= Solvation shell
A <solvation shell> is the region of <solvent> constituents surrounding a <solute>, with local arrangements influenced by their interactions. The first <solvation shell> contains the closest neighbors under a stated distance or interaction criterion. A <coordination number> counts those neighbors; it requires such a criterion and can fluctuate. Neither a drawn ring nor a few illustrated <molecules> determines a measured <coordination number>.
In a <liquid>, a <solvation shell> is an evolving neighborhood: <solvent> <molecules> reorient and exchange with the surroundings. Different <ions> can produce different local dynamics. <Spectroscopic measurement of hydration dynamics> provides an experimental way to distinguish some of those environments without pretending that a shell is a fixed cage.
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