Four molecular substances are listed below along with their condensed structural formulas. All are molecular solids at sufficiently low temperatures.
(1) For each substance, state the strongest type of intermolecular force that acts between its molecules in the solid phase (e.g., London dispersion forces, dipole–dipole interactions, hydrogen bonding).
(2) Rank the four substances in order of increasing melting point (1 = lowest, 4 = highest). Provide a clear justification that relates the ranking to the dominant intermolecular forces identified in part (a).
(3) The measured melting points of substances I and II are –114 °C and –138 °C, respectively. Using molecular–level reasoning, explain why the difference between these experimental values is smaller than might be predicted solely from the types of forces named in part (a).