What is Lewis Theory Definition Examples and Applications
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
Ligands are compounds that give away electrons. It is already known that bases release electrons and are characterized as Lewis bases.
Metals
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.
Categorization
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 of Acids and Bases in Chemistry
1. What is Lewis theory in chemistry?
Lewis theory states that a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor.
This theory, proposed by Gilbert N. Lewis, focuses on electron pairs rather than protons (H+), making it broader than Brønsted–Lowry theory.
- A Lewis base donates a lone pair of electrons.
- A Lewis acid accepts that electron pair to form a coordinate covalent bond.
- Example: NH3 + H+ → NH4+
2. What is a Lewis acid?
A Lewis acid is a species that accepts an electron pair to form a covalent bond.
Lewis acids are typically electron-deficient or positively charged.
- They often have an incomplete octet (e.g., BF3).
- They may carry a positive charge (e.g., Al3+).
- Example reaction: BF3 + NH3 → F3B←NH3
3. What is a Lewis base?
A Lewis base is a species that donates an electron pair to form a covalent bond.
Lewis bases contain at least one lone pair of electrons.
- Common examples: NH3, H2O, OH-, Cl-.
- The donated electron pair forms a coordinate (dative) bond.
- Example: H+ + :NH3 → NH4+
4. How is Lewis theory different from Brønsted–Lowry theory?
Lewis theory defines acids and bases based on electron pair transfer, while Brønsted–Lowry theory defines them based on proton (H+) transfer.
- Brønsted–Lowry acid: proton donor.
- Brønsted–Lowry base: proton acceptor.
- Lewis acid: electron pair acceptor.
- Lewis base: electron pair donor.
5. Can you give an example of a Lewis acid–base reaction?
A classic Lewis acid–base reaction is BF3 + NH3 → F3B←NH3.
- BF3 is the Lewis acid because boron has an incomplete octet.
- NH3 is the Lewis base because nitrogen has a lone pair.
- A coordinate covalent bond forms when NH3 donates its lone pair.
6. Why is BF3 a Lewis acid?
BF3 is a Lewis acid because the boron atom has only six valence electrons and can accept an electron pair.
- Boron forms three B–F bonds.
- It lacks a complete octet.
- It accepts a lone pair from a donor such as NH3.
7. What is a coordinate covalent bond in Lewis theory?
A coordinate covalent bond is a covalent bond in which both bonding electrons come from the same atom.
- The donor atom is the Lewis base.
- The acceptor atom is the Lewis acid.
- It is often shown with an arrow (→) from donor to acceptor.
8. Are all Brønsted–Lowry acids also Lewis acids?
Yes, all Brønsted–Lowry acids are also Lewis acids because a proton (H+) accepts an electron pair.
- A proton has no electrons.
- It accepts a lone pair from a base.
- Example: HCl + H2O → H3O+ + Cl-
9. What are common examples of Lewis acids and bases?
Common Lewis acids are electron-deficient species, while common Lewis bases contain lone pairs.
- Lewis acids: H+, BF3, AlCl3, Fe3+
- Lewis bases: NH3, H2O, OH-, CN-
10. What is the importance of Lewis theory in chemistry?
Lewis theory is important because it explains acid–base reactions that do not involve protons and helps describe coordination chemistry and catalysis.
- Explains formation of complex ions like [Cu(NH3)4]2+.
- Used in understanding catalysts such as AlCl3 in organic reactions.
- Provides a broader definition of acids and bases.
