Which of the following molecules will engage in the strongest dispersion (London) forces?

• A. CH_4 (methane)
• B. CO_2 (carbon dioxide)
• C. NH_3 (ammonia)
• D. C_8H_{18} (octane)

Answer: (D)

Verified step by step guidance

1. Understand that dispersion (London) forces are a type of van der Waals force that arise due to temporary fluctuations in electron density, creating instantaneous dipoles even in nonpolar molecules.
2. Recognize that the strength of dispersion forces generally increases with the size (molar mass) and surface area of the molecule because larger molecules have more electrons and a more easily polarizable electron cloud.
3. Compare the given molecules by their molar masses and molecular sizes: CH_4 (methane), CO_2 (carbon dioxide), NH_3 (ammonia), and C_8H_{18} (octane).
4. Note that methane, carbon dioxide, and ammonia are relatively small molecules with fewer electrons, while octane is a much larger hydrocarbon with a significantly higher molar mass and more electrons.
5. Conclude that because octane has the largest size and molar mass among the options, it will experience the strongest dispersion forces due to its greater polarizability.