Chapter 1 – Solid State

The study of properties-functions, structures, and synthesis of solid materials is termed as Solid State Chemistry. It is also referred to as material chemistry. Solid State Chemistry is studied to understand the properties and characteristics of solids. The functions and properties are a consequence of the arrangement and alignment of constituents as well as the type of constituents. To learn more, you need to understand the types of solids and their properties.

There are definitively two types of Solid State. Solid states are differentiated based on the arrangement of constituting particles:

1. Crystalline Solid State
2. Amorphous Solid State

Below are all types and subtypes of solid states discussed briefly.

Crystalline Solid State

Crystalline solids are the substances wherein the constituent particles are arranged in a regular-ordered long-ranged arrangement. You can imagine constituent particles of Crystalline Solid State arranged in fixed order arranged in a geometric shape. Each shape is made up of a large number of smaller crystals.

Example: Quartz, Sodium chloride, Diamond, etc.

Characteristic properties of crystalline solids

• Crystalline solids are anisotropic in nature, meaning, a few physical properties such as electrical resistance or refractive index exhibit different values when measured from different directions.
• Crystalline solids have sharp melting points (high heat of fusion).
• Crystalline solids consist of smaller crystals arranged repetitively and regularly to provide a definite geometric shape.
• Since the constituents of crystalline solids are arranged in a uniform pattern, if the solid is cleaved (cut), the resulting parts are smooth and have a definite plane.
• Crystalline solids are true solids.

Types of crystalline solids:

o   Metallic solids

o   Ionic solids

o   Molecular solids

o   Covalent solids

Metallic solids

The constituents are metallic elements.

• These solids have metal ions in the pool of electrons. In this type of arrangement the electrons are delocalized (mobile) and they are evenly present in the entire crystal.
• The electric and thermal conductivity is high as a consequence of delocalized electrons moving freely.  
• These solids are malleable and ductile, a characteristic feature of metals.
• The conductibility of metallic solids is high in both solid and molten states.
• Metallic solids have a high melting point in comparison to ionic solids.

Examples of metallic solids are Iron (Fe), Nickel (Ni), Gold (Au), etc.

Ionic solids

The constituents are ions. They are arranged in a regular pattern of cations and anions bound by coulombic forces. 

• They are insulators in the solid-state but conductors in liquid or molten state due to free electric conductance.
• Ionic Solids have high melting points.
• They are brittle and hard in nature.

Example: NaCl, MgO, etc.

Molecular solids

The constituent particles in molecular solids are molecules. These are further divided into 3 sub-types.

Non-Polar Molecular Solids

The constituent particles of this type of solid have a nonpolar covalent bond. The forces holding these particles are London forces or weak dispersion forces.

• Non-conductors or insulators in all forms.
• Very low melting point due to weak forces

Example: F₂, H₂, etc.

Polar Molecular Solids

The constituent particles share polar covalent bonds, due to which they share strong dipole-dipole interactions

• They are gaseous or liquid in nature
• They are insulators and don’t conduct electricity

Example: HCl, NH₃, etc.

Hydrogen-Bonded Molecular solids

These are the solids that consist of a bond between Hydrogen, Oxygen, Fluoride, and Nitrogen atoms called hydrogen bonds. Due to the hydrogen bonds, molecules are strongly held.

• These are hard solids
• They are insulators, non-conductors.
• They have relatively lower melting points
• At room temperature, they are volatile liquids or soft solids.

Example: H₂0, H₂F, etc.

Covalent Solids

They are also called network solids. The constituent particles have covalent bonds between them across the entire solid structure.

• They can be hard or soft depending on the structure lattice.
• They can be conducting or insulating depending on the arrangements of molecules.

Example: SiO_2, CH_4, etc.

Amorphous Solid State

Amorphous solids (in Greek amorphous- no form) consist of particles arranged in an irregular shape. The definite arrangements of particles are for short-range order. Meaning the periodic arrangement and regular order is only for short distances.

• Their arrangements at times are disordered and scattered.
• Amorphous solids are pseudo or supercooled solids. Amorphous solids show irregular cleavage.
• Their melting points are not sharp
• They are isotropic just like liquids. The values of physical properties are always the same from every direction due to short-range order.  

Example: Quartz glass, rubber, plastics, etc.