Bond Polarity and Molecular Polarity

Bonds polarity meaning an asymmetric distribution of electron on the covalent bond in molecular chemistry. Polarities of the covalent bonds or molecules are expressed by the term dipole moment of a polar molecule. Therefore, calculation and interpretation of the bonds polarity chart and dipole moment provide an important tool for the unit structure determination of molecules like water, ammonia, methane, carbon dioxide, and monoxide, etc.

The application of the dipole moment helps to determine the molecular size and shape, spatial arrangement, bond polarity, and residue charge on the atoms.

Polarity and dipole moment of covalent polar bonds in water, carbon dioxide, and disulfide in chemistry

Polarity of Non-polar Bonds

For homonuclear diatomic bond, are formed between two electron of atoms that has the same polarity. Thus for such cases, the chemical bonding electron is shared equally by two nuclei of the molecule. These type of covalent bonds is called non-polar bond and the molecule is called a polar molecule.

H2, N2, O2, Cl2, Br2, I2, etc are examples of such types of molecules.

According to the above rule heteronuclear diatomic molecules are polar due to the difference in their electronegativity. Electronegativity difference is not only the tools to determine the polarity of the molecule, but it also decided by the composition and geometry.

Therefore, CO2, CS2, BeF2, BCl3, PCl5, methane, benzene, molecule are the non-polar covalent molecules due to the center of gravity of the positive and negative charge coincide in bonds.

Meaning of Polarity in Chemistry

When the center of gravity of the positive charge does not coincide with the center of gravity of the negative charge like a water molecule, polarity arises this molecule and the molecule is called polar.

Therefore, hydrogen chloride, water, ammonia, and chlorobenzene has shows polarity and the examples of polar covalent molecules in chemistry.

Dipole Moment Calculation

Calculation of dipole moment is given by the product of charge and distance of the separation or bond length. If +q amount of positive charge separates by -q of negative charge and l is the distance between two centers of the molecule.

Thus  dipole moment, µ = q × l

Therefore for perfectly non-polar compounds shows zero dipole moment and polar molecule have positive values. This calculation suggested that the higher the value of the dipole moment of a molecule, the higher will be its polarity.

Problem
P – F, S – F, Cl – F, and F – F which of the following bonds shows the lowest polarity?

Answer

The electronegativity difference between the two fluorine atoms is zero. Thus the dipole moment of the fluorine among these molecules is zero.

Polarity Chart of Molecules

In HCl, due to the greater polarity of a chlorine atom, the bonding electron pair shifted towards chlorine atom. Thus chlorine atom acquires a small negative charge and hydrogen atom acquires a small positive charge. Therefore the dipole moment chart of the molecule suggested the polarity of the molecules.

Molecule Dipole moment
Carbon dioxide, CO2 0 Debye
Carbon disulfide, CS2 0 Debye
Carbon tetrachloride, CCl4 0 Debye
Boron trichloride, BF3 0 Debye
Water, H2O 1.85 Debye
Hydrogen sulfide, H2S 1.10 Debye
Ammonia, NH3 1.46 Debye
Nitrogen trifluoride, NF3 0.20 Debye

Unit of the Dipole Moment

In the CGS system, the charge expressed in esu and the length in cm.

Thus CGS unit of μ = esu cm

Again the charges in the order of 10-10 esu and distance of separation of charge in order of 10-8 cm.

Thus the order of µ
= 10-10 × 10-8
= 10-18 esu cm
This value is known as Debye or simply D.

∴ 1 Debye = 10-18 esu cm

In the SI units, the charge expressed in coulomb and length = meter.

Thus SI unit of the dipole moment
= coulomb × meter (c × m).

1 debye in coulomb-meters

μ = q × l.

Thus the CGS unit of µ
= 4.8 × 10-10 × 10-8 esu cm
= 4.8 D

But the SI unit of µ
1.6 × 10-19 × 10-10 coulombs × meter
= 1.6 × 10-30 C × m

∴ 4.8 Debye = 1.6 ×10-30 coulomb meter

∴1 Debye = 3.336 × 10-30 coulomb meter

Dimension of the Dipole Moment

Unit of µ = unit of charge × unit of length.
∴ CGS unit of µ = esu × cm

Coulomb’s Law,
F = q1q1/Dr2
Thus (esu)2 = dyne × cm2
= gm cm sec-2 × cm2

Therefore, the dimension of dipole moment(μ)
M1/2 L5/2 T-1