Dipole Moment, Fajan’s Rules and Resonance

Introduction

Dipole moment is a vector quantity that gives us a measure of the polarity of a covalent bond. It is calculated as the product of charge and the distance between atoms.

We usually see the occurrence of dipole moments in any system that has a separation between the charges. Dipole moment usually arises in ionic as well as covalent bonds. 

In chemistry, the formation of molecules happens when atoms bond together by means of sharing or donating electrons. 

These bonds are formed so as to fulfil the electron requirements of atoms to create stable electronic configurations. 

Such bonds are of two types – Ionic and Covalent.

Ionic and Covalent Bonds

Ionic bonds are those bonds between two atoms where one bond requires electrons while the other has a surplus. In such cases, the atom with the most electrons donates its electrons to the lesser ones to form a stable molecule. 

For example, when a water (H2O) molecule is formed, it consists of two hydrogen atoms that donate their electron to an oxygen atom which requires two additional electrons to stabilise, creating a water molecule.

On the other hand, covalent bonds are formed when two electron-deficient atoms share their existing electron to fulfil the requirement of each other. In such a case, both the atoms share an electron to form a bond. A maximum of three bonds can be created between two atoms through covalent bonding, with each bond being denoted by a single line between them (-). 

Based on this, electric dipoles also work. The electric dipole, being a pair of two equal (and opposite) charges, is separated by a small distance, causing an attraction between the charges of the atom, leading to an electric dipole moment.

For example, when a carbon atom in CO2 shares 4 electrons with two oxygen atoms, both share 2 electrons each (O=C=O).

The Dipole Moment

A dipole moment occurs in systems that have a separation of charge, which means that a dipole moment can arise in ionic and covalent bonds. Their occurrence is due to the difference in electronegativity between two chemically bonded atoms. 

Before we understand what a dipole moment is, let us get an idea of what electronegativity means. 

Electronegativity refers to the ability of an atom to attract electrons towards itself. Generally, electronegativity increases as we move left to the right on the periodic table and decreases downwards.

When a covalent bond is formed between two atoms with different electronegativities, it has been observed that the more electronegative atom will attract the shared pair of electrons towards itself. This attraction of electrons towards one atom will induce a certain ionic nature into the covalent bond and is termed polarity. Hence, all polar bonds will be those that have some percentage of ionic nature within them, and nonpolar bonds will be those that are 100 percent covalent.

The polarity of a bond is measured in terms of its dipole moment. The dipole moment is calculated as m = q x 2d, where q is the charge, 2d is the distance between atoms, and m is the dipole moment.

Magnetic Dipole Moment

The magnetic strength of an object that produces a magnetic field is known as the magnetic moment in the object.

A magnetic moment is precisely also a magnetic dipole moment, as the component of the magnetic moment can be referred to as a magnetic dipole in this case. 

Magnetic north and south poles separate a magnetic dipole by a small distance. The magnetic dipole moments have their units in metre-kilogram-second-ampere.

The magnetic dipole moments also have dimensions of current times area or could be represented as energy divided by magnetic flux density.

Fajan’s Rule

In an ionic bond, the atom donating an electron is a cation, while the one that gains an electron is called an anion. 

It is observed that the electron cloud of the anion will experience an attraction from the cation. This is called Polarisation of anion by cation. The ability of an anion to get polarised is called polarisability. On the other hand, the cation’s ability to polarise an anion is called its polarising power. 

Since the electron cloud begins shifting to a position between the two atoms, the bond starts to acquire a covalent nature. Fajan’s Rule gives us a way to calculate this nature. The amount of covalent nature depends on the following factors:

• Charge on atoms

The greater the charge on cation and anion, the greater the polarisation. 

For example, the covalent nature increases as follows

NaCl<MgCl2<AlCl3

• Size of the ions

The covalent nature depends on the size of the cation and anions. 

The greater the size of the anion, the greater will be the polarisation. This is because a bigger cation will have less charge density, resulting in a weaker attractive force. An example is an increase in covalent nature observed below:

CaF2 < CaCl2 <CaBr2 < CaI2

On the other hand, an increase in the size of the cation decreases its covalent nature. This is because a bigger cation will have less density of charge, which will result in a weaker attractive force. The covalent nature decreases as observed below:

BeCl2 > MgCl2 > CaCl2 > SrCl2 > BaCl2

• Nature of cation

Cations with configuration ns2 np6 nd0 show greater polarisation than ns2 np6 cations. The d subshell provides a weaker shielding effect than its s and p counterparts. As a result, cations with d subshells have greater polarising power.

This effect can be seen when justifying the greater covalent nature of CuCl than NaCl.

Bond Nature and Stability (Resonance)

How does the nature of a bond affect the stability of a molecule? 

The answer is resonance.

Resonance is defined as the ability of an organic molecule to display and exist as independent resonating structures. The more resonating structure, the greater will be the stability of the molecule.

Resonance involves the breakage of old bonds and the formation of new bonds between atoms. The structures formed due to resonance always exist simultaneously in every specimen of the compound and contribute to the properties exhibited.

The nature of the bond helps us in understanding resonance. This is because the ionic bonds and covalent bonds break up differently. A covalent bond breaks up by giving each atom an electron. An ionic bond breaks by forming a cation and an anion.

A covalent bond is also much more stable than an ionic bond, and hence, more difficult to break. Thus understanding the nature of the bonds in a compound helps us understand the stability it may have.

Conclusion

Dipole moment has a variety of applications. Right from using it for finding the distinction between polar and nonpolar molecules to finding the shapes of molecules, the Dipole moment is an important phenomenon.

The concept in itself is fascinating and essential. The more familiar you are with it, the better you can solve its questions in your competitive exams.