Dipole Moment

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 typically arises in ionic as well as covalent bonds. 

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).

Dipole Moment occurrence

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.

Dipole moment formula

For the measurement of the extent of polarity, Pauling introduced the concept of Dipole moment. It is denoted by ‘µ’. The multiplication of positive or negative charge (q) and the distance (d) between two poles are called a dipole moment.

µ = q × d (magnitude of charge × distance)

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.

Applications of dipole moment

1. To determine polarity and geometry of molecule:

If µ = 0 compound is non-polar and symmetrical.

e.g. CO2, BF3, CCl4, CH4, BeF2 etc.

If µ ≠ 0, the compound will be polar and unsymmetrical.

H2O, SO2, NH3, Cl2O, CH3Cl, CHCl3 etc.

2. To calculate % ionic character:

% ionic character = Experimental value of μ÷theoretical value ofμ ×100

1. To distinguish cis-form or trans-form:

2. The dipole moment of the cis-isomer is normally higher than trans-isomers. For example;

2. If two groups have opposite inductive effects, then the trans-isomer will have a greater dipole moment. For example;

4. To locate the position of substituents in aromatic compounds.

µ =1 ÷bond angle

1. If the same substituents are present in the symmetrical position of µ of benzene ring compounds will be zero.

2. As the angle between vectors decreases, the value of µ increases.

e. The dipole moment of H2O is 1.85D, which is the resultant µ of two O-H bonds. µ of H2O > µ of H2S because the electronegativity of oxygen is higher than sulphur.

f. Angular structure of the molecule has a greater dipole moment.

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 more familiar you are with the concept of dipole moment, the better you can solve questions based on it.