Polarising Power

A chemical compound can never be completely ionic or completely covalent in nature. 100% ionic or 100% covalent bonds do not exist. The ability of a cation to distort the electron cloud of its nearby anion and pull it towards itself is known as the polarising power of the cation. How easily any cation can polarise an anion is termed the cation’s polarisability.

Generally, two electrons are shared between two atoms in a covalent bond. These two atoms can be of similar types (like hydrogen gas, chlorine gas) and sometimes can be of different types. When the bond is formed between similar atoms, then the shared pair of electrons occupy the space equidistant to both of them, i.e. the pair of electrons is exactly between them. When both the atoms are different, the electrons are not exactly between them but slightly nearer to the atom, which is more electronegative. This generates polarity in the bond. This gives some ionic character to the bond and results in the polarisation of the bond.

The covalent bond between two similar atoms is known as a nonpolar covalent bond. A covalent bond between two different atoms is known as a polar covalent bond. 

An ionic bond is formed by the complete transfer of an electron from one element to another. They are polar in nature and have high melting and boiling points. Since no compound can be 100% ionic or 100% covalent, ionic compounds have some covalent character, just like the polar covalent compounds, as explained above. This concept is explained by Fajan’s Rule.

Polarisation – Introduction to Polarising Power and Polarizability

• The presence of a cation distorts the electron cloud of an anion. This is known as polarisation. And the tendency of a cation to do so is termed as polarising power of the cation. 

• That end of the electron cloud of anion, which is towards the cation, becomes relatively more negative than the end away from it. The cation also gets slightly polarised by the electron cloud of the anion. Now, the major portion of electron density lies within the centre of both the atoms, and this is somewhat like the formation of covalent bonds. The extent of polarisation depends on the polarising power of cation.

• Fajan’s Rule states that how much polarisation occurs depends upon two factors –

1. Polarising power of the cation

2. Polarizability of an anion 

Polarising power of the cation is directly proportional to the charge on a cation and inversely proportional to the size of the cation.

In other words, the greater the charge, the higher the polarising power.

The smaller the cation, the higher the polarising power.

The polarizability of an anion is directly proportional to the charge on the anion and directly proportional to the size of an anion. In other words, it is easy to polarise an anion if it is large and has a high charge.

Fajan’s Rule

In an ionic compound, the smaller the cation and the larger the anion, the greater that ionic compound will be covalent in nature.

If both positive and negative charges are high in an ionic compound, the greater that ionic compound will be covalent in nature. 

When both cations have the same size and charge, the cation with a pseudo noble gas configuration (18 e- in valence shell) has more polarising power than the cation with a noble gas configuration(8 e-in valence shell).    

Ionic compounds In which polarising power of cation is high and Polarizability of anion is high such ionic compounds have high covalent character.

Conclusion

All covalent bonds have some partial ionic character, and all ionic bonds do have some covalent character. Fajan’s Rule tells us about the presence of covalent character in ionic compounds. The higher the polarising power of the cation and the higher the polarisability of the anion, the greater the ionic bond’s polarisation. Hence, greater is the presence of covalent character in an ionic compound.