Metal ions in aqueous solution
![Schematic representation of the aqua ion [Na(H2O)6]+. The oxygen atoms are arranged at the vertices of a regular octahedron centered on the sodium ion](/Images/godic/202412/21/Na+H2O.svg3202.png")
![First and second solvation shells of an octahedral aqua ion. Up to 12 water molecules may be present in the second shell (only two are shown in this diagram) linked by hydrogen bonds to the molecules in the first shell.](/Images/godic/202412/21/Secondary_solvation.svg3202.png")
![Jahn-Teller distorted octahedral structure of [Cu(H2O)6]2+](/Images/godic/202412/21/Hexaaquacopper(II)-3D-balls3202.png")
![face-capped trigonal prism structure](/Images/godic/202412/21/Tri-capped_prism3202.png")
A metal ion in aqueous solution (aqua ion) is a cation, dissolved in water, of chemical formula [M(H2O)n]. The solvation number, n, determined by a variety of experimental methods is 4 for Li and Be and 6 for elements in periods 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have solvation number of 8 and 9. The strength of the bonds between the metal ion and water molecules in the primary solvation shell increases with the electrical charge, z, on the metal ion and decreases as its radius, r, increases. Aqua ions are subject to hydrolysis. The logarithm of the first hydrolysis constant is proportional to z/r for most aqua ions. The aqua ion is associated, through hydrogen bonding with other water molecules in a secondary solvation shell. Water molecules in the first hydration shell exchange with molecules in the second solvation shell and molecules in the bulk liquid. The residence time of a molecule in the first shell varies among the chemical elements from about 100 picoseconds to more than 200 years. Aqua ions are prominent in electrochemistry.