Final answer:
Methane (CH₄) has the weakest intermolecular forces, followed by ethane (CH₃CH₃), with propane (CH₃CH₂CH₃) having the strongest, due to increasing size and dispersion forces.
Step-by-step explanation:
The student asked to place the following compounds in order of increasing strength of intermolecular forces: CH₄ (methane), CH₃CH₂CH₃ (propane), CH₃CH₃ (ethane). The primary intermolecular force in nonpolar molecules, such as these hydrocarbons, is the London dispersion force (LDF), which increases with the size and molar mass of the molecule.
The molecule CH₄ is the smallest and has the weakest dispersion forces. Ethane (CH₃CH₃) has two carbon atoms compared to methane's one, thus it exhibits stronger dispersion forces. Propane (CH₃CH₂CH₃) has three carbon atoms and shows even stronger dispersion forces due to the increased number of electrons and larger surface area for interactions. Therefore, the order of increasing intermolecular forces and thus boiling points for these compounds is: CH₄ < CH₃CH₃ < CH₃CH₂CH₃.