Dipole Moment ($\mu$)
Polarity of Bonds & Molecules | Chemical Bonding
1. Definition & Formula
Dipole Moment ($\mu$): The product of the magnitude of the charge ($q$) and the distance of separation ($d$) between the centers of positive and negative charges.
$$ \mu = q \times d $$
- It measures the degree of polarity in a covalent bond.
- It is a Vector Quantity. Direction is from positive center ($\delta+$) to negative center ($\delta-$). (In chemistry convention, arrow points towards the more electronegative atom).
Units:
- CGS: Debye (D).
- SI: Coulomb-meter (C·m).
- Conversion: $1 \, D = 3.33564 \times 10^{-30} \, C \cdot m$.
2. Dipole Moment & Molecular Geometry
The net dipole moment of a polyatomic molecule is the vector sum of all individual bond dipoles and lone pair dipoles.
A. Non-Polar Molecules ($\mu_{net} = 0$)
Symmetrical molecules where bond dipoles cancel each other out.
- Linear: $CO_2, BeF_2$ (Dipoles equal and opposite).
- Trigonal Planar: $BF_3, BCl_3$ (Resultant of two cancels the third).
- Tetrahedral: $CH_4, CCl_4$.
- Symmetrical trans-isomers: trans-1,2-dichloroethene.
B. Polar Molecules ($\mu_{net} \neq 0$)
Unsymmetrical molecules or those with lone pairs.
- Bent/V-shape: $H_2O, SO_2$.
- Pyramidal: $NH_3, PCl_3$.
- Cis-isomers: cis-1,2-dichloroethene.
3. Important Comparison: $NH_3$ vs $NF_3$
Both are pyramidal with one lone pair. Fluorine is more electronegative than Nitrogen, yet $\mu_{NH_3} > \mu_{NF_3}$.
| Molecule | $NH_3$ | $NF_3$ |
|---|---|---|
| Direction of Bond Dipoles | H $\to$ N (Towards N) | N $\to$ F (Away from N) |
| Direction of LP Dipole | Upwards (Away from N) | Upwards (Away from N) |
| Resultant | Vectors Add Up (Reinforce each other) |
Vectors Subtract (Oppose each other) |
| Net Dipole | High ($1.47 D$) | Low ($0.23 D$) |
4. Applications
A. Percentage Ionic Character
Calculated by comparing observed dipole moment with theoretical dipole moment (assuming 100% ionic).
$$ \% \text{ Ionic Character} = \frac{\mu_{observed}}{\mu_{calculated}} \times 100 $$
Where $\mu_{calc} = q \times d$ (Assuming $q = 1.6 \times 10^{-19} C$).
B. Distinguishing Isomers
- Cis vs Trans: Generally $\mu_{cis} > \mu_{trans}$ ($\mu_{trans}$ often 0 for symmetric alkenes).
- Ortho vs Meta vs Para: For disubstituted benzenes (like dichlorobenzene):
$\mu_{ortho} (60^\circ) > \mu_{meta} (120^\circ) > \mu_{para} (180^\circ, \mu=0)$.
Practice Quiz
Test your knowledge on Dipole Moment.
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