Transcript Dipole Moment & Polarity
DIPOLE MOMENT & POLARITY 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 = qd A unit of dipole moment is the debye (D). 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 The dipole moment, m, is given by = Qd where Q is the charge and d is the separation distance of the charge - 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 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. = 0 Molecule 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. =0 symmetrical trigonal planar geometry .e.g. BF3 b) D.M.> 0 unsymmetrical – Pyramidal, trigonal pyramid 5. Penta-atomic molecule a) 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. HP(OH)2 7. Hepta atomic molecule – D.M. = 0 Symmetrical octahedra e.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.