An ion is an atom or molecular group that has a net electric charge, either positive or negative. Anions have a negative charge (with extra electrons) and are attracted to anodes; cations are positively charged (have lost electrons) and are attracted to cathodes. Ionization refers to the process by which ions are created. Ions are often noted in writing with a superscripted positive or negative sign: H+, for instance, refers to a hydrogen cation.
Nonaqueous ions or gases with charged particles are called a plasma, which is the fourth state of matter after solid, liquid, and gas.
Ionization energy is that energy that is necessary to detach an electron from its atom or molecule and create an ion. The first electron removed may not require much energy, but each subsequent removal will require greater energy, particularly when the outer atomic orbital has been exhausted of electrons.
Faraday first hypothesized that ions existed in 1830, after observing molecules travelling to anodes and cathodes.
Without ions, there could not be life. Sodium, potassium, and calcium ions play essential roles in everyday biochemical reactions, and the advanced nervous system of mammals is entirely dependent upon ionic reactions.
Developing on what was noted earlier, an ion is either an atom, atomic group or compound, or subatomic particle that has a net positive or negative electronic charge. Examples of ions include free electrons, Cl-, H+, OH-, etc. In addition to negatively charged ions,called anions, and positively charged ions, called cations there are single-atom ions, called monatomic ions, and molecular ions, called polyatomic ions. Polyatomic ions with oxygen are often called oxyanions.
An ion is formed when an electron is gained or lost. Because an electron by itself has a negative charge, its loss results in a cation and its addition results in an anion. Anions are attracted to anodes, and cations are attracted to cathodes.
Plasmas are collections of non-aqueous gaslike ions, and are typically superheated gases or are found in the vacuum of space. Astrophysical plasma containing electrons and protons only may make up 99.9% of the visible universe, making ions the most prevalent atomic form in the universe.
Web Resources On Ion
General Chemistry: Atoms and Ions
Positive and Negative Ions
Book Resources On Ion
Ion Channels of Excitable Membranes by Bertil Hille
Handbook of Ion Chromatography by Weiss & Weiss