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:
Charge to size ratio
Covalent or ionic bonds
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 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 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 also have large sizes and get polarized easily, having a less negative charge. Examples: I-.