page_title: Ion
section_title: Anions and cations
hierarchical_section_title: Ion / Characteristics / Anions and cations
context_page_description: An ion is an atom or molecule that has a net electrical charge. Since the charge of the electron is equal and opposite to that of the proton, the net charge of an ion is non-zero due to its total number of electrons being unequal to its total number of protons. A cation is a positively charged ion, with fewer electrons than protons, while an anion is negatively charged, with more electrons than protons. Because of their opposite electric charges, cations and anions attract each other and readily form ionic compounds.
  Ions consisting of only a single atom are termed atomic or monatomic ions, while two or more atoms form molecular ions or polyatomic ions. In the case of physical ionization in a fluid, "ion pairs" are created by spontaneous molecule collisions, where each generated pair consists of a free electron and a positive ion. Ions are also created by chemical interactions, such as the dissolution of a salt in liquids, or by other means, such as passing a direct current through a conducting solution, dissolving an anode via ionization.
context_section_description: Since the electric charge on a proton is equal in magnitude to the charge on an electron, the net electric charge on an ion is equal to the number of protons in the ion minus the number of electrons.
  An anion (−) (/ˈænˌaɪ.ən/), from the Greek word ἄνω (ánō), meaning "up", is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged).
  A cation (+) (/ˈkætˌaɪ.ən/), from the Greek word κάτω (káto), meaning "down", is an ion with fewer electrons than protons, giving it a positive charge.
  There are additional names used for ions with multiple charges.  For example, an ion with a −2 charge is known as a dianion and an ion with a +2 charge is known as a dication.  A zwitterion is a neutral molecule with positive and negative charges at different locations within that molecule.
  Cations and anions are measured by their ionic radius and they differ in relative size: "Cations are small, most of them less than 10⁻¹⁰ m (10⁻⁸ cm) in radius. But most anions are large, as is the most common Earth anion, oxygen. From this fact it is apparent that most of the space of a crystal is occupied by the anion and that the cations fit into the spaces between them."
caption: Hydrogen atom (center) contains a single proton and a single electron. Removal of the electron gives a cation (left), whereas addition of an electron gives an anion (right). The hydrogen anion, with its loosely held two-electron cloud, has a larger radius than the neutral atom, which in turn is much larger than the bare proton of the cation. Hydrogen forms the only charge-+1 cation that has no electrons, but even cations that (unlike hydrogen) still retain one or more electrons are still smaller than the neutral atoms or molecules from which they are derived.