The freezing point of helium is -270 °C. The freezing point of xenon is -128°C. Both of these are in the noble gas family. Which of the following statements is supported by these data? A) Helium and xenon form highly polar molecules. B) As the molecular weight of the noble gas increases, the freezing point decreases. C) The London dispersion forces between the helium molecules are greater than the London dispersion forces between the xenon molecules. D) The London dispersion forces between the helium molecules are less than the London dispersion forces between the xenon molecules. E) None of these.
The correct answer and explanation is:
Correct Answer: D) The London dispersion forces between the helium molecules are less than the London dispersion forces between the xenon molecules.
Explanation:
Helium and xenon are both noble gases, meaning they exist as individual atoms rather than forming molecules under normal conditions. Because they are nonpolar and monoatomic, the only type of intermolecular force that exists between their atoms is London dispersion forces (a type of van der Waals force). These forces arise due to temporary fluctuations in electron distribution that create instantaneous dipoles.
London dispersion forces become stronger as the number of electrons in an atom increases, because larger atoms have more easily polarizable electron clouds. Helium has only 2 electrons, while xenon has 54 electrons. This means xenon has a much more polarizable electron cloud than helium, leading to stronger London dispersion forces between xenon atoms.
The freezing point of a substance is influenced by the strength of the intermolecular forces between its particles. Stronger intermolecular forces mean that atoms or molecules are held together more tightly, requiring more energy (i.e., higher temperatures) to separate them. Therefore, xenon’s stronger dispersion forces allow it to freeze at a much higher temperature (-128°C) than helium, which only freezes at a very low temperature (-270°C).
This information supports choice D, because the London dispersion forces between helium atoms are weaker than those between xenon atoms.
Let’s quickly rule out the other options:
- A) is incorrect because noble gases do not form molecules and are nonpolar.
- B) is incorrect; as molecular weight increases, the freezing point increases, not decreases.
- C) is incorrect; helium has weaker, not stronger, London forces than xenon.
- E) is incorrect because D is correct.
Hence, the correct and supported statement is D.