Dipole: The Tiny Magnet Inside Molecules
The Heart of the Matter: Electronegativity and Bond Polarity
At the core of every dipole is a simple tug-of-war. Imagine two atoms are holding hands (forming a chemical bond), and they are both trying to pull the shared electrons closer to themselves. The strength with which an atom pulls on these bonding electrons is called its electronegativity.
When the two atoms in a bond are identical, like in a molecule of oxygen (O$_2$), they have the same electronegativity. The tug-of-war is a perfect tie, and the electrons are shared equally. This is called a nonpolar covalent bond.
However, when the atoms are different, one is usually stronger than the other. For example, in a hydrogen chloride (HCl) molecule, the chlorine atom has a much higher electronegativity than the hydrogen atom. Chlorine wins the tug-of-war and pulls the bonding electrons closer to its nucleus. This creates a slight negative charge (denoted as $\delta-$) on the chlorine end and a slight positive charge ($\delta+$) on the hydrogen end. This type of bond is a polar covalent bond, and the molecule itself becomes a dipole.
Key Formula: The polarity of a bond can be quantified. The difference in electronegativity ($\Delta$EN) between two atoms predicts the bond type.
- $\Delta$EN = 0: Nonpolar Covalent Bond
- 0 < $\Delta$EN < 1.7: Polar Covalent Bond
- $\Delta$EN $\geq$ 1.7: Ionic Bond
From Bond Polarity to Molecular Dipoles
Just because a molecule has polar bonds does not automatically mean it has an overall dipole. We must also consider the molecule's shape or geometry. The individual bond dipoles are like tiny arrows pointing from the positive to the negative end of the bond. To find the overall molecular dipole, we have to add these arrows together, both their direction and strength.
If the molecule is symmetrical, the bond dipoles can cancel each other out. A classic example is carbon dioxide (CO$_2$). It is a linear molecule: O=C=O. Each C=O bond is very polar, with the oxygen being negative and the carbon being positive. However, because the two bonds point in exactly opposite directions, their dipoles are equal in magnitude but opposite in direction, so they cancel each other. The result is that CO$_2$ is a nonpolar molecule.
Now, consider water (H$_2$O). It has a bent shape. The two O-H bond dipoles do not point in opposite directions. When we add them together, they do not cancel out. This leaves the oxygen side of the molecule with a net negative charge and the hydrogen side with a net positive charge. Therefore, water is a polar molecule with a significant overall dipole.
| Molecule | Formula | Shape | Bond Dipoles Cancel? | Overall Polarity |
|---|---|---|---|---|
| Carbon Dioxide | CO$_2$ | Linear | Yes | Nonpolar |
| Water | H$_2$O | Bent | No | Polar |
| Boron Trifluoride | BF$_3$ | Trigonal Planar | Yes | Nonpolar |
| Ammonia | NH$_3$ | Trigonal Pyramidal | No | Polar |
Dipoles in Action: Shaping the Physical World
The presence or absence of a molecular dipole has dramatic effects on the physical properties of substances. These effects are visible in our everyday lives.
Boiling and Melting Points: Polar molecules have stronger attractions between them than nonpolar molecules. The positive end of one molecule is attracted to the negative end of its neighbor. This is called a dipole-dipole force. It takes more energy (heat) to pull the molecules apart, which raises the boiling and melting points. For instance, water (H$_2$O, polar) boils at 100 $^\circ$C, while carbon dioxide (CO$_2$, nonpolar) sublimes at -78.5 $^\circ$C. A special, very strong type of dipole-dipole force is the hydrogen bond, which is crucial for the properties of water, DNA, and proteins.
Solubility: The rule of thumb is "like dissolves like." Polar substances tend to dissolve well in other polar solvents, like water. Nonpolar substances dissolve well in nonpolar solvents, like oil. This is why oil and water do not mix; the water molecules are strongly attracted to each other via their dipoles and do not want to make room for the nonpolar oil molecules. This principle is used in soaps, where one end of the soap molecule is polar (and attracted to water) and the other end is nonpolar (and attracted to grease), allowing it to wash away oily dirt.
Biological Molecules: The three-dimensional shapes of proteins and the double helix of DNA are held together by interactions involving molecular dipoles. The specific pairing of the bases in DNA (A with T, G with C) is governed by hydrogen bonding, a direct result of the dipoles within those molecules.
Common Mistakes and Important Questions
If a bond is polar, does that always mean the molecule is polar?
What is the difference between a dipole-dipole force and a hydrogen bond?
Can a molecule with nonpolar bonds ever be polar?
Footnote
1 EN (Electronegativity): A measure of an atom's ability to attract shared electrons in a chemical bond. It is a dimensionless quantity, often measured on the Pauling scale.
2 CPI: While not used in this article, this note demonstrates the format. For an unfamiliar term like "Intermolecular Force," the footnote would define it as: the forces of attraction between molecules, which are weaker than the chemical bonds within molecules.