DIPOLE MOMENT |Definition,Units interconversion, calculation|

All ionic compounds and heteroatomic molecules have charge separation due to electronegativity difference. This charge separation generate dipole moment in molecules. 

 How charge separation occurs in molecules? 

 When atoms in molecules have different electronegativity values, more electronegative atoms attracts the bonded pair more towards itself it results in the formation of two opposite charged poles in molecules.In this way charge separation occurs.

 

 There will be strong dipole moment if there is more charge separation in molecules. The charge separation depends on the electronegativity value. Its mean The magnitude of  dipole moment depends on the electronegativity difference. The three parameters - charge separation, electronegativity and dipole moment- are directly interdependent.

 DEFINITION:

When two opposite unit electrical charges(q) are separated by a distance(r) a dipole created and amount of  dipole is called dipole moment(µ). 

Dipole moment may be defined as the product  of electric charge (q) and the distance (r) between the positive and negative changes.

µ= qxr

 Dipole moment is a vector quantity as it has direction as well as magnitude. Its direction is from positive to negative  charge.

Dipole moment, a vector quantity, in HCl. Direction from positive to negative end. 

UNITS:

S.I unit for dipole moment:

To understand the S.I unit suppose a hypothetical molecule ( A⁺-B^-) in which unit positive (consider a proton)  is separated from unit negative charge(consider an electron)  separated by distance r = 100 pm. We can calculate unit of dipole moment according to suppose system's given data:

As we suppose electron and proton  hence the value of  q is the value of charge on an electron and proton.

 Data:

q = 1.6022 x 10^{-19 C}

r = 100 pm (1pm = 10^-12m)

r = 100 x 10^-12 m

Formula:

µ= q x r

µ = 1.6022 x 10 ^-19 C)( 100 x 10^-12 m)

µ =1.6022 x 10^-29 m.C

Commonly used unit for dipole moment ( Debye)

Another unit used historically for Dipole moment is Debye (D) can understand as if we take two opposite charges of same magnitude 10^-10statcoulumb (also known as esu as electrostatic unit) separated by a distance of 1A^o = 10^-10 m 

 r =  1A^o = 10^-10 m

( When distance in taken in meters).

q = 10^-10 esu

µ= q x r

µ = 10^-10 esu x 10^-10m =10 ^-20 esu.m

 If we take distance in cm

r=  10^{-8 cm}

q=  10^{-10 esu}

µ= q x r

µ = 10^-10 esu x 10^{-8 c}m =10 ^{- 18} esu.cm

prove 1D = 3.33x 10 ^-30 C.m

1columb = 3x 10⁹ esu

1esu= 1/3x 10⁹C or 0.333 x 10 ^-9 C

1D=10 ^-20 esu.m (prove above)

1 esu = 0.33x10^-9 C

1D= 10 ^-20esu.m= 10^-20 x 0.33x 10^-9 =0.33x 10 ^{-29  C.m}

1D= 3.33x 10 ^-30 C.m

1 Debye = 3.336 x 10 ^-30 C. m. So the dipole moment of above hypothetical system in debye units is-

3.33 x 10 ^-30 m.C = 1D

 1.6022 x 10 ^-29 m.C = 1 / 3.336 x 10 ^-30 x 1.6022 x 10 ^-29= 4.8 D