Solid state chemistry

Solid state chemistry is a fascinating branch of chemistry that focuses on the study of solids, particularly the synthesis, structure, and properties of solid state substances. This includes metals, minerals, ceramics, semiconductors, and other materials. Understanding these substances involves considering not only their atomic structure, but also their crystal structure and physical properties.

Introduction to solids

To understand solid state chemistry, we start with matter in the solid state. Solids differ from gases and liquids in that they have a definite shape and volume. This is because the atoms or molecules in a solid are tightly packed together in a definite arrangement.

Unlike gases and liquids, where particles move around freely, particles in solids are fixed and vibrate at fixed positions. This is due to strong intermolecular forces that hold them in place. Solids can be further classified into crystalline and amorphous types.

Crystalline and amorphous solids

Crystalline solids have a highly ordered structure, with atoms or molecules arranged in a repetitive pattern. This regular arrangement results in the formation of a crystal lattice. In contrast, amorphous solids lack such long-range order. Glass and plastic are typical examples of amorphous solids.

Crystal lattice and unit cell

A crystal lattice is a three-dimensional grid where points represent the positions of atoms. Within each crystal lattice, we identify a basic repeating unit known as the unit cell. The unit cell serves as the building block of the crystal; by repeating this cell in space, the crystal structure is completely defined.

The features of the unit cell vary in size and shape, depending on the chemical content of the solid. There are seven crystal systems based on the axes and the angles between them: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and rhombohedral.

Seven crystal systems

These seven crystal systems describe all possible symmetries of crystal structures. They include:

• Cube: All sides equal, all angles 90°.
• Quadrilateral: Two sides equal, all angles 90°.
• Orthorhombic: all sides different, all angles 90°.
• Monoclinic: all sides different, two angles 90°, one not 90°.
• Triclinic: All sides and angles different.
• Hexagonal: Two sides equal, base angle 90°, other angles 120°.
• Rhombus: All sides equal, no angle is 90°.

Types of crystals

Depending on the nature of the bonding forces, crystals are classified into four types: ionic, covalent (network), molecular and metallic crystals.

Ionic crystals

Ionic crystals form when large numbers of anions and cations come together. The electrostatic attraction between the oppositely charged ions gives the solid significant strength. Sodium chloride (NaCl) is a common example.

Na+ + Cl- → NaCl

Covalent (network) crystals

In covalent or network crystals, atoms are held together by covalent bonds in a continuous network. An example of this is diamond, where each carbon atom forms four strong covalent bonds with neighboring carbon atoms.

C (diamond) – four covalent bonds with C

Molecular crystals

Molecular crystals consist of molecules that are held together by weak forces such as van der Waals or hydrogen bonds. Water ice is a familiar example, in which water molecules are held together in a lattice by hydrogen bonds.

H2O – hydrogen bonds

Metal crystals

Metal crystals contain metal atoms packed close to each other in a regular pattern. They are bound together by a group of mobile electrons that allow metals to conduct electricity. Examples include copper (Cu) and iron (Fe).

Cu, Fe – metallic bonding

Defects in solids

Despite the ordered arrangement in crystalline solids, imperfections or defects may be present. These defects can significantly affect the properties of the materials. Defects are mainly classified as point defects, line defects and plane defects.

Point defects

Point defects are localized disruptions in the crystal structure. These are further classified into the following categories:

• Vacancy defect: An atom or ion is missing from its normal lattice site.
• Interstitial defect: An extra atom or ion is present in the lattice structure.
• Substitution defect: An atom or ion is replaced by a different type of atom.

Line faults

Line defects are also known as dislocations and occur when an entire row of atoms becomes distorted. They are responsible for a wide range of mechanical properties in materials, including ductility and strength.

Plane defects

Plane defects or grain boundaries occur where two crystals with different orientations meet. These boundaries can change the electrical and mechanical properties of the material.

Properties of solids

The physical properties of solids are determined by the type of bond and the arrangement of the atoms. These properties include electrical conductivity, thermal conductivity, hardness, and optical properties.

Electrical conductivity

In metals, the free motion of electrons under an electric field causes them to conduct electricity. Semiconductors such as silicon have conductivity between metals and insulators. They can conduct electricity under certain conditions.

Thermal conductivity

Metals are usually good thermal conductors because heat is conducted by the movement of electrons. However, in nonmetals, heat is often conducted by lattice vibrations, also called phonons.

Rigidity

Hardness is a measure of a solid substance's resistance to deformation. The Mohs scale is commonly used for mineral hardness, with diamond being the hardest natural substance.

Optical properties

The interaction of solids with light determines their optical properties, such as color and transparency. For example, the band gap in semiconductors affects their ability to absorb certain wavelengths of light, producing color.

Synthesis of solids

Solid state chemistry also includes methods for synthesizing new materials with desired properties. These include:

Solid state reactions

In these reactions, solid reactants are converted into solid products, often involving heating and mixing to facilitate diffusion. The reactions are usually slow because of the immobility of the atoms.

Sol-gel process

The sol-gel process is a chemical technique in which a 'sol' (stable colloidal solution) is converted into a 'gel' (more solid substance). This process is widely used in making glass and ceramics.

Crystallization

Crystallization is the process by which a solid substance forms, where atoms or molecules are highly organized into a structure called a crystal. This can occur from deposition from a solution, a molten substance, or, more rarely, directly from a gas.

NaCl (aq) → NaCl (s)

Solid state chemistry forms the basis for understanding and developing advanced materials, which are vital in technology and industry. From increasing the efficiency of semiconductors to developing stronger materials, this field plays a vital role in technological advancement.

Comentarios