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 (d) and the distance (d) that separates the centers of positive and negative charge. m = dd A unit of dipole moment is the debye (D).

d is the separation distance of the charge 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

Polarity and shape The shape of the molecule directly influences the overall polarity of the molecule. By knowing the polarity the shape of molecule can be ascertained. Homonuclear Diatomic mol- AA D.M. = 0 Molecule has symmetrical dumbbell shape 2. Heteronuclear diatomic mol – AB. Some value of D.M. but still are symmetrical dumbbell shape.

Net dipole 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 (m = 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. Net dipole

4) Tetra-alomic molecule D.M. =0 symmetrical trigonal planar geometry .e.g. BF3 D.M.> 0 unsymmetrical – Pyramidal, trigonal pyramid

5. Penta-atomic molecule D.M.=0 Symmetrical 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 atoms e.g. 7. Hepta atomic molecule – D.M. = 0 Symmetrical octahedra

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.