London Dispersion Forces Notes (Transcript-Based)

London dispersion forces are attractive interactions that arise from instantaneous dipoles in one species and induced dipoles in a neighboring species.
They are present between all molecules and neutral species.
These forces help hold atoms together at low temperatures.

A molecule on the left can have a temporary dipole (instantaneous uneven distribution of electrons).
We denote partial charges as $\delta^-$ (partial negative) and $\delta^+$ (partial positive).
The adjacent molecule experiences an induced dipole due to the fluctuating electron density of the first molecule.
There is an attractive force between the $\delta^+$ of the instantaneous dipole and the $\delta^-$ end of the induced dipole in the neighboring molecule.
This attractive interaction is what we call London dispersion forces.

Another way to represent it: an instantaneous uneven distribution of electrons in a (nonpolar) atom or molecule induces a dipole in a neighboring atom or molecule (e.g., helium–helium interaction).
In the example with helium, the left atom’s instantaneous dipole induces a dipole in the right atom, resulting in an attractive force between the $\delta^+$ of the left and the $\delta^-$ of the right.
A top-view description: the right helium atom can have a temporary dipole; this induced dipole interacts with the left atom’s charges, producing an attraction between the two species.

Question summary: an instantaneous uneven distribution of electrons in a helium atom interacts with a nonpolar helium atom, leading to an induced dipole in the neighboring helium atom.
Conclusion: an induced dipole exists on the neighboring helium atom.
How many molecules must be present for London dispersion forces to exist?
Answer: C
- Rationale: you must have at least two atoms or molecules in an electrostatic field for an interaction to occur.

As atoms come together, the kinetic energy increases; the potential energy decreases.
The attractive Coulombic interaction helps the atoms approach because it lowers potential energy as the dipoles interact.
When they come even closer, the electron clouds begin to overlap, leading to repulsion and a change in the energy balance.
The statement from the scenario: as the atoms approach, kinetic energy increases and potential energy decreases; as they reach a closest approach, kinetic energy begins to decrease as they stop and start moving apart.
If the atoms get very close, electron-electron repulsion raises potential energy and limits further approach.
In a closed system, the total energy remains constant: $E_{\text{total}} = KE + PE = \text{constant}.$

The simulation examines the relationship between kinetic energy and potential energy as two electron clouds interact.
Initially, attractive Coulombic interactions cause the atoms to approach due to London dispersion forces.
As the atoms get closer, the electron clouds begin to overlap, causing repulsion and a rise in potential energy.
The kinetic energy initially increases during approach (as potential energy decreases), then starts to decrease as the distance reduces further and the system moves toward separation.
If left in a closed system, the total energy remains constant and the motion may oscillate.
Real systems are not perfectly isolated; the simulation’s closed-system assumption is a simplification.

Energy conservation in a closed system: $E_{\text{total}} = KE + PE = \text{constant}$
Coulombic interaction qualitatively: $F \propto \frac{q<em>1 q</em>2}{r^2}$
Conceptual representation: the interaction involves a positive end of one dipole attracting the negative end of the neighboring induced dipole.
Relationship during approach: when distance decreases due to attraction, \Delta KE > 0\quad\Rightarrow\quad \Delta PE < 0 to conserve total energy.

London dispersion forces are universal, occurring in all atoms and molecules, including noble gases like helium in transient interactions.
They contribute to the cohesion of nonpolar molecules and can influence physical properties such as boiling/condensation behavior at low temperatures.
The idealized closed-system simulation is a simplification; in real environments, energy exchange with surroundings occurs and systems are not perfectly isolated.

Links to electrostatics: interactions between partial charges and dipoles.
Van der Waals interactions: London dispersion is a component.
Conceptual bridge to energy landscapes: balancing attraction (lower PE) and repulsion (higher PE) as distance changes, with KE adapting accordingly.