Solvation shell

A solvation shell is a shell of any chemical species that acts as a solvent and surrounds a solute species. When the solvent is water it is often referred to as a hydration shell or hydration sphere.

A classic example is when water molecules form a sphere around a metal ion. The electronegative oxygen atom contained in the water molecule is attracted electrostatically to the positive charge on the metal ion. The result is a solvation shell of water molecules that surround the ion. This shell can be several molecules thick, dependent upon the charge of the ion.