A metal ion is an atom of a metal that has lost one or more electrons, resulting in a positive charge. This process is known as ionization. Metal ions are commonly found in various chemical compounds and play essential roles in biological systems, such as in the function of enzymes and proteins. Metal ions can vary in charge and size, depending on how many electrons they lose. Common examples include sodium ion (Na⁺), calcium ion (Ca²⁺), and iron ion (Fe²⁺). These ions are crucial in many industrial processes, including electroplating and catalysis.