Question

Many lithium salts are hygroscopic (absorb water), but the corresponding salts of the other alkali metals are not. Why are lithium salts different from the others?

Short answer

Lithium salts are hygroscopic due to their high charge density, resulting from the small ionic radius of lithium ions. This high charge density leads to stronger electrostatic interactions between lithium ions and anions in the salt, as well as stronger interactions with water molecules in the environment. On the other hand, other alkali metal salts have lower charge densities due to their larger ionic radii, resulting in weaker interactions with both the anions and water molecules, making them non-hygroscopic.

Step-by-step solution

Understanding Hygroscopic Behavior

Hygroscopic substances are those that readily absorb water from their surroundings. To understand why lithium salts are different from other alkali metal salts, we need to examine the properties of lithium salts and compare them to the other alkali metal salts.

Properties of Lithium Salts

Lithium is the smallest and lightest alkali metal, with the atomic number 3 and atomic symbol Li. It is a soft, silvery-white metal which reacts rapidly with water to form lithium hydroxide (LiOH) and release hydrogen gas. When lithium forms salts, its ionic radius is much smaller than the other alkali metals. As a result, the charge density (charge per unit volume) of the lithium ion is much higher compared to other alkali metal ions. This high charge density results in stronger electrostatic interactions between lithium ions and the anions in the salt, as well as stronger interactions with water molecules in the environment.

Properties of Other Alkali Metal Salts

Other alkali metals (sodium, potassium, rubidium, and cesium) form salts in a similar manner to lithium. However, their ionic radii are larger, and therefore their charge densities are lower. This results in weaker electrostatic interactions between the alkali metal ions and the anions in the salt, and weaker interactions with water molecules in the environment.

Comparing Lithium Salts to Other Alkali Metal Salts

As previously mentioned, the high charge density of lithium ions results in stronger interactions with both the anions in the salt and water molecules in the environment. This means that the lithium salts have a higher affinity for water and are more likely to attract and hold onto water molecules, making them hygroscopic. In contrast, the lower charge densities of the other alkali metals result in weaker interactions with both the anions in the salt and water molecules in the environment. This means that the corresponding salts of these alkali metals have a lower affinity for water and are less likely to absorb water, making them non-hygroscopic.

Conclusion

Lithium salts are hygroscopic because of their high charge density, which leads to stronger interactions with water molecules and allows them to absorb water from the environment more readily. In contrast, the lower charge densities of other alkali metal salts result in weaker interactions with water molecules, making them non-hygroscopic.