Question

Would you expect the boiling points to increase or decrease in the following series? Explain. (a) \(\mathrm{Kr}, \mathrm{Ar}, \mathrm{Ne}\) (b) \(\mathrm{Cl}_{2}, \mathrm{Br}_{2}, \mathrm{I}_{2}\)

Short answer

(a) Increase from Ne to Kr; (b) Increase from Cl2 to I2.

Step-by-step solution

Understanding the Trend Based on Intermolecular Forces

The boiling point of a substance is influenced by the strength of intermolecular forces. Stronger intermolecular forces result in higher boiling points. In Group 18, the noble gases experience van der Waals forces, also known as London dispersion forces. These forces are greater in larger atoms because they have more electrons and exhibit more polarizability.

Analyzing Series (a): Krypton, Argon, Neon

Krypton (Kr), Argon (Ar), and Neon (Ne) are all noble gases. As you move from Neon to Argon to Krypton, the size of the atom increases, which results in greater van der Waals forces. Therefore, since Krypton is larger, it will have the highest boiling point, followed by Argon, and then Neon.

Understanding Halogen Trends for Series (b)

Halogens are diatomic molecules, and their molecular size increases down the group. As with noble gases, as the size of the molecule increases, the strength of van der Waals forces also increases, leading to higher boiling points for larger halogens.

Analyzing Series (b): Chlorine, Bromine, Iodine

In the series Chlorine ( ext{Cl}_2 ext{) , Bromine ( ext{Br}_2 ext{) , and Iodine ( ext{I}_2 ext{) , the atomic size increases from Chlorine to Iodine. With a larger molecular size, Iodine will have the strongest van der Waals forces and thus the highest boiling point, followed by Bromine and then Chlorine.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction that occur between molecules. These forces play a crucial role in determining the physical properties of substances, including boiling points. There are various types of intermolecular forces, such as hydrogen bonding, dipole-dipole interactions, and van der Waals forces. In this article, we will focus primarily on van der Waals forces, particularly when considering trends in boiling points for noble gases and halogens. Boiling points depend significantly on the strength of these intermolecular forces. The stronger the intermolecular forces, the more energy is required to break them, leading to higher boiling points. Conversely, weaker intermolecular forces will result in lower boiling points.

Noble Gases

Noble gases, found in Group 18 of the periodic table, are characterized by their lack of reactivity. This is due to their full valence electron shell, which makes them stable and less likely to form chemical bonds. The common noble gases include Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). Since noble gases are monoatomic and do not form molecules, the primary intermolecular force acting between them is the van der Waals force, specifically known as London dispersion forces.

• As you move down the group from Helium to Radon, the atomic size increases because of the addition of more electron shells.
• This increase in size enhances the polarizability of the atoms, leading to stronger van der Waals forces.
• Consequently, boiling points increase down the group, from Neon to Argon to Krypton, for example.

Van der Waals Forces

Van der Waals forces are attractive forces that exist between all atoms and molecules. They are one of the weakest types of intermolecular forces but play a significant role in determining the physical properties of nonpolar compounds. These forces arise due to temporary dipoles that form when electrons move around the nucleus. The strength of van der Waals forces is influenced by the following factors:

• Size of particles: Larger atoms or molecules have more electrons and a greater electron cloud. This makes them more polarizable, resulting in stronger van der Waals forces.
• Shape of particles: The surface area available for contact can also affect these forces. A larger surface area allows for stronger attractions.

In summary, van der Waals forces are crucial for understanding boiling point trends, particularly for nonpolar or noble gas elements.

Halogens

Halogens are in Group 17 of the periodic table and include Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). These elements are highly reactive and typically exist as diatomic molecules (e.g., Cl extsubscript{2}, Br extsubscript{2}). In halogens, as you move down the group, the molecular size increases due to the addition of more electron shells. Like noble gases, this increase results in stronger nonpolar van der Waals forces between the molecules.

• The increase in atomic size and stronger van der Waals forces lead to higher boiling points. For example, Iodine, being the largest halogen in the series Cl extsubscript{2}, Br extsubscript{2}, I extsubscript{2}, has the highest boiling point.
• Chlorine has the smallest molecular size and thus the weakest van der Waals forces, resulting in the lowest boiling point among the three.

Understanding the behavior of van der Waals forces in halogens helps explain why, despite being diatomic and similar in reactivity, they exhibit noticeable differences in boiling points.