SOLID STATE, Classification of solids and Properties

  Solids: 

Solids are a state of matter in which particles are closely packed together and have a fixed shape and volume. The particles in a solid are held in place by strong intermolecular forces and do not have the freedom to move around freely like they do in liquids or gases. Solids are typically more rigid and less compressible than liquids and gases. Some common examples of solids include metals, minerals, and ice.

Classification of solids:

Solids are classified into two categories:

Property	Crystalline solid	Amorphous solid
1. Shape	Crystalline solids have sharp edges and well-defined planes.	Amorphous solids do not show regularity in their external form.
2. Order	They have regularity in the arrangement of constituent particles. They, exhibit long-range order.	There is no regularity in the arrangement of constituent particles. They possess short-range order.
3. Melting points	They have a sharp melting point ie. they change abruptly into liquid at a fixed temperature	They do not have a sharp melting point. They gradually soften over a range of temperatures.
4. Cleavage property	Crystalline solids show clean cleavage when cut with a sharp-edged tool.	Amorphous solids undergo irregular cleavage.
5. Heat of fusion	They have a characteristic enthalpy of fusion.	They do not have a characteristic enthalpy of fusion.
6. Nature	Crystalline solids are regarded as true solids	Amorphous solids are regarded as pseudo-solids or supercooled liquids.
7. Anisotropy	They are anisotropic and have different physical properties in different directions.	They are isotropic and have the same physical properties in all directions.

  

Single crystalline solids:  Single crystalline solids, also known as single crystals, are solids that are composed of a single, continuous crystal lattice. Single crystals are characterized by an ordered arrangement of atoms, molecules, or ions in a three-dimensional pattern that repeats throughout the crystal. Single crystals have a well-defined internal structure and a well-defined melting point, and they tend to be more rigid and stronger than polycrystalline solids. Example: Diamond, etc.

Polycrystalline solids: Polycrystalline solids, also known as polycrystals, are solids that are composed of many small crystals, also known as grains, that are bonded together. Polycrystalline solids do not have a single, continuous crystal lattice like single crystals do, but rather have a mosaic arrangement of grains with different orientations.

Crystallography: Crystallography is the study of the arrangements of atoms, molecules, or ions in crystals and the diffraction patterns that are produced when X-rays or other forms of radiation are passed through them. It is a branch of science that plays a key role in the fields of materials science, chemistry, and engineering, and has numerous applications in areas such as drug design, mineralogy, and electronics.

Crystallites: The crystalline part of an amorphous solid are known as Crystallite.

Classification of crystals:

Crystals are classified into four types. Depending upon the nature of constituent particles and the nature of binding forces.

(i) Ionic crystals                                                                 (ii) Covalent crystals

(iii) Metallic crystals                                                         (iv)  Molecular crystals

(i) Ionic crystals: Ionic crystals are solid materials that are composed of ions, which are atoms or molecules that have a positive or negative charge due to the loss or gain of electrons. Ionic crystals are held together by the electrostatic forces between the ions, which are much stronger than the van der Waals forces that hold atoms together in covalent crystals. Example - NaCl, KCl, etc.

Properties of ionic crystals: 

• Ionic crystals are hard and brittle. 
• These are very slightly soluble or insoluble in non-polar solvents like benzene, CCI, etc. They have high melting and boiling points.
• They are insulators in a solid-state.
• Ionic crystals are soluble in water and in other polar solvents. These are very slightly soluble or insoluble in non-polar solvents like benzene, CCI, etc.

 (ii) Covalent crystals: Covalent crystals are solid materials that are composed of atoms that are held together by covalent bonds, which are chemical bonds formed by the sharing of electrons between atoms. Covalent crystals are characterized by a strong, three-dimensional network of covalent bonds that gives them a high melting point and a high boiling point. Example Diamond.

Properties of covalent crystals:

• The covalent crystals are hard due to continuous network 
• They are poor conductors of electricity and are insulators.
• They have high enthalpies of fusion.
• Due to the multidimensional network, they have very high melting points.

(iii)  Metallic Crystals: Metallic crystals are solid materials that are composed of metallic elements and are characterized by their metallic properties, such as high electrical and thermal conductivity, malleability, and ductility. Metallic crystals are held together by metallic bonds, which are formed by the electrostatic attraction between the positively charged metal ions and the delocalized electrons that are shared by all of the atoms in the crystal.

Properties of metallic crystals:

• Metallic crystal is lustrous.
• They are good conductors of electricity.
• They are ductile and malleable.
• In this valence electrons are non-directional.  

 (iv)  Molecular crystals: Molecular crystals are solid materials that are composed of molecules that are held together by intermolecular forces, such as van der Waals forces and hydrogen bonds. Molecular crystals are characterized by a regular, repeating arrangement of molecules, known as a crystal lattice, and they typically have a low melting point and a low boiling point.

Properties of molecular crystals:

Due to low cohesive forces, these crystals are soft and have low melting and boiling points.

The major binding force between these particles is vander Waals forces.

They have high enthalpies of fusion.

They are of volatile nature. 

They are good insulators. 

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