Classification of Solids: In a solid substance, the constituent particles are held together by strong forces of attraction, and because of this, the particles of solids are packed in proper order or symmetry. Therefore, a solid has a definite shape, a distinct boundary, a fixed volume, and negligible compressibility. Some solids are strong, while some solids may break under force. In this article, we will explore different classifications of solids.

Solid is one of the forms of matter which can be classified under various categories. The classification of solids based on different binding forces to the classification of crystalline solids, the article focuses on all properties and categorisation of solids. Read the article to know more.

Classification of Solids

Solids have been classified as crystalline solids and amorphous solids. Let’s study about both these types in detail.

Crystalline Solids

The solids having sharp edges and well-defined planes are called crystals or crystalline solids. Crystalline solids are also called true solids. Sodium chloride, quartz, gold, copper, and iron are some examples of crystalline solids.

Amorphous Solids

The solids which lack in well-defined structure and shape are known as amorphous solids. Amorphous solids are also called supercooled liquids or pseudo solids. Glass, rubber, and plastics are some examples of amorphous solids.

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Uses of Amorphous Solids

i. Amorphous solids are useful as inorganic glasses, fabrication of glass apparatus for laboratory, and making kitchen wares.
ii. Amorphous silica is used as a photovoltaic to convert sunlight into electricity.

Difference between Crystalline Solids and Amorphous Solids

Property	Crystalline solids	Amorphous solids
Structure	Crystals have a long-range order.	Amorphous substances have short-range order.
Melting point	Crystalline solids have a sharp and characteristic melting point.	Amorphous solids either decompose or change gradually into liquid over a range of temperatures on being heated.
Cleavage	Crystals when cut with a sharp edged tool, split into two pieces and the newly generated surfaces are plain and smooth.	Cleavage of amorphous solids gives irregular fractures.
Cleavage of amorphous solids gives irregular fractures.	Crystalline substances are anisotropic. They have different electrical and optical properties along different planes of the same crystal.	Amorphous substances are isotropic and have identical electrical and optical properties in all directions.

Classification of Crystalline Solids

Crystalline solids have been classified into the following four main categories on the basis of binding forces between their constituent particles.

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1. Ionic Solids

In an ionic solid, the cations and anions are the constituent particles. These ions are systematically arranged in three-dimensional space. The strong electrostatic force of attraction between cations and anions is the binding force in an ionic solid. The ions are arranged alternatively in the crystal as \({{\rm{A}}^ + }{{\rm{B}}^ – }{{\rm{A}}^ + }{{\rm{B}}^ – }{{\rm{A}}^ + }{{\rm{B}}^ – }{{\rm{A}}^ + }{{\rm{B}}^ – }.\) This type of arrangement repeats in three-dimensional space.

Example of ionic solids- Sodium chloride \(\left( {{\rm{NaCl}}} \right){\rm{,}}\) potassium chloride \(\left( {{\rm{KCl}}} \right){\rm{,}}\) and lead bromide \(\left( {{\rm{PbB}}{{\rm{r}}_{\rm{2}}}} \right)\) are ionic solids.

Characteristics

Because of the strong electrostatic force of attraction, the ionic solids show the following properties:
i). Ionic solids are hard, brittle, and have high melting points and very high enthalpy of fusion.
ii). Ionic solids are insulators in solid state. But in the molten state and in aqueous solutions, they conduct electricity.

2. Metallic Solids

The metals like iron, copper, gold, silver, sodium, potassium exist in the solid state at room temperature. In a metallic crystal, the atoms are held together by a strong force of attraction called metallic bonding. In the metallic crystals, the metal occupies the fixed positions, but their valence electrons move.

Characteristics

i). They are hard but malleable and ductile.
ii). They are good conductors of heat as well as electricity.
iii). They are lustrous.
iv). They have high melting and boiling points.

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3. Covalent Solids

The solid substances in which covalent bonds bond the atoms throughout the crystal are known as covalent solids.
Covalent solids are also known as-
i). Atomic solids
ii). Network solids
iii). Macromolecular crystals
iv). Giant molecules

Examples- Asbestos, silicon carbide, mica, graphite, and diamond are examples of macromolecular crystals.

Characteristics

i). They are very hard. For example- Diamond is the hardest naturally occurring substance (except graphite is soft).
ii). They have high melting points.
iii). They are poor conductors of heat as well as electricity (except graphite which is an atomic solid and is a good conductor of electricity).
iv). They have high heat of fusion.

4. Molecular Solids

These are solid substances in which the molecules are the constituent particles. Iodine \(\left( {{{\rm{I}}_{\rm{2}}}} \right){\rm{,}}\) ice \(\left( {{{\rm{H}}_{\rm{2}}}{\rm{O}}} \right){\rm{,}}\) dry ice (solid carbon dioxide), and naphthalene are examples of molecular solids.

Types of Molecular Solids

Molecular solids are further classified into three subgroups as nonpolar molecular solids, polar molecular solids, and hydrogen-bonded molecular solids.

Nonpolar Molecular Solids

In such types of solids, nonpolar molecules are constituent particles. The force responsible for holding these molecules is Van der Waals’s force of attraction or London dispersion force. This molecular force of attraction is weak.

Examples- Dry ice (solid \(\left( {{\rm{C}}{{\rm{O}}_{\rm{2}}}} \right){\rm{,}}\)), iodine \(\left( {{{\rm{I}}_{\rm{2}}}} \right){\rm{,}}\) solid hydrogen \(\left( {{{\rm{H}}_{\rm{2}}}} \right){\rm{,}}\) naphthalene, and wax.

Properties

Because of the weak intermolecular force of attraction, the nonpolar molecules show the following characteristics-
i. They are soft.
ii. They have a low melting point.
iii. They have a low enthalpy of fusion.
iv. They are poor conductors.
v. They are volatile and electrical insulators.
vi. Some nonpolar molecules solids sublime and give their characteristic smell.

Polar Molecular Solids

In such types of solids, polar molecules with permanent dipole moments are constituent particles. The force responsible for holding polar molecules is called dipole-dipole interaction, which is relatively stronger as compared to dispersion forces.

Examples- Solid ammonia \(\left( {{\rm{N}}{{\rm{H}}_3}} \right)\) and solid sulphur dioxide \(\left( {{\rm{S}}{{\rm{O}}_{\rm{2}}}} \right).\)

Properties

i). These solids are soft.
ii). They are poor conductors of heat and electricity.
iii). They have higher melting points than nonpolar molecules.
iv). They have lower melting points than ionic solids.
v). Some molecular solids sublime and give their characteristic smell.

Hydrogen-Bonded Molecular Solids

This type of bonding is possible in all those solids whose molecules contain hydrogen atoms bonded to highly electronegative atoms such as \({\rm{F,}}\,{\rm{O,}}\) or \({\rm{N}}{\rm{.}}\) These solids are called molecular solids because the constituents of the crystals are molecules.

Examples- Many organic compounds with hydroxyl \(\left( {{\rm{OH}}} \right)\) groups such as phenols, alcohols, and carboxylic acids crystallize in a manner involving hydrogen bonding.

Properties

i). They are hard solids.
ii). They are poor conductors of heat and electricity.
iii). They have a low melting point.
Thus, we can conclude different types of crystalline solids based on the type of attraction forces from the table given below:

Types of Crystalline Solids Based on the Type of Attracting Force

Type of solid	Constituent particles	Attracting force	Examples (in solid state)	Physical nature	Melting point	Electrical conductivity
1. Ionic solids	Cations and anions	Electrostatic (Coulombic)	\({\rm{NaCl,}}\,{\rm{MgO,}}\,{\rm{ZnS,}}\,{\rm{Ca}}{{\rm{F}}_{\rm{2}}}\)	Hard but brittle	High	Insulator as solid but conductor in molten state and in aqueous solution
2. Metallic solids	Positive ions in a sea of electrons	Metallic bonding	Copper, silver, gold, iron	Hard, but malleable and ductile	Fairly high	Conductor in solid and molten states
3. Covalent or Network solids	Atoms	Strong covalent bonding	Graphite diamond, quartz, silicon carbide	Soft Hard	Very high	Conductor Insulators
4. Molecular (i) Nonpolar molecules (ii) Polar molecules (iii) Hydrogen bonded	Molecules Molecules Molecules Molecules	Dispersion force Dipoledipole interaction Hydrogen bonding	\({{\rm{I}}_{\rm{2}}}{\rm{,}}\) dry ice, \({{\rm{H}}_{\rm{2}}}{\rm{,}}\)   \({\rm{Ar,}}\,{\rm{CC}}{{\rm{l}}_{\rm{4}}}{\rm{N}}{{\rm{H}}_{\rm{3}}}{\rm{,}}\,{\rm{S}}{{\rm{O}}_{\rm{2}}}\) Ice, alcohol, phenol, proteins	Soft Soft Hard	Very low Low Low	Insulator Insulator Insulator

Classification of Solids Based on Their Electrical Conductivity

On the basis of their ability of current conduction, the solids have been classified as follows:
i. Conductors
ii. Semiconductors
iii. Insulators
The conductivity of metals is large, whereas the conductivity of an insulator is very less. Between these two are semiconductors.

Conduction of Electric Current in Metals

A metallic atom has more valence orbitals than the number of valence electrons. Therefore, the electrons find more space to move and conduct electricity easily. This can easily be understood by band theory which is also called the molecular orbital theory.

According to this theory, the atomic orbitals of metallic atoms form molecular orbitals. The group of molecular orbitals is called a band, and the difference in the energy of these orbitals is called a gap.
In metals, valence bands are partially occupied by the electrons, and the conducting band of higher energy is unoccupied.

The gap between the conducting band and the valence band in metals is not very large. That is, if this band is partially filled or it overlaps with a higher energy unoccupied conduction band, the electrons can flow easily under the influence of the electric field and thus conduct electricity.

Conduction of Electric Current in Semiconductors

In semiconductors, the energy gap between the valence band and conduction band is not very large. Therefore, The valence electrons have a tendency to jump into the conduction band. Thus when an electric field is applied, some electrons move to the conduction band and conduct electricity.

Conduction of Electric Current in Insulators

When the energy gap between the valence band and conduction band is very large, the valence electrons cannot jump into the conduction band. Therefore, they do not conduct electricity, and such substances are called insulators.

Classification of Solids With Respect to Magnetic Properties

The solids can be classified on the basis of their magnetic properties as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, and ferrimagnetic.

Diamagnetism

This property is caused by an induced magnetic field of the orbital electron. It is present more or less in all substances, but it is more pronounced in materials like \({{\rm{H}}_2},\,{\rm{KCl}},\,{\rm{NaCl}}\) and \({\rm{Ti}}{{\rm{O}}_{\rm{2}}}\) in which all the electrons have paired spins. The magnetic field weakly repels the substances that have diamagnetic properties.

Paramagnetism

It is caused by the spin and orbital angular momentum of electrons. Paramagnetism is shown by all the solids in which atoms, ions, or molecules have unpaired electrons. Paramagnetic materials are weakly attracted to a magnetic field, and it is shown only in the presence of the magnetic field. \({{\rm{O}}_2},\,{\rm{C}}{{\rm{r}}^{3 + }},\,{\rm{C}}{{\rm{u}}^{2 + }},\,{\rm{F}}{{\rm{e}}^{3 + }}\) show paramagnetism

Ferromagnetism

This property is caused by particles arranged on the lattice and the electrons with parallel spins. Iron \(\left( {{\rm{Fe}}} \right),\) cobalt \(\left( {{\rm{Co}}} \right){\rm{,}}\) and nickel \(\left( {{\rm{Ni}}} \right)\) are ferromagnetic elements. They are attracted very strongly by a magnetic field. It is a permanent property of the material at a given temperature.

Antiferromagnetism

This property is caused by particles arranged on two lattices with spins on one lattice antiparallel to those on the other lattice. This results in the cancellation of magnetic moments on the lattice. The materials like \({\rm{MnO}},\,{\rm{MnSe}},\,{\rm{KMnF}}{{\rm{e}}_3}\) are antiferromagnetic.

Ferrimagnetism

The particles on interpenetrating lattices cause this magnetic property with the unequal number of electrons and with antiparallel spins. This represents a situation with a net magnetic moment. \({\rm{FeO}}.{\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3} \equiv {\rm{F}}{{\rm{e}}_3}{{\rm{O}}_4}\) is an example of ferrimagnetic material.

Summary

In this article, we studied the different types of classification of solids. We also studied the classification of crystalline solids and the classification of solids based on their magnetic property and electrical conductivity. We now know the difference between crystalline solids and amorphous solids and also the difference between different crystalline solids.

Frequently Asked Questions (FAQs) – Classification of Solids

Q.1. What are solids? Classify them.
Ans: In a solid, the constituent particles are held together by strong forces of attraction, and because of this, the particles of solids are packed in proper order or symmetry. Therefore, a solid has a definite shape, a distinct boundary, a fixed volume, and negligible compressibility. Some solids are strong, while some solids may break under force.
Solids have been classified as crystalline solids and amorphous solids.

Crystalline Solids- The solids having sharp edges and well-defined planes are called crystals or crystalline solids. Crystalline solids are also called true solids. Sodium chloride, quartz, gold, copper, and iron are some examples of crystalline solids.

Amorphous Solids- The solids which lack in well-defined structure and shape are known as amorphous solids. Amorphous solids are also called supercooled liquids or pseudo solids. Glass, rubber, and plastics are some examples of amorphous solids.

Q.2. What are the four types of crystalline solids?
Ans: The four types of crystalline solids are as follows-
i. Ionic solids
ii. Metallic solids
iii. Covalent solids
iv. Molecular solids

Q.3. What are the two types of solids?
Ans: There are two types of solids which are crystalline solids and amorphous solids.

Q.4. What are the two major classifications of crystalline solids?
Ans: The major classification of crystalline solids are as follows-
i. Ionic solids
ii. Metallic solids
iii. Covalent solids
iv. Molecular solids

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