DIPOLE MOMENT & POLARITY

A polar bond has separate centers of positive and negative charge. A molecule with separate centers of positive and negative charge is a polar molecule.

The dipole moment (m) of a molecule is the product of the magnitude of the charge (q) and the distance (d) that separates the centers of positive and negative charge.

m = qdA unit of dipole moment is the debye (D).

Dipole Moment & PolarityDipole Moment & Polarity

Depending on the Electronegativity of atoms A & B in an AB molecule , it is possible

to predict whether A-B bond in polar or non-polar covalent bond or an ionic bond.

a) (XA-xB)=0 - Non-polar covalent bond, Molecule is homonuclear A-A or B-B .

b) (xB – xA) = relatively small – e.g. O-H, Cl-H Bond is covalent with some ionic

character/polarity (Polarity – separation of charges) Polar covalent bond.

c) (xB – xA) = Very large – complete transfer of electron; the bond is ionic, A+-B -

Units

if l = 1 A0 µ = 10-10 x e.s.u.µ = 10-10 x 10-8 in CGS units = 10-18 e.s.u. cm = 1 D Debye)

The dipole moment, m, is given by

= Qd where Q is the charge and d is the separation distance of the charge

The shape of the molecule directly influences the overall polarity of the molecule. By knowing the polarity the shape of molecule can be ascertained.

1. Homonuclear Diatomic mol- AA D.M. = 0Molecule has symmetrical dumbbell shape

2. Heteronuclear diatomic mol – AB. Some value of D.M. but still are symmetrical dumbbell shape.

3. Triatomic molecule a) D.M. =0 symmetrical and linear Example – 1. BeCl2

2. CO2 has polar bonds, but is a linear molecule; D.M. of C-O is 2.3 D, the bond dipoles cancel each other and it has no net dipole moment ( = 0 D). Bonds are polar but molecule, is non-polar

b) D.M. >0 unsymmetrical shape e.g. H2 O

•Two O-H bonds, as there is some D.M. so the mol is angular The H─O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule. The bond dipoles do not cancel (m = 1.84 D), so water is a polar molecule.

O === C === O

4) Tetra-alomic molecule a)D.M. =0symmetrical trigonal planar geometry .e.g. BF3

b) D.M.> 0 unsymmetrical – Pyramidal, trigonal

pyramid

5. Penta-atomic molecule a)D.M.=0Symmetrical tetrahedral – all four groups same

e.g. symmetrical PtCl4

Methane

b) D.M.>0

unsymmetrical, Tetrahedral but with D.M.

e.g. CH3 Cl

all 4 bonds are not identical in polarity

3 C-H & 1 C-Cl, So values are not

cancelled

6. Hexa atomic molecule –a) D.M. = 0 Symmetrical arrangement of bonds. PCl 5 is non polar. All the bonds cancel out each other. There are no net dipole. b) D.M.>0 Different atomse.g. HP(OH)2

7. Hepta atomic molecule –D.M. = 0 Symmetrical octahedrae.g. SF6

Induced dipole moment

Under the inference of electric field a non-polar molecule shows some charge separation. As the electron cloud is mobile it gets polarized. This induced polarity is temporary and is given as

µi =αE where E- Electric field and α- Molar polarisability

The polarisability of molecule increases with size of ion e.g. I- is more polarisable than Cl- ion.