Solid state


Solid is one of the three states of matter in which the intermolecular forces of attraction are very strong, and they contain minimal empty spaces between the structures. For this reason, solids have fixed shapes and volume. The reason behind why solids are tough is that the constituent particles of solids have fixed positions and are not free to move from one place to another(as in the case of liquids and gases)  i.e., they possess rigidity.



Properties of solids

  1. Unlike liquids and gas, all solids have definite mass, volume, and shape hence they have a higher density.

  2. Solids are not compressible because the molecules within have no scope for the free expansion consequently, they become hard, rigid and compact.

  3. Solids have strong intermolecular forces of attraction due to their short intermolecular distance between the particles.

  4. Solids have definite mass, volume and shape due to which they have a compact arrangement of constituent particles.

  5. The force between the constituent particles (atoms, molecules or ions) is very strong resulting in the shorter intermolecular distance between their molecules.




Classification of solids according to the arrangement of constituent particles

Solids can be categorized based on their properties and the type of bond found in their structure. Basing on the various properties, solids can further be grouped as either crystalline and amorphous solids. In amorphous solids, the structures are irregular over a long distance, and their properties are not as sharp as the crystalline form. Crystallines are regular throughout and have sharp properties.



Crystalline solids 





Features of crystalline solids

  1. Crystalline solids have definite geometrical shapes.

  2. They have extended range order.

  3. Crystalline solids have sharp melting points.

  4. When measured along  different directions in the same crystal, crystalline solids are anisotropic.

  5. They have definite and characteristic heat of fusion.

  6. Crystalline solids form plain and smooth surfaces when cut using sharp edge tools.




Classification of crystalline solids

In general, all solids can be classified based on molecular structure and conductivity. On the basis of the nature of intermolecular forces, crystalline solids are divided into Molecular Solids, Ionic solids, Metallic solids and covalent solids. Molecular solids are those that contain molecules as particles. They are further divided into three categories: non-polar molecular solids, Polar Molecular Solids and Hydrogen-Bonded Molecular Solids



Classification of crystalline solids based on the structure

Molecular Solids

Non-Polar Molecular Solids are formed by the atoms that share a non-polar covalent bond. The atoms or molecules are packed with the help of London forces. They are soft, and they don’t conduct electricity hence they are insulators. Furthermore, they have a very low melting point , some of them are H2, Cl2, and I2. Polar Molecular Solids are those solids held together by polar covalent bonds. The atoms are bonded by stronger dipole-dipole interactions. They are in their gases or liquids form at room temperature, and they do not conduct electricity. Unlike non-polar molecules, they have a higher melting point than the non -polar molecular solids. For example, HCl, SO2, NH3 etc. Hydrogen-Bonded  Molecular Solids comprises polar covalent bonds in which the molecules are bonded together with the help of strong hydrogen bonding. They are pretty hard, and they do not conduct electricity. Hydrogen-bonded molecular solids have a low melting point. For example, H2O (Ice).



Ionic Solids

The constituent particles of ionic solids are ions, and they are formed by the strong columbic forces between cations and anions. Ionic solids are hard and brittle in nature, and they act as an insulator in a solid-state, but they are capable of conduction while in a molten and aqueous state. They have a strong melting point. Some of the examples are  NaCl, MgO, ZnS,



Metallic Solids

Metallic solids are made up of metal atoms held together by strong metallic bonds. The bonds act like huge molecular orbitals that span across the whole solid. In other words, the electrons in metallic solids are delocalized hence they are good conductors in both solid and molten state. They are malleable and ductile and have a higher melting point when compared to the ionic solids. For example, Fe, Cu, Ag, Mg, etc.



Covalent or Network Solids

These are the crystalline solids of the non-metal form containing covalent bond between the adjacent atoms. Network solids are the hardest among the crystalline solids, diamonds and graphite are the best examples.



Classification of solids based on their electrical conductivities


A conductor is a substance which is usually a solid which allows electricity to pass through it and its conductivity ranges between 104-107 ohm-1 m-1.



A semiconductor is a solid whose conductivity ranges from 10-6- 104 ohm-1m-1. Some electrons in such substances move to the conduction band because the forbidden gap is small. Their conductivity increases with increase in temperature since more electron(s) can move to the conduction band.


Types of semiconductors

Semiconductors are divided into two categories. These include:

  1. Intrinsic

Intrinsic semiconductors refer to solids/substances in which the forbidden gap is small. Few electrons can excite into the conduction band which can lead to some conductivity. Intrinsic semiconductors experience low electrical conductivity. A good example is silicon and germanium.


  1. Extrinsic semiconductors

An extrinsic semiconductor is formed by adding impurities to an intrinsic semiconductor. Electrical conductivity in such substances is high. Doping refers to the process of adding the recommended amount of impurities in a semiconductor with the aim of increasing its electrical conductivity. Extrinsic semiconductors are further classified as either n-type or p-type. The n-type semiconductors are formed when silicon, for example, is doped with an electron(s) rich impurity like those in group 15 elements. The presence of negatively charged holes increases the electrical conductivity of these semiconductors. On the other hand, the p-type semiconductors are formed when silicon is doped with an electron(s) deficient impurity like the group 13 members of the periodic table. The presence of positively charged holes increases the electrical conductivity of the p-type semiconductors.


  1. Insulators

Insulators are substances/solids with very low conductivity that ranges from 10-20- 10-10 ohm-mm-1. The electron(s) are unable to jump into the conduction band due to the very large forbidden gap in the structure.


Amorphous solids

Features of amorphous solids

  1. Amorphous solids have an irregular shape

  2. These solids are isotropic. (they have uniform physical properties in all directions)

  3. Unlike crystalline solids, amorphous solids don't have a specific heat of fusion.

  4. Due to their tendency to flow, though very slowly, amorphous solids are said to be pseudo/super solids.

  5. Amorphous solids form two pieces with irregular surfaces when cut using sharp edge tools.

  6. Amorphous solids are always softened when exposed to a range of temperatures.

  7. They have don’t possess any symmetry.



Classification of solids depending on the type of bond present in the structure

Solids are classified into four main types according to the type of bond present in their structures as discussed below.


Type of solids

Constituent particles

Melting point

Electrical conductivity


Physical nature





Conduct only in the molten and aqueous state

Electrostatic forces

Hard but brittle

NaF, CaCl2, MgSO4 etc.


Positive ions in a sea of delocalized electrons

Fairly high

Conducts both in the solid and molten state

Metallic bond

Malleable and ductile

Na, Al, Mg, Be, Li, etc.


  1. Polar  


Very low




Solid ammonia, HCl, solid SO2, etc

  1. Non-polar




Dispersion forces


Argon, hydrogen gas, carbon dioxide, etc.

  1. Hydrogen




Hydrogen bond


Water molecule

Covalent/ network


Fairly high

Conducts both in the solid and molten state


Malleable and ductile

Silicon dioxide, diamond, graphite, etc.




Structure of crystalline solids


Crystal lattice




The crystal lattice is the regular arrangement of the ions/atoms/molecules inside the crystalline solid. A unit cell is the smallest part or crystal lattice which can be recurrent in all directions to produce the crystal lattice. In this unit cell, small spheres can be used to represent the ions/atoms/molecules. The following parameters can be used to determine the type of lattice to be formed:

  1. The length of the edges along the three axes in the Cartesian plane.

  2. The interfacial angles.

  3. The location of atoms/ions/molecules with respect to each other in the crystal lattice.





Seven crystal systems

The seven crystal systems are known as primitive unit cells /crystal habits as illustrated below.



Glass -a supercooled liquid

The glass is an amorphous solid. Just like liquids, it tends to flow, though very slow. This can be seen from the glass panes of windows or doors of very old buildings which are thicker at the bottom than at the top. Therefore, glass is considered as a supercooled liquid.





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