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Lucid Understanding Of, “Dipole Moment Of A Molecule”:

Updated on September 29, 2015

What is meant by, “Dipole”?

The molecule of a compound which contains two dissimilar atoms is called, "hetero-nuclear diatomic molecule". In such a molecule the shared electron pair of a covalent bond is not equally shared between two constituent atoms. It remains closer to the more electronegative atom. Such uneven distribution of electron is known as, "dispersion of electron cloud".

This results in the separation of charge due to which, a more electronegative atom of the bond acquires partial negative charge (denoted by ∂-), while a less electronegative atom of the bond acquires equal magnitude of partial positive charge (denoted by ∂+).

Thus, such covalent bond has two poles within it: its one end is a positive pole while the opposite end is a negative pole.

This is known as, “dipole”.

What is meant by, “Polar Bond”?

What is meant by, “Polar Bond”?

A covalent bond containing dipoles is known as a “polar bond”, and this property is known as “polarity of bond”.

The covalent compound containing such polar bond shows partial ionic character.

For example, the covalent bond between hydrogen and chlorine in the molecule of HCl is a polar bond. Here chlorine atom is more electronegative than hydrogen. Hence shared electron pair of this bond remains closer to chlorine atom. This gives partial negative charge to chlorine atom while equal amount of partial positive charge to hydrogen atom. (See the figure given below.)

Due to polarity of bond inter molecular attraction among the constituent particles of compound increases. Hence some of the physical properties of compound like: melting point, boiling point, solubility etc. are affected by such phenomenon.

What is called, “Dipole Moment”?

The intensity of opposite charges developed in polar bond is known as: “degree of polarity of a covalent bond” which is also known as, “Dipole Moment of a covalent bond”.

Thus the dipole moment of a polar bond may be regarded as, “degree of dispersion of shared electron cloud in a particular direction of the bond producing dipoles”.

As dipole moment is a vector quantity, it has both: (a) magnitude as well as (b) direction.

Dipole moment of a molecule is symbolized by Greek letter-μ and can be determined either through an experiment or by calculation.

As it is a vector quantity, it is denoted by a crossed arrow, with its tail pointed towards positive end while head towards negative end.

Dipole moment of H-Cl molecule is shown in the following figure.

Pictorial Representation of Dipole Moment in the molecule of HCl

Source

Which Types of Molecules can show Dipole Moment?

As discussed above, only such molecules which contain "hetero covalent bond" can show dipole moment.

This is because only dissimilar atoms can have difference in their electronegativity values which gives rise to dispersion of electron cloud.

However, molecules containing "homo covalent bond" (means bond formed between two similar atoms), can not show dipole moment. This will be clear from following two tables.

Dipole Moment Values of Some Homo Atomic Molecules

Name of molecule (its formula)
Dipole Moment in Debye unit
 
Hydrogen (H2)
0.00
 
Chlorine (Cl2)
0.00
 
Nitrogen (N2)
0.00
 
Bromine (Br2)
0.00
 
Iodine (I2)
0.00
 
Due to absence in difference in electronegativity values of constituent atoms, homo atomic molecules show zero dipole moments.

Dipole Moment Values of Some Hetero Atomic Molecules

Name of molecule (its formula) )
Dipole Moment in Debye unit
 
Hydrogen chloride (HCl)
1.04
 
Hydrogen cyanide (HCN)
2.93
 
Water (H2O)
1.85
 
Hydrogen sulphide (H2S)
1.10
 
In hetero atomic molecules, dipole moment exists due to difference in electronegativity values of constituent atoms.

Is it important to study the Dipole Moment of all Compounds?

Study of dipole moment is useful when,

(a) Compound under consideration is a covalent compound which contains one or more covalent bonds or

(b) It is an ionic compound with some covalent character.

Here it must be noted that, all ionic compounds possess high values of dipole moment. This is because of large difference in electronegativity values between constituent atoms.

For example the value of dipole moment of potassium chloride (KCl) is as high as 10 D or 3.336 X 10-29 Cm.

However, the study of dipole moment of ionic compounds has only limited application like in calculation of their percentage ionic character. It can not give any indication regarding:

(a) Their shapes

(b) Bond angle

(c) Orientation of constituent atoms in space etc.

Quantitative Aspects of Dipole Moment

How to determine the degree of polarity of the given bond?

Of course by measuring its dipole moment value!

Quantitatively, the degree of polarity or dipole moment of a given covalent bond depends upon following factors:

(a) Difference in electronegativity values of constituent atoms. More is the difference higher is the value of dipole moment. This is clear from dipole moment values of various hydrogen halides given in the following table.

(b) Distance between the two charges (which is also called, “bond length”). More is the distance more is dispersion of the electron cloud and higher is the value of dipole moment.

Mathematically, dipole moment is the product of: charge X distance.

It can be computed by applying following formula:

μ = q X d;

Where: μ = dipole moment,
q = charge on either atom, and
d = bond length.

(Note: Though both positive as well as negative charges exist in a polar bond, for computation of dipole moment, magnitude of only positive charge is taken into consideration.)

Values of Dipole Moments of Various Hydrogen Halides

Name of molecule (formula)
Dipole moment in Debye
Electronegativity values of halogen atoms (on Pauling scale)
Bond-length (in Angstrom)
 
Hydrogen fluoride (HF)
1.91
F=3.92
H-F=0.92
 
Hydrogen chloride (HCl)
1.04
Cl=3.16
H-Cl=1.27
 
Hydrogen bromide (HBr)
0.78
Br=2.96
H-Br=1.41
 
Hydrogen iodide (HI)
0.38
I=2.66
H-I=1.61
 
Combined effects of electronegativity and bond-length determine overall dipole moments. As the electronegativity of halogen increases, value of dipole moment also increases. However, increase in electronegativity value outweighs the decrease in bond

Comparison of Dipole Moments of HF and HCl

It is interesting to know that both HF and HCl are polar molecules containing common atom hydrogen.

However, passing from HF to HCl, dipole moment is found to decrease from 1.91 D to 1.04 D. As electronegativity and bond-length for halogens are inversely proportional to each other, the equation: μ = q x d suggests two opposite trends as below:

(a) Due to higher electronegativity value of Fluorine (4.0) than that of Chlorine (3.0), dipole moment of H-F should be greater as value of “q” will be more in the said equation;

(b) But on the basis of bond-length values, dipole moment of H-F should have lesser value than that of HCl, because “F” is smaller than “Cl” giving lower value for “d” in the equation.

However, the actual value of dipole moment for HF is higher. How to attribute this controversy?

This can be explained by following consideration.

Here, the ‘magnitude of increase’ in difference in electronegativity values is more than the ‘magnitude of decrease’ in bond-length value. Hence increase in electronegativity outweighs the decrease in bond-length, and as an overall effect, there is increase in the dipole moment of HF.

Two units of Dipole Moment

In the equation μ = q X d, if charge is expressed in terms of electrostatic unit (abbreviated as e. s. u.) and if distance is expressed in angstrom unit-A0 (1A0=10-8 cm); then the value of dipole moment computed thus, is expressed in unit: “Debye” having symbol of "D".

(The unit Debye is selected in the honor of a scientist whose name was, “Peter Debye”. He first introduced the idea of “electric dipole moment in molecules” in 1923. He was awarded Nobel Prize of chemistry for the year 1936.)

Here, 1 D is defined as "10-18 e. s. u. cm." or "10-10 e. s. u. A0".

However in international system of units (which is called S.I. unit), the charge is expressed in coulomb while distance in meter. Hence in S.I. units, the value of dipole moment is given in terms of: “Coulomb. Meter” (Cm).

Thus, in S.I. system of units, the value of 1 D = 3.336 X 10-30 Cm".

The numerical-1 given below will make this clear.

Meaning of the Terms, “e. s. u.” and "Angstrom"

In international system of units, the electric charge on single electron is taken as: 1.602 x 10-19 coulombs.

However, in old system this charge was expressed in terms of electrostatic unit-e. s. u., having a value of: 4.8 X 10-10 e. s. u.

Likewise, in international system of units, the quantity distance is expressed in terms of meter-m, but in old days, small distances were expressed in terms of angstrom-A0 unit.

1A0 = 10-10 m = 10-8 cm.

Dipole Moment in "Debye" unit

In a polar bond, the intensity of electric charge produced on either atom of the bond is extremely small. It is in the order of 10-10 e. s. u.

Similarly the distance between two atoms of the bond is also very small in the order of 10-8 cm or 1A0.

Due to this, the product "q x d" (means dipole moment) is also a small quantity which is in the order of 10-18.

Based on this, the definition of 1 Debye is given as: “product of 10-10 e. s. u. charge and 10-8 centimeter distance is called 1 Debye”.

Mathematically:

1 D = (10-10 e. s. u.) X (1 A0)

= 10-10 e. s. u. A0

= (10-10 e. s. u.) X (10-8 cm)

= 10-18 e. s. u. cm.

This means, if in a given polar bond, the charge of one electron is separated by one angstrom distance, then its dipole moment will be,

μ = charge X distance

= (4.8 x10-10 e. s. u.) x (10-8 cm)

= 4.8 x 10-18 e. s. u. cm

= 4.8 Debye.

[Remember the following formula for calculation:

Dipole moment (μ) in Debye unit =

Charge on positive atom of the bond in e. s. u. unit x distance between two charges in cm/ 10-18.

Refer numerical-1 given below.

Numerical-1: Calculation of Dipole Moment of HBr

In a polar molecule of HBr, the charge on hydrogen is found to be: 0.56 X 10-10 e. s. u.; while the distance between hydrogen and bromine is found to be: 1.41 A0. Calculate the value of dipole moment of HBr in both units.

(a) Dipole moment (μ) in Debye unit = (charge in e. s. u.) X (distance in cm)

= (0.56 X 10-10 e. s. u.) X (1.41 X 10-8 cm)

= 0.7896 X 10-18 e. s. u. cm.

≈ 0.79 D. (because 10-18 e. s. u. cm. = 1 D).

(b) Dipole moment (μ) in Cm unit = (charge in Coulomb) X (distance in m)

= [{(0.56 X 10-10 X 1.602 X 10-19)} / (4.8 X 10-10)] X (1.41 X 10-10) Cm

= 2.635 X 10-30 Cm.

Difference between Dipole Moment of a single bond and that of a molecule containing several bonds

Though, dipole moment is a characteristic property of individual covalent bond, it is also expressed for entire molecule, particularly when a given molecule contains more than one polar bond.

As dipole moment is a directional property, its value for a molecule containing more than one polar covalent bond, can be determined by vector addition of individual values of dipole moments of all the bonds. This will be clear from following table.

Table Showing Dipole Moment Values of Molecules Containing More Than One Polar Bond

Name of molecule (its formula)
Name of polar bond (Its number)
Value of dipole moment of molecule
Ammonia (NH3)
N-H (3)
1.47 D
Nitrous oxide (N2O)
N-O (2)
0.17 D
Carbon dioxide (CO2)
C=O (2)
0.00 D
Carbon tetrachloride (CCl4)
C-Cl (4)
0.00 D
Methyl chloride (CH3Cl)
C-H (3); C-Cl (1)
1.87 D
Dichloromethane (CH2Cl2)
C-H (2); C-Cl (2)
1.57 D
Chloroform (CHCl3)
C-H (1); C-Cl (3)
1.01 D
Boron trifluoride (BF3)
B-F (3)
0.00 D
Phosphorus pentachloride (PCl5)
P-Cl (5)
0.00 D
Sulphur hexafluoride (SF6)
S-F (6)
0.00 D
Beryllium dichloride (BeCl2)
Be-Cl (2)
0.00 D
The vector addition of individual bond dipole values gives the resultant dipole moment of entire molecule.

Applications of Dipole Moment

The value of dipole moment helps to determine several properties of a substance as listed below.

(1) To determine the polarity of a bond.

(2) To determine % ionic character of a given covalent bond.

(3) To determine shapes of molecules.

(4) To identify geometrical isomers.

(5) To identify ortho, meta and para isomers of aromatic compounds.

All these applications are discussed below in detail.

(1) To Determine Polarity of a Covalent Bond

The value of dipole moment is a measure of polarity of bond: more is the value of μ more is
the polarity of given bond.

This enables us to compare polarity of two different molecules.

For example μ (HCl) = 1.03 D & μ (HF) = 1.91 D; suggests that HF molecule is more polar than HCl.

(2) To Determine Percentage of Ionic Character of a Given Covalent Bond

The percentage of ionic character of a given covalent bond can be determined from its dipole moment value. For this purpose, calculation is carried out as per following steps:

(a) Experimental value of dipole moment is obtained from data. This is called, "μ-experimental".

(b) Theoretical value of dipole moment is calculated assuming that the bond under consideration is 100% ionic called. This is called, "μ-ionic or "μ-theoretical".

(c) % ionic character = {μ-experimental / μ-ionic)} x 100.

[Note: 100% ionic bond means 1 unit charge (4.8 X 10 -10 e. s. u.) is separated by the distance given as actual bond length. This information is provided in data. This means,

μ-ionic = (4.8 X 10 -10 e. s. u.) X (actual bond-length).

This will be clear from the following example.

Example: Calculate % ionic character for HCl molecule.

Given: (a) The bond-length of H-Cl molecule is 1.275 A0 and

(b) Measured dipole moment is 1.03 D.

Here, μ-ionic = (4.8 X 10-10 e. s. u.) X (1.275 X 10-8 cm)

= 6.12 X 10-18 e. s. u. cm

= 6.12 D

μ-experimental = 1.03 D.

Hence, % ionic character of HCl = {(1.03) / (6.12)} X 100

= 16.83 % ≈ 17%.

(3) To Determine Shapes of some Molecules

As the dipole moment value of a molecule is vector sum of individual dipole moments of different bonds within it, a net dipole moment of molecules can give important hint about the shape of molecules.

For example:

(a) Dipole moment value of BF3 = 0.00 D, while that of NH3 = 1.49 D. This suggests that the shape of BF3 molecule must be triangular planar while that of NH3 molecule must be triangular pyramidal.

(b) Dipole moment value ofCO2 = 0.00 D, while that of SO2 = 1.60 D. This suggests that the shape of CO2 molecule must be linear while that of SO2 molecule must be bent.

Relation of Shape of Molecule with its Dipole Moment Value

In carbon dioxide, due to its linear shape, individual dipole moments of two C=O bonds cancels each other giving net value of zero. But due to bent shape sulphur dioxide shows dipole moment.
In carbon dioxide, due to its linear shape, individual dipole moments of two C=O bonds cancels each other giving net value of zero. But due to bent shape sulphur dioxide shows dipole moment. | Source

(4) To identify Geometrical Isomers

Geometrical isomers are difficult to distinguish from each other due to their similar physical as well as chemical characteristics.

However, they differ in their value of dipole moments. Based on this they can be easily identified.

Generally the trans isomer has lower value of μ, due to its symmetrical geometry.

For example, dipole moment of trans-1, 2-dichloroethene = 0.00 D, while

that of cis-1, 2-dichloroethene = 1.90 D.

(5) To identify ortho, meta and para isomers of aromatic compounds.

Due to their characteristic geometry, ortho, meta and para isomers of aromatic compounds show different values of dipole moment.

Based on this they can be easily identified / distinguished from each other as follows.

Generally the ortho isomer shows highest value of μ, while para isomer shows the lowest value. However meta isomer generally shows intermediate value of μ.

For example,

(a) Dipole moment of orthodichloro benzene = 2.54 D,

(b) Dipole moment of meta dichlorobenzene = 1.48 D, while

(c) Dipole moment of paradichlorobenzene) = 0.00 D.

Dipole Moments of Various Isomeric Dichlorobenzene

Due to both electron withdrawing "Cl" groups on opposite ends, dipole moments of two C-Cl bonds cancel each other in para isomer. In ortho isomer, due to orientation, effect of two C-Cl bonds reinforce each other to give highest value.
Due to both electron withdrawing "Cl" groups on opposite ends, dipole moments of two C-Cl bonds cancel each other in para isomer. In ortho isomer, due to orientation, effect of two C-Cl bonds reinforce each other to give highest value. | Source

Test Your Knowledge on Dipole Moment


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