Question

Why are lithium salts commonly hydrated and those of the other alkali ions usually anhydrous?

Short answer

Lithium salts are hydrated due to the small size and high charge density of lithium ions.

Step-by-step solution

Understand the properties of lithium

Lithium, being the smallest alkali metal, has high polarizing power due to its small ionic radius. This causes it to have a significant attraction for water molecules, which tend to surround the lithium ion in solutions or solid forms, leading to the formation of hydrated salts.

Compare with other alkali metals

Other alkali metals like sodium, potassium, rubidium, and cesium have larger ionic radii. Their larger size means less charge density and, consequently, less polarizing power compared to lithium. This reduced polarization results in weaker attraction for water molecules, meaning they usually do not form hydrated salts as easily as lithium does.

Conclusion on hydration tendency

The hydration tendency of lithium compared to other alkali metals is primarily due to its small size and high charge density. These factors facilitate strong interactions with water molecules, causing lithium salts to be commonly hydrated, while the larger alkali ions form anhydrous salts.

Key concepts

Polarizing Power

When discussing the polarizing power of ions, it refers to the ability of an ion to distort the electron cloud of an adjacent atom or ion. This is particularly important when considering the interaction of metal ions with nonmetal ions or molecules like water.

Lithium ions, being small in size, possess a high polarizing power. This means that lithium can strongly attract the electrons of water molecules, leading to a distortion in the electron cloud around the water molecules. The result is a strong attraction known as an ionic bond with the oxygen in water.

• Lithium, with greater polarizing power, forms stronger bonds with water, while larger alkali metals have lower polarizing power.
• This increased bond strength is due to lithium's small ionic size. It allows it to effectively pull electrons away from other atoms or molecules.

This high polarizing power of lithium is what makes its salts notably hydrated, unlike other alkali metals with lower polarizing power.

Ionic Radius

The term "ionic radius" describes the size of an ion's radius when it becomes an ion, either by losing or gaining electrons. This concept is crucial to understanding why different elements behave distinctly in chemical reactions or when forming compounds.

Lithium has the smallest ionic radius of all the alkali metals. A smaller ionic radius leads to an increase in charge density, as the charge is concentrated over a smaller area. In the case of lithium, this allows it to draw in and tightly hold onto water molecules.

• The smaller the radius, the closer the water molecules can be to the lithium ion. This proximity enhances the strength of interaction between them.
• Lithium's compact ion allows it to penetrate closer to the electron clouds of water, inducing hydration of its salts.

This is why lithium salts are more likely to be hydrated compared to its larger fellow alkali ions like sodium, which have larger ionic radii.

Charge Density

Charge density is the measure of electric charge per unit area, which is determined by both the charge of the ion and its size. For lithium, with its small ionic radius, this results in a high charge density.

High charge density enhances the interaction strength with polar molecules such as water. As lithium has only one positive charge compressed in a relatively small area, its charge density significantly influences how it interacts with surrounding molecules.

• Higher charge density means a stronger electric field radiating from the ion.
• This strong electric field attracts other polar or charged species, like water molecules, more effectively. Consequently, this leads to the formation of hydrated salts.

The high charge density of lithium means its salts are more likely to draw water molecules to themselves, resulting in the commonly observed hydration of lithium salts.