Question

Where can we expect to find strong Vander wheels for dispersion forces within a large molecule with a strong temporary dipole between neighboring molecules with small electric on clouds within a larger molecule with a weak temporary dipole or between neighboring molecules with large electron clouds

Answer 1

Short answer: where the electron clouds are largest and most easily distorted — i.e., between neighboring species (or parts of the same molecule) that have large, polarizable electron clouds and can get into close contact. Those give the strongest London (dispersion) forces.

Why:
- Dispersion forces arise from instantaneous/induced dipoles and scale with polarizability (how easily an electron cloud is distorted). Larger atoms/molecules with more electrons and more diffuse outer electrons are more polarizable → stronger dispersion.
- They also depend strongly on distance and contact area (roughly ∝ 1/r^6). Flat or elongated surfaces that can approach closely give larger dispersion than compact or highly branched shapes.
- Permanent or strong temporary dipoles can enhance attraction by induction or dipole–induced dipole effects, but a “strong temporary dipole” usually comes from a large, polarizable electron cloud anyway.

Examples and comparisons:
- Heavy noble gases: Xe ≫ Ar ≫ He (Xe has much stronger dispersion).
- Long, linear alkanes have stronger dispersion than equally massive but highly branched alkanes (more contact area).
- Aromatic rings and molecules with pi‑electron clouds are highly polarizable and give strong dispersion.
- Intramolecular dispersion can be significant when a large molecule folds so electron‑rich regions lie close together.

So of your listed options, the strongest dispersion is expected between neighboring molecules (or parts of a molecule) with large electron clouds (and/or when a large molecule has strongly polarizable regions that come into close contact). Small electron clouds or weakly polarizable fragments give much weaker dispersion.