Dipole Moment
Polarity of bonds :
Covalent bond is formed between the two atoms by mutual sharing of electrons; and ionic bond by transfer of electrons but in reality no bond or a compound is either completely covalent or ionic.
When covalent bond is formed between two similar atoms e.g., H2, O2, Cl2, N2 or F2 shared pair of electrons is equally attracted by the two atoms. Due to this reason electron pair is situated exactly between the two identical nuclei. The molecular orbital constituting the covalent bond is symmetrically distributed around the atoms. Such a covalent bond is called a non-polar covalent bond.
In case, the covalent bond is formed between two dissimilar atoms, one of which has a larger value of electronegativity, the bonding pair of electrons is displaced towards the more electronegative atom or you can say electron cloud containing the bonding electrons gets distorted and the charge density concentrates around the more electronegative atom.
Due to the unequal distribution of electron charge density, the more electronegative atom acquires a partial negative charge (indicated as δ-) whereas the less electronegative atom acquire a partial positive charge (indicated as δ+). Thus, a covalent bond develops a partial ionic character as a result of the difference of electronegativies of the atoms comprising the bond. Such a bond is called polar covalent bond.
For example, the bond between hydrogen and chlorine atoms in HCl molecule is polar because the shared electron pair is displaced towards chlorine atom which is more electronegative.
Electronegativity difference and percentage ionic character
Dipole Moment :
Dipole moment is defined as the product of the magnitude of the charge and the distance between the centres of positive and negative charges. It is represented by a Greek letter ‘μ ’. Mathematically , it is expressed as
Dipole moment ( μ ) = Charge (q) Distance of separation (d)
Dipole moment is usually expressed in Debye (D).
Calculation of dipole moment (μ )
In C.G.S. units
μ = q x d
μ = dipole moment
q = charge in esu units
d = distance between two charges
For example , dipole moment of HCl is 1.03 10-18 esu – cm and is expressed as 1.03 D.
Dipole moment and molecular structures :
1. Dipole molecules:
In diatomic molecules the dipole moment of the bond gives the dipole moment of the molecule. For example, dipole moment of HCl molecule is the same as that of H-Cl bond ( μ =1.03D). More the electronegativity difference between the atoms, more will be the dipole moment of such molecules.
For example, the dipole moment of HF is 1.91 D while that of HCl is 1.03 D; because fluorine is more electronegative than chlorine.
2. Polyatomic molecules:
In polyatomic molecules, the dipole moment not only depends upon the individual dipole moments of bonds known as bond dipoles but also on the spatial arrangement of various bonds in the molecule. In such case, the dipole moment of a molecule is the vector sum of the dipole moments of various bonds.
For example, both CO2 and H2O are triatomic molecules but dipole moment of CO2, is zero whereas the dipole moment of H2O is 1.85 D because CO2 is a linear molecule in which the two C=O bonds are oriented in the opposite directions at an angle of 1800.
In tetra-atomic molecule, for example in BF3, the dipole moment is zero whereas in NH3 the dipole moment is 1.47 D. In BF3 , the B-F bonds are oriented at an angle of 1200 to one other, the three bond moments give a net sum of zero because the resultant of any two is equal and opposite to the third (parallelogram law of forces).
Dipole moment of molecules with lone pairs
To explain the observed value of dipole moment, sometimes the orbital dipole of the Ione pair has to be considered. For example, dipole moment of NF3 is 0.23 D while that of NH3 is 1.47 D although N-F bond is more polar than N-H bond. In fact in NH3 , the orbital dipole due to Ione pair is in the same direction as the resultant dipole moment of three N-H bonds. Whereas in case of NF3, the orbital dipole due to Ione pair is in teh opposite direction to the resultant dipole moment of three N-F bond. Thus Ione pair moment partly cancels the resultant of N-F bond moment consequently decreasing the net dipole moment value. The Ione pair moment and bond pair moments of NH3 and NF3 are shown below :
