Intermolecular Forces

What does dipole mean?

When you have a separation of charge where the positive and negative charges are at two different parts

What are London Dispersion Forces (LDF’s)?

A weak type of intermolecular forces produced by small instantaneous dipoles that occur in nonpolar molecules which induce a dipole in neighboring atoms

How does the instantaneous dipole occur to produce LDF’s?

1. Electrons moving but evenly distributed (symmetrical) to minimize repulsion

2. At an instant the electron cloud wobbles so electrons become unevenly distributed

3. This creates a temporary dipole

Why do non-polar molecules lack a permanent dipole?

Because the average distribution of electrons in a non-polar molecule is symmetrical

What is the strength of London Dipole forces influenced by?

Polarizability

What is polarizability?

How easily an atom or molecule is able to shift its electrons within its electron cloud and create a temporary dipole for the purpose of London dispersion forces

What does high polarizability mean?

Electrons are more easily shifted to create a temporary dipole and stronger LDF’s

What is polarizability influenced by?

• The number of electrons (larger electron cloud) hence molar mass

• Branching/compactness

How can you increase polarizability?

• having a larger electron cloud hence molar mass

• having less branching and compactness

How does the size of electron clouds effect the boiling point?

The larger the electron cloud, the more polarizable the atom or molecule and the higher the boiling point as LDF’s are stronger, and vise versa

How do branching and compactness effect the boiling point?

The less branching and compactness, (more surface area for intermolecular contact) the more polarizable the atoms or molecule and the higher the boiling point as LDF’s are stronger, and vise versa

In what molecules or atoms do LDF’s occur in?

London Dispersion Forces (LDFs) occur in all molecules and atoms, but they’re especially important in nonpolar substances