Dipole Moment & Polarity

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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.