Lewis theory refers to an immensely influential theory of categorizing acids and bases. It was proposed in 1923 by Gilbert Lewis. Any chemical compound that attaches itself to a pair of electrons in another molecule is regarded as acid while the molecule that donates its electron pairs is regarded as a base. Acids and bases react and neutralize each other. This reaction is called neutralization. After neutralization, a new compound is created in which the chemical bond (that is formed from the electron pair) comes from a single reactant. As per the Lewis acid base theory, metal ions, nonmetallic (sulfur, nitrogen, phosphorous) oxides, hydrogen ion donors, proton donors, and compounds such as silica, alumina, and aluminum chloride are characterized as acids.
What is Kossel Lewis Theory?
The scientists Kossel and Lewis developed the theory of chemical bonds or the theory of electronic valency which is popularly known as Kossel Lewis theory. This theory 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 principle can be used to explain how chemical bonds are formed between any two atoms. In the atomic system, to attain stability, the atom has to attain an octet configuration in its outermost shell. This is achieved by gaining, losing, or sharing electrons.
What are Hard Acid and Hard Base?
Based on Lewis acid theory, one can easily study the chemical interactions between metals and ligands. Ligands are substances that donate their electrons. From the previous sections, we know that substances that donate their electrons are characterized as Lewis bases.
Thus, bases, hard or soft are also called ligands. Metals form ions that accept electron pairs from electron-pair donors. Since metals accept electrons, they are characterized as Lewis acids. Therefore, acids, hard or soft are used to refer to metal ions.
What is Softness?
Softness is a quality that indicates how amenable to polarization an acid or a base is. If any chemical species is larger and more electrically neutral it is softer as compared to a chemical species that is smaller in size and less electrically neutral. Size and charge density are directly proportional to the softness of a chemical species.
Now that we know what Lewis acids and bases are, we need to categorize them to understand them better. One way in which these chemical species can be categorized is based on how hard or soft they are. All Lewis acids and bases exist on a spectrum of hardness and softness. While some acids and bases exist on the extreme ends, others exist in the intermediate position. Hardness or softness is determined by the following characteristics:
Charge to size ratio
Covalent or ionic bonds
Hard Acids and Bases
Hard acids and bases are highly charged, less polarizable and their chemical bonds are ionic. They also have a smaller radius.
Hard Acids: Metal ions that have a high positive charge and small sizes are hard acids. The transition metals in the 3d series are examples of hard acids.
Hard Bases: Negatively charged ions and neutral molecules that are small in size are hard bases. The second row of the periodic table consists of heteroatoms such as nitrogen and oxygen which are examples of hard bases.
Soft Acids and Bases: Soft acids and bases are less charged, highly polarizable and their chemical bonds are covalent. They also have a larger radius.
Soft Acids: In the periodic table, the transition metals like Copper with (+1) charge or elements located towards the end of 4d and 5d series such as Cd2+ and Hg2+ are categorized as soft. They also include transition metals with +2 charge and metals with d orbitals that are completely filled.
Soft Bases: Negatively charged ions and neutral molecules that are larger. For instance, I- is a soft base.
Acids and Bases Located in the Middle of the Spectrum or Intermediate: The examples include aniline and pyridine (bases) and Fe2+, and Pb2+ (acids).
Note: The hardness of the softness of an element also depends upon other factors such as oxidation state or whether the polarizability is impacted by the presence of nearby atoms. For instance, in Hydrogen, the H+ ion is a hard acid, and the H- ion is a soft base.
Generally, acids and bases that are characterized as hard have an increased density of charge, are not very amenable to polarization, and form ionic bonds more often than not with other elements. Hard acids and bases have a smaller radius and a high concentration of charge. On the other hand, acids and bases that are characterized as soft have a lower density of charge and are quite polarizable. The nature of their bond with other elements is more often than not covalent. Soft acids and bases have a low concentration of charge and a larger radius.