Dhristi JEE 2022-24

Lewis Acid Base Theory

Kossel and Lewis were two scientists who developed the theory of electron valency popularly known as Lewis theory which is based on the principle that a higher energy state is the least stable. Therefore, every system aims to achieve stability or equilibrium by losing some energy. This theory is most generally in regards to acids and bases. Before delving into other concepts related to Lewis theory, let us understand a few terms first.

Acids and Bases

Chemical compounds that attach themselves to a pair of electrons in other molecules are called acids while those molecules that release electrons and thereby bond with other molecules are called bases.


Ligands are compounds that give away electrons. It is already known that bases release electrons and are characterized as Lewis bases.


Metals are elements that accept electrons. From Lewis theory, it is known that acids accept a pair of electrons and are characterized as Lewis acids.

Softness and Hardness

Hardness is characterized by the relatively nonpolarizable donor or acceptor atoms.

In the same way, softness indicates how easily polarization takes place in an acid or base.

The larger a chemical species is, the more stable it is, therefore, less susceptible to electronegativity changes. Similarly, the small size of any chemical entity allows it to lose or accept electrons readily. Hence, it can be concluded that size and charge density are directly proportional to the softness of the chemical entity.


Lewis acids and bases can be categorized broadly into hard acids and bases, and soft acids and bases. Now the hardness or softness of acids and bases can be determined by the following characteristics:

  • Size

  • Charge density

  • Charge to size ratio

  • Polarizability 

  • Covalent or ionic bonds

Hard Acids

Hard acids have a high positive charge, are nonpolarizable and have ionic chemical bonds. They also have a small size hence, less tightly bonded. Example:  transition metals of 3d series.

Hard Bases

Hard bases are negatively charged and they also have a small size. They also have a high charge and are less polarizable. Examples: nitrogen, oxygen.

Soft Acids

Soft acids are characterized by the large size of metal ions thus, are easily polarized, have a less positive charge, and have covalent bonds. Examples: Cu1+, Hg2+.

Soft Bases

Soft bases also have large sizes and get polarized easily, having a less negative charge. Examples: I-.

FAQs on Lewis Theory

1. What is the Lewis theory of acids and bases?

The Lewis theory of acids and bases is a method of categorizing acids and bases given by G.N. Lewis. This theory is based on the principle that the higher the state of energy, the less stable the system is. Therefore, every element tries to become stable by losing or gaining electrons.

Get detailed information on Lewis's theory on Vedantu. You can also get study materials, question papers, and a lot more for your exam preparation which is available for download for free. Make use of these resources and get good marks in your Chemistry exam.

2. What are hard acids and bases?

Hard acids are those metal ions that can readily accept electron pairs due to the small size of the ions and have a high positive charge. For example, H+.

Hard bases are those that have a high negative charge and can lose electrons very easily. They also have a small size. For example, O2-.

3. What are soft acids and bases?

Soft acids are those metal ions that cannot accept electrons easily due to their large size of radii. They have a less positive charge. For example, Cu1+ ions.

Soft bases are those metal ions that do not lose electrons readily due to their large sizes which makes them more stable and have a less negative charge. For example, I.

4. What are the factors affecting the hardness or softness of acids and bases?

There are a few main factors that affect the nature of acids and bases, that is if an acid or a base will be hard or soft. Those factors are:

  • Covalent or ionic bonds

  • Size of the metal or ligand

  • Ease of polarization

  • Charge density

  • Charge to size ratio

5. Give examples of acids of bases located in the medium of the spectrum.

Some elements with some amount of positive or negative charge located in the medium range of the spectrum can also be categorized as acids or bases. Two examples of such bases are pyridine and aniline, and two acids are Fe2+ and Pb2+.

6. What Is Lewis's Theory of Bonding?

Lewis theory of bonding is essential to understand chemistry as a whole since it explains the nature of chemical bonding. This theory sought to provide a working knowledge of how elements bonded with each other and to offer a simple graphic representation of electrons within the molecules. 

The theory emphasizes the role of electrons in the valence shells in forming chemical bonds. How an element bonds to the other depends on how it fills its octets to get an electronic configuration which is that of the noble gases. Lewis observed various elements and discovered that all of them did not follow the same rules of chemical bonding. To explain this, he proposed two kinds of chemical bonds.

7. What are the Two Types of Chemical Bonds According to Lewis? 

Lewis identified two distinct ways in which electrons are used to create chemical bonds. The first type is ionic bonding which involves a complete transmission of electrons and the second type of bonding is covalent bonding which involves not a complete transfer but sharing of electrons. 

He broadened his theory of covalent bonding by proposing the theory that atoms could not only form single bonds but double or even triple bonds with other atoms in the process of sharing electrons to form a covalent bond. If the bond is a single bond then two electrons are shared between the two atoms. In a double bond, the number of electrons shared between the atoms is four. Finally, in a triple bond, there are six electrons in total that are shared between the two atoms.