1. ...molecule is said to be a “polar”
Molecular Polarity
Molecule
Polar Molecules: + -
Contain + and - ends:
Charged ends or “poles” are the result of polar bonds within the molecule and...
...the molecule’s geometry.
Dipole Moment Vector
...molecule is said to be a “polar” H Cl + -
En = 2.1
En = 3.0
En = 0.9 
Polar Covalent Bond

2. Dipole Moment:  d  = q  d H Cl+ -
q: Magnitude (: coulombs) of the charge at either end of the molecule
...larger when more electronegative elements are involved.
d: Distance between the charged ends of the molecule.

3. Largest Dipole Moments:  = q  d  F H H Cl H Br Larger H I Larger
...differences in d
Bond Length (d)  F H
small
92 pm H Cl
Larger
127 pm H Br
Larger
141 pm
Largest
161 pm H I

4. Largest Dipole Moments:  = q  d + -  F H H Cl Larger H Br H I +
...differences in En
Bond Length (d) En 
Largest + F H -
92 pm H Cl
Larger
127 pm + - H Br -
Larger
141 pm +
161 pm H I
small + -

5. Dipole Moments:  = q  d + -  F H H Cl H Br H I + -
...two conflicting predictions...
...nature tells us what’s correct
Bond Length (d) En  + F H -
 = 1.92 D
92 pm
4.0 H Cl
127 pm
3.0 + -
 = 1.08 D H Br
 = 0.78 D
141 pm
2.8 + -
161 pm
2.5 H I
 = 0.38 D + -

6. Dipole Moments:  = q  d F H H Cl H Br H I
H-X Series: Electronegativity differences are more important than bond length differences. F H
 = 1.92 D H Cl
 = 1.08 D H Br
 = 0.78 D
The length of the dipole moment vector suggests the size of the molecule’s dipole moment, . H I
 = 0.38 D