The least attractive force is the London Dispersion Force.
Understanding Intermolecular Forces
Intermolecular forces are attractive or repulsive forces between molecules. These forces determine many of the physical properties of substances, such as their boiling and melting points. There are several types of intermolecular forces, each with varying strengths.
London Dispersion Forces: The Weakest Link
London Dispersion Forces are the weakest of the intermolecular forces. They are also known as van der Waals forces. According to the reference: "The London dispersion force is the weakest intermolecular force. The London dispersion force is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles."
Here’s a breakdown:
- Temporary Dipoles: These forces arise from temporary fluctuations in electron distribution within atoms and molecules. At any given instant, electrons might happen to be unevenly distributed, creating a temporary, instantaneous dipole.
- Induced Dipoles: This temporary dipole in one atom or molecule can then induce a dipole in a neighboring atom or molecule. The positive end of the temporary dipole attracts the electrons of the neighboring atom, creating an induced dipole.
- Attraction: The temporary and induced dipoles attract each other, leading to a weak, short-lived attractive force.
Why are London Dispersion Forces so Weak?
- Temporary Nature: The dipoles are not permanent; they constantly appear and disappear as electrons move.
- Distance Dependence: The strength of London dispersion forces decreases rapidly with increasing distance between molecules.
Examples and Practical Insights
Here are some key points to consider about London dispersion forces:
- Present in all molecules: All molecules, even nonpolar ones, exhibit London dispersion forces. This is because all molecules have electrons that are constantly in motion.
- Dominant force in nonpolar substances: For nonpolar substances like methane (CH4) or diatomic molecules like hydrogen (H2), London dispersion forces are the only intermolecular forces present.
- Increase with molecular size: The strength of London dispersion forces generally increases with the size of the molecule (more specifically, with the number of electrons). Larger molecules have more electrons, leading to larger temporary dipoles and stronger attractions. For example, larger hydrocarbons (like octane) have higher boiling points than smaller hydrocarbons (like methane) because of stronger London dispersion forces.
Comparing with other Intermolecular Forces
Here's a simple comparison of intermolecular forces from weakest to strongest:
| Intermolecular Force | Strength | Occurrence |
|---|---|---|
| London Dispersion Forces | Weakest | All molecules |
| Dipole-Dipole Forces | Moderate | Polar molecules |
| Hydrogen Bonding | Strong | Molecules with H bonded to N, O, or F |
| Ion-Dipole Forces | Very Strong | Between ions and polar molecules |
