Noncovalent forces — When do attractive van der Waals interactions occur?

Introduction:
Van der Waals interactions include dispersion, induction, and orientation components. This question probes the generality of the attractive dispersion term that underlies packing in protein interiors and molecular crystals.

Given Data / Assumptions:

• Atoms or molecules are neutral overall.
• Distances are within the range where electron cloud fluctuations can interact but are not so close as to cause Pauli repulsion.
• Temperature and phase can vary.

Concept / Approach:
London dispersion forces arise from instantaneous correlated fluctuations in electron density, leading to transient dipoles that attract each other. Because all atoms and molecules exhibit electron fluctuations, the attractive component is universal. The interaction strength depends on polarizability and distance and is balanced by short range repulsion at very close approach.

Step-by-Step Solution:

Verification / Alternative check:
Molecular dynamics force fields include a universal dispersion term for all atom pairs, modulated by atom types and distance dependent parameters, reflecting this generality.

Why Other Options Are Wrong:

• Apolar molecules in liquid only: too narrow; van der Waals act in all phases.
• Polar molecules in solid only: again too narrow.
• Only if other forces are less favorable: dispersion acts regardless of other interactions.
• Only between identical atoms: dispersion occurs between any atom types.

Common Pitfalls:
Assuming polarity is required for attraction. Dispersion is present even for noble gases and saturated hydrocarbons, explaining liquefaction and crystal formation.

Final Answer:
any pair of nearby atoms.