metalion

A metal ion is a positively charged atom or atom group that forms when a metallic element loses electrons. In chemistry, metal ions are cations derived from metals and can carry a range of charges, depending on the element and its oxidation state. Transition metals, alkali and alkaline earth metals, and post-transition metals each exhibit characteristic oxidation states that influence their reactivity and coordination behavior.

In aqueous environments, metal ions typically exist as hydrated ions, written as [M(H2O)n]z+, where n is the coordination number and z is the net charge. The hydration shell stabilizes the ion in solution and influences properties such as pH, hydrolysis, and complex formation. Metal ions readily form coordination complexes with ligands, including water, hydroxide, and organic molecules. Common geometries include octahedral and tetrahedral arrangements, with coordination numbers ranging from four to six or more, depending on ionic size and charge.

Biology and medicine rely on metal ions as essential cofactors in enzymes and cellular processes. Examples include Mg2+ and Ca2+ for structural and signaling roles; Fe2+/Fe3+ in oxygen transport and redox enzymes; Zn2+ and Cu2+ in metalloenzymes; and Mn2+ in various catalytic sites. In industry and environmental science, metal ions participate in electrochemical reactions, catalysis, and materials synthesis.

Analytical techniques such as atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, and colorimetric methods are used to detect and quantify metal ions in samples. Metal ions must be managed carefully, as some—such as lead, cadmium, and mercury—can be toxic at elevated concentrations.