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A cation is a positively charged ion. It forms when an atom or molecule loses one or more electrons, resulting in a net positive charge. Cations are typically formed by metals or metal-containing species, but may also arise from protonation or alkylation of larger molecules. In contrast, anions are negatively charged ions.

Cations include monatomic cations, such as Na+, K+, Ca2+, and Fe3+, as well as polyatomic cations like ammonium (NH4+). The charge state of a cation is indicated by a positive superscript in its formula, for example Na+ and Ca2+. The charge and size of a cation influence its properties, including ionic radius, hydration energy, and mobility in solution.

In aqueous solutions, cations are surrounded by water molecules in a hydration shell, and their interactions with solvent and other ions affect solubility and reactivity. Cations play essential roles in chemistry and biology, with common physiological examples including Na+, K+, Ca2+, and Mg2+ that participate in nerve signaling, muscle function, and enzymatic activity.

In electrochemistry, cations migrate toward the cathode, where they may be reduced or discharged during electrolysis or plating processes. The presence and concentration of specific cations determine the conductivity of electrolytes and influence outcomes in electroplating, corrosion, and various analytical techniques.

Overall, cations are a fundamental category of ions characterized by positive charge, diverse chemical identities, and broad significance across chemical, physical, and biological contexts.