An ion is an atom or bonded group of atoms that carries a net electric charge. Positive ions, called cations, have fewer electrons than protons; negative ions, called anions, have more electrons than protons. Ions occur as constituents of solids, as dissolved particles in liquids, and as charged species in gases. Their charge distinguishes them from neutral atoms and molecules and strongly influences their chemical behavior. (goldbook.iupac.org)
Charge and notation
An atom becomes positively charged when it loses one or more electrons, or negatively charged when it gains electrons. Ordinary ion formation does not change the number of protons in its nucleus, so it does not change the identity of the chemical element. For example, an aluminum atom has 13 protons and 13 electrons; an Al³⁺ ion has the same 13 protons but only 10 electrons. (openstax.org)
An ion’s charge is written as a superscript after its chemical symbol or formula. Na⁺ denotes a sodium ion with one positive charge, whereas Mg²⁺ denotes a magnesium ion with two. Cl⁻ denotes chloride, and SO₄²⁻ denotes sulfate. A superscript charge applies to the entire species; subscripts instead specify the numbers of atoms within its formula. These conventions distinguish charge from chemical composition. (openstax.org)
Monatomic and polyatomic ions
Monatomic ions contain a single atom, as in Na⁺, Ca²⁺, and Cl⁻. Polyatomic ions contain several bonded atoms but behave as charged units. Examples include ammonium, NH₄⁺; hydroxide, OH⁻; nitrate, NO₃⁻; and sulfate, SO₄²⁻. Polyatomic ions may contain bonds between their constituent atoms while participating as complete units in ionic compounds. (openstax.org)
The periodic table helps predict many common monatomic charges. Group 1 metals commonly form ions with charge +1, and group 2 metals commonly form ions with charge +2. Many nonmetals form negative ions. These patterns are useful for simple compounds, but they are not universal rules: some elements form ions with several different charges, and polyatomic charges depend on the composition of the whole group. (openstax.org)
Formation and ionization
Ionization is the generation of one or more ions. It can involve electron loss, the splitting of a neutral species into oppositely charged fragments, or reactions between initially neutral molecules. Thus, ion formation is broader than simply removing an electron from an isolated atom. Successive removal of electrons from an already positive ion produces more highly charged species. (goldbook.iupac.org)
The ionization energy describes the energy needed to remove an electron from an isolated gaseous species. In gases, sufficient energy can separate electrons from atoms, producing positive ions and free electrons. A plasma contains such charged particles and conducts electricity; plasmas occur in the Sun, other stars, and interplanetary space. Their behavior is strongly affected by electromagnetic fields. (goldbook.iupac.org)
Ion formation in solution can instead involve proton transfer. In water, one molecule can transfer a proton to another, producing hydronium, H₃O⁺, and hydroxide, OH⁻. Only a small fraction of pure water molecules undergo this process at any instant, which helps explain pure water’s very low electrical conductivity. (openstax.org)
Ionic solids and dissolved ions
Oppositely charged ions attract one another. In ionic solids, this attraction contributes to the chemical bonding that holds an ordered crystal together. The formula of an ionic compound gives the simplest ratio of its constituent ions needed for electrical neutrality. Calcium chloride, CaCl₂, therefore contains two chloride ions for every calcium ion, balancing one +2 charge with two −1 charges. (openstax.org)
Many ionic solids are rigid, brittle, and have relatively high melting temperatures. They generally conduct electricity poorly while solid because their ions cannot move freely through the structure. When melted, or when dissolved in a suitable liquid, mobile ions can carry electrical current. Having charged constituents is therefore not sufficient by itself for high conductivity; their mobility also matters. (openstax.org)
In a solution, solvent molecules surround dissolved ions. Water’s negatively polarized oxygen side tends to face cations, while its positively polarized hydrogen sides face anions. These interactions help separate ions from a crystal and disperse them through the liquid. Dissolving an ionic solid releases ions already present in its structure, rather than necessarily creating them by electron transfer. (openstax.org)
An electrolyte produces mobile ions in solution. Strong electrolytes yield ions essentially completely in their dissolved portion; weak electrolytes do so only partially. Electrolyte strength is therefore distinct from solubility: a poorly soluble compound can still dissociate extensively in the small amount that dissolves. (openstax.org)
Biological roles and applications
Biological membranes maintain differences in ion concentrations between compartments. Selective ion channels allow ions such as Na⁺, K⁺, Ca²⁺, and Cl⁻ to cross a cell membrane down their electrochemical gradients. Changes in channel opening alter membrane voltage and generate action potentials in electrically excitable cells, including neurons. Channels transport ions passively, whereas ion pumps can use energy to maintain the underlying gradients. (ncbi.nlm.nih.gov)
In a lithium-ion battery, lithium ions move internally through the electrolyte between electrodes, while electrons travel through the external circuit. These coupled movements enable charging and discharging without requiring electrons to follow the ions through the electrolyte. (energy.gov)
Mass spectrometry distinguishes ionic species by their mass-to-charge ratios, conventionally written m/z. A molecular ion can be produced by adding or removing electrons without fragmenting the original molecular structure. Separately, ion exchange uses the transfer of ions between a liquid and an exchange material; cation-exchange resins can remove dissolved calcium during water softening. (goldbook.iupac.org)