Figure 2.

Schematic illustration of the difference in the hydration of a cation (pink spheres, left column) and an otherwise equivalent anion (blue spheres, right column). The ratio of the distances of the closest water fractional charge from the ion center of d₂/(d₂ – r_OH) in the case of a small pair of ions (bottom row) is considerably larger than that for a large pair of ions of d₁/(d₁ – r_OH) (top row), implying that the difference in the soft/hard character of a cation and an otherwise equivalent anion is larger for smaller ions.