shared electron pair between the two atoms gets displaced more towards fluorine since the electronegativity of fluorine (Unit ) is far greater than that of hydrogen. The resultant covalent bond is a polar covalent bond. As a result of polarisation, the molecule possesses the dipole moment (depicted below) which can be defined as the product of the magnitude of the charge and the distance between the centres of positive and negative charge. It is usually designated by a Greek letter ‘ µ ’.
Mathematically, it is expressed as follows : Dipole moment ( µ ) = charge (Q) × distance of separation (r) Dipole moment is usually expressed in Debye units (D). The conversion factor is D = .33564 × – C m where C is coulomb and m is meter. Further dipole moment is a vector quantity and by convention it is depicted by a small arrow with tail on the negative centre and head pointing towards the positive centre. But in chemistry presence of dipole moment is represented by the crossed arrow ( ) put on Lewis structure of the molecule.
The cross is on positive end and arrow head is on negative end. For example the dipole moment of HF may be represented as : This arrow symbolises the direction of the shift of electron density in the molecule. Note that the direction of crossed arrow is opposite to the conventional direction of dipole moment vector. H F In case of polyatomic molecules the dipole moment not only depend 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 in H O molecule, which has a bent structure, the two O–H bonds are oriented at an angle of . . Net dipole moment of .
× – C m (1D = .33564 × – C m) is the resultant of the dipole moments of two O–H