Ions & Anions

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass (“go”) between electrodes in a solution, when an electric field is applied. It is the transliteration of the Greek participle ἰόν, ión, “going”. An ion consisting of a single atom is an atomic or monatomic ion; if it consists of two or more atoms, it is a molecular or polyatomic ion. From: Wickipedia

Anions and cations

“Cation” and “Anion” redirect here. For the particle physics/quantum computing concept, see Anyon. For other uses, see Ion (disambiguation).

An anion (−) (play /ˈæn..ən/ an-eye-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).

cation (+) (play /ˈkæt..ən/ kat-eye-ən), from the Greek word κατά (katá), meaning “down”, is an ion with fewer electrons than protons, giving it a positive charge. Since the charge on a proton is equal in magnitude to the charge on an electron, the net charge on an ion is equal to the number of protons in the ion minus the number of electrons.


History and discovery

The word ion is the Greek ιον (going), the present participle of ιεναιienai, “to go”. This term was introduced by English physicist and chemist Michael Faraday in 1834 for the then-unknown species that goes from one electrode to the other through an aqueous medium.[1][2] Faraday did not know the nature of these species, but he knew that since metals dissolved into and entered solution at one electrode, and new metal came forth from solution at the other electrode, that some kind of substance moved through the solution in a current, conveying matter from one place to the other.

Faraday also introduced the words anion for a negatively charged ion, and cation for a positively charged one. In Faraday’s nomenclature, cations were named because they were attracted to the cathode in a galvanic device and anions were named due to their attraction to the anode.


Ions in their gas-like state are highly reactive, and do not occur in large amounts on Earth, except in flames, lightning, electrical sparks, and other plasmas. These gas-like ions rapidly interact with ions of opposite charge to give neutral molecules or ionic salts. Ions are also produced in the liquid or solid state when salts interact with solvents (for example, water) to produce “solvated ions,” which are more stable, for reasons involving a combination of energy and entropy changes as the ions move away from each other to interact with the liquid. These stabilized species are more commonly found in the environment at low temperatures. A common example is the ions present in seawater, which are derived from the dissolved salts.

All ions are charged, which means that like all charged objects they are:

  • attracted to opposite electric charges (positive to negative, and vice versa),
  • repelled by like charges
  • when moving, travel in trajectories that are deflected by a magnetic field.

Electrons, due to their smaller mass and thus larger space-filling properties as matter waves, determine the size of atoms and molecules that possess any electrons at all. Thus, anions (negatively charged ions) are larger than the parent molecule or atom, as the excess electron(s) repel each other, and add to the physical size of the ion, because its size is determined by its electron cloud. As such, in general, cations are smaller than the corresponding parent atom or molecule due to the smaller size of its electron cloud. One particular cation (that of hydrogen) contains no electrons, and thus is very much smaller than the parent hydrogen atom.

Natural occurrences

Ions are ubiquitous in nature and are responsible for diverse phenomena from the luminescence of the Sun to the existence of the Earth’s ionosphere. Atoms in their ionic state may have a different color from neutral atoms, and thus light absorption by metal ions gives the color of gemstones. In both inorganic and organic chemistry (including biochemistry), the interaction of water and ions is extremely important; an example is the energy that drives breakdown of ATP. The following sections describe contexts in which ions feature prominently; these are arranged in decreasing physical length-scale, from the astronomical to the microscopic.


Main article: Plasma (physics)

A collection of non-aqueous gas-like ions, or even a gas containing a proportion of charged particles, is called a plasma. Greater than 99.9% of visible matter in the Universe may be in the form of plasmas.[3] These include our Sun and other stars and the space between planets, as well as the space in between stars. Plasmas are often called the fourth state of matterbecause their properties are substantially different from those of solidsliquids, and gases.Astrophysical plasmas predominantly contain a mixture of electrons and protons (ionized hydrogen).

Related technology

Ions can be non-chemically prepared using various ion sources, usually involving high voltageor temperature. These are used in a multitude of devices such as mass spectrometersoptical emission spectrometersparticle acceleratorsion implanters, and ion engines.

As reactive charged particles, they are also used in air purification by disrupting microbes, and in household items such as smoke detectors.

As signaling and metabolism in organisms are controlled by a precise ionic gradient across membranes, the disruption of this gradient contributes to cell death. This is a common mechanism exploited by natural and artificial biocides, including the ion channels gramicidinand amphotericin (a fungicide).

Inorganic dissolved ions are a component of total dissolved solids, an indicator of water quality in the world.

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