1.5.3 Types of Forces Between Molecules

1. Induced Dipole–Dipole (Van der Waals / London Dispersion) Forces

Induced dipole–dipole forces, also known as van der Waals or London dispersion forces, are the weakest type of intermolecular force. They occur in all atoms and molecules, whether polar or non-polar, due to temporary shifts in the electron distribution.

Formation

• Electrons are constantly moving in their clouds, and at any given moment, they may be unevenly distributed. This causes a temporary dipole in one molecule.
• This dipole can induce a dipole in a neighbouring molecule, leading to an attraction between the two molecules.
• The process can continue, with the induced dipole in one molecule creating dipoles in additional nearby molecules.

Factors Influencing Strength

• The larger the molecule (higher $M_r$), the more electrons it has, and the stronger the van der Waals forces.
• For example, larger molecules like iodine ($I₂$) have stronger dispersion forces compared to smaller ones like hydrogen ($H₂$).

2. Permanent Dipole–Dipole Forces

Permanent dipole–dipole forces occur between polar molecules where there is a permanent separation of charge (dipole).

Formation

• In a polar molecule, one end is slightly positive ($δ⁺$) and the other is slightly negative ($δ⁻$) due to differences in electronegativity between atoms.
• The $δ⁺$end of one molecule attracts the $δ⁻$end of a neighbouring molecule, leading to electrostatic attraction between the molecules.

Comparison with Van der Waals Forces

• While polar molecules also experience van der Waals forces, the permanent dipole–dipole forces add an extra layer of attraction, making the overall intermolecular forces stronger in polar substances.

Testing Polar Liquids

You can observe dipole–dipole forces by using an electrostatically charged rod near a jet of a polar liquid, such as water. The polar molecules will be attracted to the rod, causing the liquid to deflect. Non-polar liquids like hexane will not show any deflection.

3. Hydrogen Bonding

Hydrogen bonding is a special type of dipole–dipole interaction and is significantly stronger than both van der Waals and permanent dipole–dipole forces. It occurs when a hydrogen atom is covalently bonded to a very electronegative atom ($O$, $N$, or $F$) and interacts with the lone pair of another electronegative atom in a different molecule.

Formation

• In molecules like water ($H₂O$) or ammonia ($NH₃$), the hydrogen atom bonded to oxygen ($O$) or nitrogen ($N$) has a partial positive charge ($δ⁺$), while the electronegative atom ($O$, $N$, or $F$) holds a partial negative charge ($δ⁻$).
• This highly polar bond, combined with hydrogen's small size and high charge density, results in a strong attraction between the hydrogen atom and a lone pair on a neighboring $O$, $N$, or $F$atom.

Effect on Physical Properties

• Hydrogen bonding is responsible for the unusual properties of substances like water, including its lower density in solid form (ice) and anomalously high boiling points.

Summary of Intermolecular Forces

Type of Force	Strength	Example Substances	Key Feature
Induced Dipole–Dipole	Weak	Methane ($CH₄$) and Iodine ($I₂$)	Present in all molecules; increases with size.
Permanent Dipole–Dipole	Moderate	Hydrogen chloride ($HCl$)	Attraction between permanent dipoles.
Hydrogen Bonding	Strong	Water ($H₂O$) and Ammonia ($NH₃$)	Involves H bonded to O, N, or F.

These intermolecular forces significantly influence the melting and boiling points of substances. Stronger forces require more energy to overcome, resulting in higher melting or boiling points.