State Fajan’s Rule

Inorganic scientist Kazimierz Fajan proposed the Fajans’ rule in 1923, which is used to predict whether a chemical bond would be covalent or ionic. The charge and size of the cations and anions determine this.

Ionic is sodium chloride, which has a low positive charge (+1), a big cation, and a tiny anion. However, aluminium iodide with a big anion and a high positive charge is covalent. The following factors will enhance polarisation:

The cation’s tiny size and high charge

Armstrong Z+/r+ and ionic potential

(power to polarise)

The polarizability of an anion is connected to the deformability of its electron cloud because of its vast size and high charge.

An electron configuration with an incomplete valence shell.

The cation process with a noble gas configuration has superior shielding and less polarisation power. Hg2+(r+=102pm) is more polarising than Ca2+(r+=100pm), for example.

The number of electrons transported determines the size of the charge in an ionic connection. An aluminium atom with a +3 charge, for example, has a greater positive charge. The electron cloud of the other ion, which has accepted the electrons from the aluminium positive ion, is then attracted by this positive charge.

Types of Chemical Bonds

In chemistry, chemical bonds are split into two types: ionic and covalent bonds. Despite the fact that ionic and covalent bonds are two extreme kinds of bonds, most bonds are neither entirely covalent nor purely ionic. Ionic bonds are those that entirely transfer electrons from one atom to the next. The term “covalent bonding” refers to bonds in which electrons are shared equally. As a result, bonds in nature typically have traits of both types of bonds, with one bond denoting more of its characteristics to a specific bond.

In the liquid state, ionic compounds have high melting and boiling temperatures, as well as electrical conductivity. These chemicals are soluble in polar solvents like water and nonpolar solvents like carbon tetrachloride and benzene, but only to a limited extent. Some ionic compounds, however, are quite soluble in nonpolar solvents in their molten state and may have lower melting temperatures and electric conductivity.

The shift in the properties of covalent bonds clearly demonstrates that some ionic compounds may have a less covalent nature. In the same way, certain covalent molecules are less ionic than others. The notion of anion polarisation explained these data, leading to the establishment of Fajan’s rule in 1923. The nature of chemical bonding could now be determined thanks to Fajan’s rule.

What is Fajans’ Law?

The Fajans’ law predicts whether a chemical bond will be changed to covalent or ionic. Few ionic bonds contain partly covalent properties, which Kazimierz Fajans first discussed in 1923. He was able to anticipate ionic or covalent bonding with attributes like atomic or ionic radius using x-ray and crystallography.

Two different examples can be used to demonstrate the effect variation. A link that is ionic yet has a lot of covalent nature is present in aluminium iodide. The aluminium bonding acquires a +3 charge in All3. It attracts the enormous charge of the electrons in the iodine cloud. When we look at the iodine atom, we can see that it is quite huge. As a result, the nuclear charge is mostly insulated from the outer shell electrons.

In this situation, the aluminium ion charge will tug on the iodine electron clouds, drawing it closer to itself. The negative charge of the electron cloud, which is near the aluminium atom as the iodide electron clouds, cancels out the positive charge of the aluminium cation. With the help of a covalent character, an ionic bond is formed. When compared to a cation with a pseudo inert gas like configuration, a cation with an inert gas like configuration has less polarising power.

What are the postulates of Fajan’s Rule?

Fajan’s rule is based on three primary factors or postulates: ion size, electronic configuration, and cation charge. The following are the specifics of these three factors or postulates:

The smaller the cation and the larger the anion, the greater the covalent character of the ionic bond.

The charge of the cation: when the charge of the cation grows, the covalent property of an ionic bond increases.

Electronic configuration: If two cations have the same charge and size, those with an electronic configuration of (n-1)dn nso are known to have a larger covalent character than those with an electronic configuration of ns2 np6, which is a common electronic configuration seen in alkali metals and alkali earth metals.

What are the factors that can affect the polarisation of anion?

Fajan’s rule is based on the concept of polarisation, which claims that as an anion’s polarisation rises, its covalent nature increases. The polarising power of the cation and the polarizability of the anion are the two basic elements that influence the extent of polarisation of an anion. The next sections go through these two factors in greater depth:

The ability of a cation to polarise an anion is characterised as its polarising power. The polarising power of a cation increases with increasing charge density, charge on the cation, and cation size, while decreasing with increasing anion size.

The polarizability of an anion: The capacity of an anion to undergo polarisation, or how easily an anion undergoes deformation due to the presence of a cation, is characterised as its polarizability. The polarizability of an anion grows as the size of the anion and the negative charge on the anion increases.

Conclusion

Fajans’ rule, developed by Kazimierz Fajan in 1923, is used in the inorganic branch of chemistry to determine whether a chemical connection will be covalent or ionic. It is determined by the cation’s charge and the cations and anion’s respective sizes. When we need to forecast which compound is the most ionic among a set of compounds, we use Fajan’s rule. Based on the atomic and ionic radius, Kazimierz Fajans used X-ray crystallography to predict ionic or covalent bonding in 1923.