What Is Lewis Theory? Definition, Principles & 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 in Chemistry: Concepts, Acids & Bases
1. What is the Lewis theory of acids and bases?
The Lewis theory defines acids and bases based on their ability to accept or donate a pair of electrons. A Lewis acid is any substance that can accept a pair of non-bonding electrons to form a covalent bond. Conversely, a Lewis base is any substance that can donate a pair of non-bonding electrons. The product of a Lewis acid-base reaction is called an adduct, which contains a coordinate covalent bond.
2. Can you provide a simple example of a Lewis acid-base reaction?
A classic example is the reaction between Boron Trifluoride (BF₃) and Ammonia (NH₃). In this reaction, NH₃ has a lone pair of electrons on the nitrogen atom, which it can donate. BF₃ is electron-deficient because the boron atom has an incomplete octet. Therefore, NH₃ acts as a Lewis base (electron donor) and BF₃ acts as a Lewis acid (electron acceptor) to form the adduct F₃B-NH₃.
3. How can you identify a Lewis acid or a Lewis base?
You can generally identify them based on their electronic structure:
Lewis Acids typically have an incomplete octet of electrons (e.g., BF₃, AlCl₃), are simple cations (e.g., H⁺, Mg²⁺), or have central atoms that can expand their octet (e.g., SiF₄, SnCl₄).
Lewis Bases have at least one lone pair of electrons to donate. Common examples include molecules with nitrogen (like NH₃), oxygen (like H₂O), or halogen anions (like F⁻, Cl⁻).
4. How is the Lewis theory more general than the Brønsted-Lowry and Arrhenius theories?
The Lewis theory is more comprehensive because it is not limited to reactions involving protons (H⁺) or hydroxide ions (OH⁻) in aqueous solutions. The Arrhenius theory only applies to substances that produce H⁺ or OH⁻ in water. The Brønsted-Lowry theory expands this to proton donors and acceptors, but still requires a proton transfer. The Lewis theory covers reactions without any protons, such as the reaction between BF₃ and NH₃, making it the most general of the three acid-base theories.
5. Are all Brønsted-Lowry acids also Lewis acids? Explain the difference.
No, this is a common misconception. A Brønsted-Lowry acid (a proton donor like HCl) is not itself a Lewis acid. However, the proton (H⁺) it donates is the Lewis acid because it can accept an electron pair. For example, in the reaction HCl + H₂O → H₃O⁺ + Cl⁻, HCl is the Brønsted-Lowry acid. The H⁺ from HCl is accepted by H₂O (the Lewis base), making H⁺ the actual Lewis acid. So, while Brønsted-Lowry acids *contain* a Lewis acid (H⁺), the molecule itself is defined differently.
6. What is the importance of Lewis acids in organic chemistry?
Lewis acids are extremely important as catalysts in many organic reactions. They can accept electrons from a reactant, making it more electrophilic and reactive. A prime example is their use in Friedel-Crafts reactions, where a Lewis acid like AlCl₃ is used to generate a carbocation electrophile, enabling the alkylation or acylation of aromatic rings.
7. What is the Hard and Soft Acids and Bases (HSAB) principle?
The HSAB principle is an extension of the Lewis acid-base theory that classifies Lewis acids and bases as either 'hard' or 'soft'. The principle states that hard acids prefer to bind with hard bases, and soft acids prefer to bind with soft bases, leading to more stable compounds.
Hard acids/bases are small, have a high charge density, and are not easily polarisable (e.g., H⁺, Li⁺, F⁻, OH⁻).
Soft acids/bases are large, have a low charge density, and are easily polarisable (e.g., Ag⁺, Hg²⁺, I⁻, SCN⁻).
8. Why is a water molecule (H₂O) considered a Lewis base?
A water molecule (H₂O) is considered a Lewis base because the oxygen atom has two lone pairs of electrons available for donation. In a reaction with a Lewis acid, such as a proton (H⁺), the oxygen atom can donate one of its lone pairs to form a coordinate covalent bond, resulting in the formation of the hydronium ion (H₃O⁺). This ability to donate an electron pair is the defining characteristic of a Lewis base.
9. Can a single species act as both a Lewis acid and a Lewis base?
Yes, some species can exhibit this dual behaviour, although not simultaneously in the same reaction. For example, water (H₂O) can act as a Lewis base by donating a lone pair from its oxygen atom. However, in certain reactions, it can also behave as a Lewis acid where one of its O-H bonds accepts electrons, leading to cleavage. A clearer example is a metal ion like Zn²⁺ in [Zn(OH)₄]²⁻; it acts as a Lewis acid, but the complex itself can then react in different ways. This amphoteric nature is a key concept in coordination chemistry.
