Dispersion forces would be weakest in which of the following substances?

• A. Xe
• B. C_6H_{14}
• C. I_2
• D. He

Answer: (D)

Verified step by step guidance

1. Understand that dispersion forces (also known as London dispersion forces) are a type of intermolecular force that arise due to temporary fluctuations in electron distribution within molecules or atoms, creating instantaneous dipoles.
2. Recognize that the strength of dispersion forces generally increases with the size (molar mass) and surface area of the molecule or atom because larger electron clouds are more easily polarizable.
3. Compare the given substances: Xe (xenon), C_6H_{14} (hexane), I_2 (iodine), and He (helium). Note that helium is a very small atom with only two electrons, while the others are much larger and heavier.
4. Conclude that since helium has the smallest electron cloud and lowest molar mass among the options, it will have the weakest dispersion forces.
5. Therefore, the substance with the weakest dispersion forces is He, because dispersion forces increase with atomic or molecular size and polarizability.