage

The scope of amorphous solid is limited as of now, hence we will continue our learnings in detail about crystalline solid.

Crystal Lattice and Unit Cells

A crystal lattice is a three-dimensional arrangement of atoms, ions, or molecules in a crystalline solid. A unit cell is the basic repeating structural unit that forms the three-dimensional lattice of a crystalline solid. It is the smallest portion of the crystal lattice that, when repeated in all three dimensions, generates the entire crystalline structure.

Lattice Parameter

Different crystal lattice can be classified on the basis of lattice parameters. There are two main lattice parameters mentioned below:

• Lattice Constants (a, b, c): The lengths of the edges of the unit cell along the three axes are denoted as a, b, and c. These lengths are measured in units such as angstroms (Å) or picometers (pm).
• Angles Between Edges (α, β, γ): The angles between the edges of the unit cell, typically denoted as α, β, and γ. These angles define the orientation of the edges relative to each other and are measured in degrees.

Coordination Number

Coordination number is the number of atoms or ions directly adjacent to a central atom or ion in a crystal lattice. It indicates how many other particles are in direct contact with the central particle. The value of coordination number ranges from 2 to 12, the most common are 4, 6 and 8.

Types of Unit Cell

Unit cell can be classified as Primitive and Non-Primitive. No-Primitive Unit cell can be further classified as face centered, base centered and edge centered.

• Primitive Cubic Unit Cell
• Non-Primitive Unit Cell
  • Body-centered Cubic Unit Cell
  • Face centered cubic unit cell
  • Edge Centered

Primitive Cubic Unit Cell

In Primitive Cubic unit cell the atoms are present only at the corners. The properties of primitive unit cell is discussed below:

• Coordination Number: It has a coordination number of six which means every atom in a primitive cubic unit cell is directly joined to six other atoms.
• Atoms per Unit Cell: Each cube’s corner has a single atom. The eight nearby unit cells share these corner atoms. Hence, effective number of atoms per unit cell is 1.
• Edge Length Relationship: The equation a = 2r describes the relationship between the atomic radius (r) and the edge length (a) of the cubic unit cell.

Non-Primitive Unit Cell

Non-Primitive Unit Cells have atoms at corners of the unit cell as well as other places of the unit cell such as body center, face center or edge center. Based on this Non-Primitive unit cells are classified as follows

• Body-centered Cubic Unit Cell
• Face centered cubic unit cell

Body-Centered Cubic Unit Cell

In BCC unit cell, an atom is present in each cube corner and an extra atom is present in the center of a body-centered cubic arrangement. A body-centered cubic unit cell has the following essential characteristics:

• Coordination Number: Because it is physically related to eight nearby atoms, every atom in a body-centered cubic unit cell has a coordination number of eight.
• Atoms per Unit Cell: In a body-centered cubic lattice, a cube has two atoms per unit cell: one at each corner and one in the center.
• Connection between Atomic Radius and Edge Length: The equation a = 4r/√3 relates the atomic radius (r) to the edge length (a) of the cubic unit cell.

Face Centered Cubic Unit Cell

In FCC, there is extra atoms at the center of each face and atoms at each cube’s corner. A face-centered cubic unit cell has the following essential characteristics:

• Coordination Number: Because it is physically coupled to twelve nearby atoms, every atom in a face-centered cubic unit cell has a coordination number of twelve.
• Atoms per Unit Cell: The number of atoms in a face-centered cubic lattice is four per unit cell, with one atom located at each corner and one at the center of each face.
• Volume Occupied: The atoms in a face-centered cubic unit cell fill the whole volume of the unit cell and occupy the cube’s faces and corners.
• Edge Length Relationship: The equation a = 2√2 r describes the relationship between the atomic radius (r) and the edge length (a) of the cubic unit cell.

Seven Crystal System

Based on the configuration of lattice parameters, there are seven crystal system. These seven crystal system are tabulated below:

Crystal System	Lattice Parameters	Examples
Cubic System	Edge length: All Equal Angle between Edges: 90°	Diamond NaCl
Tetragonal	Edge Length: Two equal and one different Angle between Edges: 90°	CaSO4 SnO2
Orthorhombic	Edge Length: All different Angle between Edges: 90°	Rhombic Sulfur KNO3
Monoclinic	Edge Length: All different Angle between Edges: Two 90°, one different	Monoclinic Sulphur Na2SO4.10H20
Triclinic	Edge Length: All different Angle between Edges: All different	K2Cr2O7 H3BO3
Hexagonal	Edge Length: Two equal, one different Angle between Edges: Two equal to 90° and one 120°	Graphite ZnO
Trigonal System (Rhombohedral System)	Edge Length: All Equal Angle: All Equal to each other but not equal to 90°	Calcite cinnabar

Bravais Lattice

Bravais Lattice is a system of arrangement of atoms inside a unit cell observed by Auguste Bravais. The above mentioned 7 Crystal System which and different possible combinations of unit cells leading to 14 possible Bravais lattices.

14 Bravais Lattice

The list of 14 bravais lattice is mentioned below:

• Cubic System
  • Simple cubic lattice (SC)
  • Body-centered cubic lattice (BCC)
  • Face-centered cubic lattice (FCC)
• Tetragonal System
  • Simple tetragonal lattice
• Orthorhombic System
  • Simple orthorhombic lattice
  • Base-centered orthorhombic lattice (B-centered)
  • Face-centered orthorhombic lattice (F-centered)
• Monoclinic System
  • Simple monoclinic lattice
• Triclinic System
  • Simple triclinic lattice
• Hexagonal System
  • Hexagonal lattice
• Rhombohedral System:
  • Rhombohedral lattice

Voids in Crystal Lattice

Voids are the gaps or empty spaces that exist inside a crystal lattice structure. They are sometimes referred to as interstitial spaces or interstices. The atoms surrounding these gaps may be used to categorize them into two basic types: octahedral and tetrahedral voids.

Octahedral Voids

• Six atoms or ions grouped in the shape of an octahedron surround a space known as an octahedral void.
• Octahedral voids are found in the center of the unit cell’s edges in a face-centered cubic (FCC) lattice.
• Octahedral voids appear in a body-centered cubic (BCC) lattice near the body center of the unit cell.

Tetrahedral Voids

• Four atoms or ions organized in the shape of a tetrahedron surround a space known as a tetrahedral void.
• Tetrahedral voids are found in the middle of the unit cell’s faces in an FCC lattice.
• Tetrahedral voids are present in the space between lattice points in a BCC lattice but are absent from the lattice points themselves.

Defects in Solid

Deviations from the ideal, flawless crystal structure are referred to as defects in solids. The mechanical, electrical, and physical characteristics of materials can be significantly impacted by these flaws. There are several types of defects such as point defect, line defect, surface defect and volume defect. Here we will learn in detail about Point Defects

Point Defects

Point defects are a type of defect in solid materials where there are irregularities or deviations from the ideal atomic arrangement at a single point within the crystal lattice. They are further classified as follows

• Vacancy Defects: Atoms or ions absent from their normal lattice positions are known as vacancy defects. Thermal vibrations or atom loss during crystal formation may cause this effect
• Interstitial Defects: Atoms or ions move to the interstitial sites leaving normal lattice sites are known as interstitial defects. Smaller atoms or ions may do this in the crystal lattice.
• Substitutional Defects: Substitutional defects are when one type of atom or ion is substituted for another.
• Frenkel Defect: It is a type of point defect commonly found in ionic solids. It occurs when an ion is displaced from its normal lattice site into an interstitial site within the crystal lattice, creating both a vacancy and an interstitial defect simultaneously.
• Schottky Defect: In a Schottky defect, pairs of cation and anion vacancies are created within the crystal lattice.

Frequently Asked Questions on Solid State

What is a solid?

A state of matter that is known to have a definite shape and volume, where particles are in a close-packed arrangement.

How many crystal systems are there?

There are seven crystal system

Do ionic solids differ from molecular solids in what ways?

Ionic solids are composed of ions bonded by electrostatic forces while molecular solids are made of discrete molecules connected by intermolecular forces.

Why metals are good conductors of electricity?

Metals have delocalized electrons that can move freely; this, in turn, facilitates highly efficient electronic conduction.

What is an example of a polymorphic solid?

Carbon can be found in various forms like graphite or diamond.

What is difference between the solids and the liquids and gases with reference to shape and volume?

Solids have a fixed shape and volume, while liquids not have fixed shape but have fixed volume, and gases do not have fixed volume and shape

What is the essential difference between the crystalline and the amorphous solids?

Crystalline solids exhibit a highly ordered structure that is repeated, whereas amorphous solids lack the distinct long-range order.