A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula [M(H2O)n]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have higher solvation numbers (often 8 to 9), with the highest known being 11 for Ac3+. 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 ionic radius, r, increases. Aqua ions are subject to hydrolysis. The logarithm of the first hydrolysis constant is proportional to z2/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. Introduction to metal aqua ions {| class=wikitable style=text-align:center |+Elements that form aqua ions |- | H || colspan=30 | | |- | Li|| Be ||colspan=24| || || || || || || |- | Na|| Mg ||colspan=24| | Al || || || || || |- | K|| Ca|| colspan=14| |Sc||Ti||V||Cr||Mn||Fe||Co||Ni||Cu |Zn||Ga ||Ge*|| || || || |- | Rb|| Sr|| colspan=14| ||Y||Zr||||Mo||||Ru||Rh||Pd||Ag |Cd||In||Sn||Sb* || || || |- |Cs|| Ba|| La |Ce||Pr||Nd|| Pm ||Sm||Eu||Gd||Tb||Dy||Ho||Er||Tm||Yb||Lu |Hf||||||||||Ir||Pt||Au |Hg||Tl||Pb||Bi||Po*|| || |- |Fr*||Ra*||Ac | Th||Pa||U||Np||Pu |Am||Cm||Bk |Cf||Es*||Fm*||Md*||No*||Lr*|||||||||||||||||||||||||||||| |} * No experimental information regarding aqua ion structures Most chemical elements are metallic. Compounds of the metallic elements usually form simple aqua ions with the formula [M(H2O)n]z+ in low oxidation states. With the higher oxidation states the simple aqua ions dissociate losing hydrogen ions to yield complexes that contain both water molecules and hydroxide or oxide ions, such as the vanadium(IV) species [VO(H2O)5]2+. In the highest oxidation states only oxyanions, such as the permanganate(VII) ion, , are known. A few metallic elements that are commonly found only in high oxidation states, such as niobium and tantalum, are not known to form aqua cations; near the metal–nonmetal boundary, arsenic and tellurium are only known as hydrolysed species. Some elements, such as tin and antimony, are