Notes on Instantaneous and Induced Dipoles (Transcript-Derived)

A temporary dipole arises when electron distribution in a molecule becomes momentarily uneven due to random motion of electrons. This is described as an instantaneous or temporary dipole.
The example situation in the transcript: the electrons move to one side at a given moment, creating a brief separation of positive and negative regions.
This dipole is not permanent; it is a fleeting fluctuation as electrons continuously move and redistribute themselves.
Dipole moment definition (simple): $\mu = q \; d$ where $q$ is the charge and $d$ is the separation distance between charges.

An induced dipole is created in a neighboring molecule when it experiences the electric field from a nearby dipole.
In the transcript: the temporary dipole is described as inducing the other two to become dipoles. This captures the idea that a fluctuating dipole can polarize nearby molecules.
Induced dipole moment depends on the polarizability of the molecule: $\mu{\text{ind}} = \alpha \; E{\text{ext}}$ where $\alpha$ is the polarizability and $E_{\text{ext}}$ is the external electric field.
The external field from the instantaneous dipole at a distance $R$ is approximated as: $E{\text{ext}} \approx \frac{1}{4\pi \varepsilon0} \frac{\mu_{\text{inst}}}{R^3}$
The induced dipole tends to align with the external field, leading to an attractive interaction between the molecules.
The induced dipole, like the temporary dipole, is also not permanent; as the electron distribution changes, the induced dipole can disappear.

Step 1: Random electron motion creates an instantaneous dipole in one molecule (the electrons shift briefly to one side).
Step 2: This instantaneous dipole generates an external electric field that acts on neighboring molecules.
Step 3: Neighboring molecules with finite polarizability $\alpha$ develop induced dipoles: $\mu{\text{ind}} = \alpha E{\text{ext}}$
Step 4: The interaction energy between the instantaneous dipole and the induced dipole is negative (attractive), leading to a net attraction between the molecules.
Step 5: Because electron motion is dynamic, the dipoles are temporary; there is nothing to keep them from shifting back to non-polar states.
Step 6: This whole sequence explains why nonpolar molecules experience attractive forces even in the absence of permanent dipoles.
Step 7: The overall effect is a dispersion (London dispersion) force that arises from these instantaneous and induced dipoles.

Instantaneous dipole moment: $\mu_{\text{inst}}$ (time-dependent; averages to zero over long times).
Induced dipole moment: $\mu{\text{ind}} = \alpha \; E{\text{ext}}$
External field from instantaneous dipole at distance $R$: $E{\text{ext}} \approx \frac{1}{4\pi \varepsilon0} \frac{\mu_{\text{inst}}}{R^3}$
Energy of interaction (approximated as interaction of induced dipole with external field): $U \approx - \mu{\text{ind}} \; E{\text{ext}} \ \approx - \alpha \left( \frac{1}{4\pi \varepsilon0} \frac{\mu{\text{inst}}}{R^3} \right)^2 \ = - \frac{ \alpha \; \mu{\text{inst}}^2 }{16 \pi^2 \varepsilon0^2 \; R^6 }$
General London dispersion energy between two molecules is often summarized as: $E{\text{disp}}(R) = - \frac{C6}{R^6}$ where $C_6$ depends on the polarizabilities of the interacting species (and other quantum factors).
Key point: dispersion forces scale with distance as $R^{-6}$ and with polarizability (larger $\alpha$ generally means stronger dispersion).
Time-averaged perspective: because $\mu_{\text{inst}}$ fluctuates with time, the instantaneous energy contributions fluctuate, but the overall attraction is a real, observable consequence of these fluctuating dipoles.

Noble gases (e.g., He, Ne, Ar) are nonpolar; their weak cohesion at low temperatures is largely due to London dispersion forces.
Dispersion forces explain why nonpolar molecules can condense and why larger atoms/molecules (with greater polarizability) have higher boiling points than smaller ones within the same group.
These forces are universal and operate even when permanent dipole moments are absent.
In materials science and biochemistry, dispersion forces contribute to molecular packing, solubility, and protein-ligand interactions, especially where other stronger forces are absent or scarce.

Metaphor: a momentary gust of electron movement creates an instant dipole, which acts like a tiny wind that briefly nudges neighboring molecules to polarize; this effect propagates transiently through a system and then dissipates as electrons rearrange.
Core idea: attraction between nonpolar entities arises from fluctuations and induced responses, not from permanent charges.

Connects to electrostatics: dipoles, fields, and interactions between charge distributions.
Highlights polarizability as a fundamental property controlling how easily electron clouds distort under external fields.
Demonstrates that attractive forces can arise from second-order (fluctuation-induced) interactions, not only from first-order permanent dipoles.

The temporary dipole -> induced dipole mechanism underpins London dispersion forces, an essential component of intermolecular forces.
These forces are always present, even in nonpolar substances, and become more significant with larger, more polarizable atoms/molecules.
Understanding this concept helps explain trends in boiling points, solubility in nonpolar solvents, and the behavior of gases and condensed phases.