The freezing point of helium is –270°C. The freezing point of xenon is –112°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 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 :
The correct answer is 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, which means they are monoatomic and generally non-reactive under normal conditions. They exist as individual atoms rather than molecules, and their intermolecular forces arise from weak London dispersion forces (a type of van der Waals force).
- London Dispersion Forces:
These forces occur due to temporary fluctuations in electron distribution, leading to the formation of instantaneous dipoles. The strength of these forces depends on two main factors:
- Polarizability: Larger atoms with more electrons are more polarizable, meaning their electron clouds can be more easily distorted to create these instantaneous dipoles.
- Size and Atomic Number: Heavier atoms, with more electrons, experience stronger dispersion forces because they have more electrons that can contribute to temporary dipoles.
- Helium vs Xenon:
- Helium (He) is the lightest noble gas with only 2 electrons, and its atomic size is very small. Due to its small size and limited electron cloud, the London dispersion forces between helium atoms are very weak.
- Xenon (Xe), on the other hand, is much heavier, with 54 electrons, making its electron cloud more polarizable. As a result, xenon atoms experience stronger London dispersion forces than helium atoms.
- Freezing Point:
The stronger the London dispersion forces, the higher the temperature needed to overcome them and keep the substance in a gaseous state. This explains why xenon, with stronger intermolecular forces, has a much higher freezing point (–112°C) than helium (–270°C).
Thus, Option D is supported by the data: xenon has stronger London dispersion forces than helium due to its larger size and greater polarizability.