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Hygroscopic substances are those that have the ability to attract and hold water molecules from the surrounding environment through either absorption or adsorption. This means that they can readily absorb water from the atmosphere around them.

Lithium (Li) is the smallest alkali metal and is part of the first group in the periodic table. Other alkali metals include sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). These metals have similar properties, such as low melting points, low densities, and high reactivities. However, the ionic radius of lithium is significantly smaller than the ionic radii of the other alkali metals. This results in a stronger electrostatic attraction between lithium ions and its counterions in salts.

Lattice energy refers to the energy required to separate an ionic compound into its constituent ions in the gas phase. It is a measure of the stability of an ionic crystal lattice. Since lithium has a smaller ionic radius and greater charge density compared with the other alkali metals, the lattice energy of lithium salts is generally higher than those of other alkali metal salts.

Hydration enthalpy refers to the energy released when ionic compounds dissolve in water. This energy is released due to the attraction between water molecules and the ions in a compound. The hydration enthalpy of lithium salts is significantly higher than those of other alkali metal salts. This is because smaller lithium ions have a stronger electrostatic attraction to water molecules, resulting in greater hydration enthalpy.

For a substance to be hygroscopic, the energy released when water molecules are absorbed by the substance must be greater than the energy required to remove water molecules from the atmosphere and the energy required to break the ionic lattice of the anhydrous substance (lattice energy). Since lithium salts have higher lattice energy and higher hydration enthalpy than other alkali metal salts, the energy released during the hydration process is more significant, making anhydrous lithium salts more likely to absorb water from the environment. To conclude, anhydrous lithium salts are more hygroscopic than other alkali metal salts because of their higher lattice energy and hydration enthalpy. This results from the smaller ionic radius and greater charge density of lithium ions, leading to stronger attractions with counterions in salts and with water molecules during hydration.