Question

List the three types of van der Waals forces in decreasing order of strength.

Short answer

Hydrogen Bonding > Dipole-Dipole Interactions > London Dispersion Forces

Step-by-step solution

- Understand van der Waals Forces

Van der Waals forces are weak intermolecular forces that come into play when atoms or molecules are in close proximity to each other. They include three common types: dipole-dipole interactions, London dispersion forces, and hydrogen bonding.

- Identify the Types

The three types of van der Waals forces are: (1) Dipole-Dipole Interactions, (2) London Dispersion Forces (also known as instantaneous dipole-induced dipole forces), and (3) Hydrogen Bonding.

- Rank by Strength

The strength of these forces varies. Hydrogen bonding is generally the strongest among them, followed by dipole-dipole interactions, and then London dispersion forces.

- List in Decreasing Order of Strength

Based on the identified order: Hydrogen Bonding > Dipole-Dipole Interactions > London Dispersion Forces.

Key concepts

Dipole-Dipole Interactions

Dipole-dipole interactions are a type of van der Waals force that occur between polar molecules. These molecules have a permanent dipole moment, which means they have regions with a slight positive charge and others with a slight negative charge. The positive end of one molecule attracts the negative end of another, creating a dipole-dipole interaction. This force is stronger than London dispersion forces but weaker than hydrogen bonding.

• Example: HCl (hydrogen chloride) has a dipole-dipole interaction because of the difference in electronegativity between hydrogen (H) and chlorine (Cl).

These interactions play a significant role in determining the boiling and melting points of substances containing polar molecules. Similar to a magnet, the molecules align themselves such that opposite charges attract.

London Dispersion Forces

London dispersion forces, also known as instantaneous dipole-induced dipole forces, are the weakest type of van der Waals forces. They occur between all atoms and molecules, whether they are polar or non-polar. These forces arise from temporary fluctuations in the electron distribution within atoms and molecules, which create temporary dipoles.

Despite being the weakest, London dispersion forces are ubiquitous and significant, especially in large atoms and molecules where electrons are more easily polarizable.

• Example: In noble gases like Argon (Ar) and nonpolar molecules like O₂ (oxygen), London dispersion forces are the primary type of intermolecular force.

The strength of these forces increases with the size of the electron cloud, making them more relevant for larger, heavier atoms and molecules.

Hydrogen Bonding

Hydrogen bonding is the strongest type of van der Waals force. It occurs when hydrogen atoms are bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F). The unique properties of hydrogen, combined with these electronegative atoms, create an exceptionally strong dipole-dipole interaction.

In a hydrogen bond, the hydrogen atom behaves almost as if it were 'shared' between two electronegative atoms. This creates a strong and specific attraction.

• Example: Water (H₂O) molecules exhibit extensive hydrogen bonding. This is why water has a high boiling and melting point compared to other molecules of similar size.
• DNA structure: Hydrogen bonds between the base pairs hold the double helix structure together.

Because of its significant impact on physical properties, hydrogen bonding is vital in biological and chemical systems, influencing everything from solubility to the structure of proteins and nucleic acids.