Classification of Crystalline Solids: Incompressibility, rigidity, and mechanical strength are all characteristics of solids. This means that the molecules, atoms, or ions that make up a solid are packed tightly together. They cannot move at random because they are bonded together by strong, cohesive forces. As a result, all molecular, atomic, and ionic arrangements in solids are ordered.

Some solids, such as sodium chloride, sulphur, and sugar, have distinct geometrical forms in addition to being incompressible and rigid.  Such substances are said to be crystalline solids. To learn more about crystalline solids, read the below article.

Classification of Crystalline Solids

The solid in which the constituent particles are arranged in a regular fashion containing long-range order is known as crystalline solid. Different crystalline solids may have different structural units (atoms, molecules, or ions) and different types of cohesive forces that hold them together in the crystal. The crystalline solids can be classified into different categories based upon the nature of the constituent particles and the binding forces present between them, as shown below;

Ionic Solids

Positive and negative ions make up the constituent particles in ionic crystalline solids (i.e., cations and anions). For example, in the case of \({\rm{NaCl,}}\) the ions \({\rm{N}}{{\rm{a}}^{\rm{ + }}}\) and \({\rm{C}}{{\rm{l}}^ – }\) are arranged in three-dimensional space. Coulombic forces of attraction hold each ion of giving a sign to all ions of opposite sign.

These forces are very strong, and, therefore, the amount of energy required to separate ions from one another is very high.

Accordingly, the ionic solids have the following characteristics:

1. Due to the strong electrostatic force of attraction, they have high melting and boiling points.
2. The heat of vaporisation of ionic crystals is high.
3. Ionic solids are hard because the ions are densely packed due to strong electrostatic forces of attraction. They are brittle, however, because their stability is dependent on the preservation of their geometric pattern.
4. The vapour pressures of ionic crystals at ordinary temperatures are very low.
5. Ionic crystals are soluble in water and also in other polar solvents. They are insoluble in non-polar solvents such as benzene and carbon tetrachloride.
6. Ionic crystals are solid-state insulators. This is because ions in ionic solids are imprisoned in fixed positions in the crystal lattice and are unable to move when an electric field is applied. However, when melted, they become good electrical conductors. This is because when an electric field is applied to a molten state, the well-ordered organisation of ions in the crystals is broken, and the ions are to move around in the liquid medium.
7. Examples of ionic solids are Salts like \({\rm{NaCl}},\,{\rm{KCl}},\,{\rm{LiF}},\,{\rm{ZnS,}}\) etc.

Molecular Solids

In molecular solids, the constituent particles are molecules that do not carry any charge. The forces binding the molecules together are of two types: (i) Dipole-dipole forces (ii) van der Waals forces.

Depending upon the nature of molecules, these are further subdivided into the following three types;

Non-polar Molecular Solids

These are crystalline solids in which the component particles are either noble gas atoms (helium, neon, argon, and so on) or non-polar molecules (\({{\rm{H}}_2},\,{\rm{C}}{{\rm{l}}_2},\,{{\rm{I}}_2},\,{\rm{C}}{{\rm{H}}_4},\) and so on).

Weak dispersion forces or London forces (a form of van der Waals force) exists between them in the case of helium, as do momentary dipole-induced dipole forces.

The main characteristics of non-polar molecular solids are as follows:

i). Because of the weak intermolecular forces present in these solids, they are generally soft.
ii). They are usually gaseous or liquid at room temperature and pressure due to weak intermolecular forces.
iii). They are non-conductors of electricity as there is no ions present.
iv). They have low melting and boiling points because they are soft.

Polar Molecular Solids

Polar molecular solids are crystalline solids whose constituent particles are polar molecules such as \({\rm{HCl}},\,{\rm{S}}{{\rm{O}}_2},\) and so on. Dipole-dipole forces of attraction are the forces that hold these molecules together. These intermolecular forces of attraction are comparatively stronger than London dispersion forces.

Following are the main characteristics of polar molecular solids:

I. These solids are soft.
II. They exist as gases or liquids under room temperature and normal pressure due to their low melting and boiling points.
III. Their melting and boiling points are higher than non-polar molecular solids, but not by much.
IV. They are non-conductors of electricity.

Hydrogen-bonded Molecular Solids

The constituent particles of hydrogen-bonded molecular solids are molecules that contain a hydrogen atom linked to a highly electronegative atom of small sizes, such as \({\rm{F,}}\,{\rm{O,}}\) or \({\rm{N,}}\) as in \({{\rm{H}}_2}{\rm{O}},\,{\rm{N}}{{\rm{H}}_3},\) and so on. As a result, the strong hydrogen bonds that exist between these molecules serve as intermolecular forces of attraction.

The main characteristics of Hydrogen-bonded molecular solids are as follows:

I. Their melting and boiling points are often greater than those of the polar and non-polar molecular solids.
II. Under room temperature and normal pressure, they exist as volatile liquids or soft solids.
III. They are non-conductors of electricity.

Covalent Solids

Covalent solids are crystalline solids in which the constituent particles are non-metal atoms connected to adjacent atoms by covalent bonds. As a result, a network of covalent bonds is formed. Therefore, they form massive molecules. Diamond is one of the most well-known examples of this type of crystal, in which covalent bonds bind the carbon atoms together to form a three-dimensional structure. Silicon carbide is another well-known example.

Each carbon atom is covalently bonded in diamond by sharing electrons to four other atoms involving sp3 hybrid orbitals. Thus, each carbon atom is surrounded by four others at the four corners of a regular tetrahedron. This gives rise to a rigid three-dimensional network. This is why diamond is the hardest substance known, with a high density and melting point. The entire crystal is regarded as one large carbon molecule and is called a macromolecule

Exception: Graphite is a covalent solid that is soft and has strong electrical conductivity. Because of its unique structure, it exhibits exceptional behaviour. Carbon atoms are organised in different layers in this case. Every carbon atom in each layer is bonded to three other carbon atoms. As a result, the fourth electron of each carbon atom is to move about.

Graphite becomes a good conductor of electricity due to the presence of these electrons in different layers. Furthermore, the distance between neighbouring layers is higher than the length of the carbon-carbon bond. As a result, these layers have not adhered to one another and can readily slide over one another. As a result, graphite is soft and acts as a good solid lubricant.

The main characteristics of covalent solids are as follows:

I. They do not conduct electricity since they are insulators. Except for graphite which is a good conductor of electricity.
II. These solids are extremely hard and brittle due to the strong and directional nature of covalent bonds. Except for graphite which is soft and is used as lubricants and lead in a pencil.
III. Their melting points are extremely high, and they may even decompose before melting.

Metallic Solids

Metals are characterised by high electrical and thermal conductivity, bright lustre, malleability, ductility and high tensile strength. In the case of metallic solids, the constituent particles are positively charged metal ions and electrons. These are made from metal atoms because metal atoms have low ionisation energy and can easily lose their valence electrons, leaving behind positively charged ions (called kernels). These electrons can easily flow through the metal crystal-like water in the sea. Hence, we call it a sea of electrons.

Electron Sea Model

To account for the nature of bonding in metals, H. A. Lorentz proposed a model known as the ‘electron sea’ model. According to this model, a metal behaves like a collection of positive ions (kernels) submerged in a sea of mobile electrons. As a result, each electron is associated with a number of positive ions, and each positive ion is associated with a number of electrons.

The force that binds a metal atom to a number of electrons within its sphere of influence is known as a metallic bond.

The main Characteristics of metallic solids are as follows:

1. Due to the presence of mobile electrons, they have high electrical conductivity. These electrons move readily in an electric field and thus conduct electricity throughout the metal from one end to the other.
2. The existence of these mobile electrons is also responsible for the high thermal conductivity. When the electrons in one region of metal are heated, they gain a lot of kinetic energy. These electrons flow quickly through the crystal and to other areas of the metal because they are.
3. They possess lustre and colour in some cases. This can also be explained on the basis of highly mobile electrons.
4. Due to the close packing of positive ions in the crystal, most metals have high melting points and densities.
5. They exhibit a high degree of malleability and ductility: unlike ionic crystals, the positions of positive ions can be changed without breaking the crystal because of the uniform charge distribution given byly moving electrons. As a result, metals are easily deformed.

Summary

From this article, we can conclude that the solid in which the constituent particles are arranged in a regular fashion containing long-range order is known as crystalline solid. The crystalline solids can be classified into different categories based on the constituent particles’ nature and the binding forces present between them. These include Ionic Solids, Metallic Solids, molecular solids, and Covalent Solids.

Frequently Asked Questions – Classification of Crystalline Solids

1. What are molecular solids? How are they classified?
Ans: The solids in which the constituent particles are molecules that do not carry any charges are called molecular solids.
Depending upon the nature of molecules, molecular solids are classified into the following classes;
1) Non-polar molecular solids
2) Polar molecular solids
3) Hydrogen bonded molecular solids

2.What are the seven crystal systems?
Ans: The seven crystal structures are
1) Cubic system
2) Orthorhombic system
3) Tetragonal system
4) Monoclinic system
5) Triclinic system
6) Hexagonal system
7) Rhombohedral system

3.What are the types of crystal structures?
Ans: The types of the crystal structure are;
1) Ionic crystals
2) Molecular crystals
3) Covalent crystals
4) Metallic crystal

4. What are the \(4\) types of crystalline solids?
Ans: The four types of crystalline solids are;
1) Ionic solids
2) Molecular solids
3) Covalent solids
4) Metallic solids