An ion (/ˈən, -ɒn/) is an atom or a molecule in which the total number of electrons isn't equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge. Ions can be created, by either chemical or physical means, via ionization.

In chemical terms, if a neutral atom loses one or more electrons, it has a net positive charge and is known as a cation.

If an atom gains electrons, it has a net negative charge and is known as an anion.

Ions consisting of only a single atom are atomic or monatomic ions; if they consist of two or more atoms, they're molecular or polyatomic ions. Because of their electric charges, cations and anions attract each additional and readily form ionic compounds, such as salts.

In the case of physical ionisation of a medium, such as a gas, what're known as "ion pairs" are created by ion impact, and each pair consists of a free electron and a positive ion.

History of discovery

The word ion is the Greek ἰόν, ion, "going", the present participle of ἰέναι, ienai, "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 additional through an aqueous medium. Faraday didn't know the nature of these species, but he knew that after metals dissolved into and entered a solution at one electrode, and new metal came forth from a solution at the additional electrode, that a few kind of substance moved through the solution in a current, conveying matter from one place to the other.

Faraday additionally 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.

Svante Arrhenius put forth, in his 1884 dissertation, his explanation of the fact that solid crystalline salts disassociate into paired charged particles when dissolved, for which he would win the 1903 Nobel Prize in Chemistry. Arrhenius' explanation was that in forming a solution, the salt dissociates into Faraday's ions. Arrhenius proposed that ions formed even in the absence of an electric current.

Characteristics

Ions in their gas-like state are highly reactive, and don't occur in large amounts on Earth, except in flames, lightning, electrical sparks, and additional plasmas.

These gas-like ions rapidly interact with ions of opposite charge to give neutral molecules or ionic salts. Ions are additionally 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 additional to interact with the liquid. These stabilised 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 consists of a single proton - very much smaller than the parent hydrogen atom.

Anions and cations

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 (−) (/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).

A cation (+) (/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.

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 ions are measured by their ionic radius and they differ in relative size: "Cations are small, most of them less than 10-8 cm in radius. But most anions are large, as is the most common Earth anion, oxygen. From this fact it is obvious that most of the space of a crystal is occupied by the anion and that the cations fit into the spaces between them."

In terms of an angstrom Å, a cation has radius less than .8 Å while an anion has radius greater than 1.3 Å.

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 might have a different colour from neutral atoms, and thus light absorption by metal ions gives the colour 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 adenosine triphosphate (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.

Astronomical

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 might be in the form of plasmas. These include our Sun and additional stars and the space between planets, as well as the space in between stars. Plasmas are often called the fourth state of matter because their properties are substantially different from those of solids, liquids, and gases. Astrophysical plasmas predominantly contain a mixture of electrons and protons (ionized hydrogen).

Ions can be non-chemically prepared using various ion sources, usually involving high voltage or temperature. These are used in a multitude of devices such as mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters, and ion engines.

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

As signalling 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 gramicidin and amphotericin (a fungicide).

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

Detection of ionising radiation

Schematic of an ion chamber, showing drift of ions. Electrons drift faster than positive ions due to their much smaller mass.
Avalanche effect between two electrodes. The original ionisation event liberates one electron, and each subsequent collision liberates a further electron, so two electrons emerge from each collision: the ionising electron and the liberated electron.

The ionising effect of radiation on a gas is extensively used for the detection of radiation such as alpha, beta, gamma and X-rays. The original ionisation event in these instruments results in the formation of an "ion pair"; a positive ion and a free electron, by ion impact by the radiation on the gas molecules. The ionization chamber is the simplest of these detectors, and collects all the charges created by direct ionization within the gas through the application of an electric field.

The Geiger–Müller tube and the proportional counter both use a phenomenon known as a Townsend avalanche to multiply the effect of the original ionising event by means of a cascade effect whereby the free electrons are given sufficient energy by the electric field to release further electrons by ion impact.

Chemistry

Notation

Denoting the charged state

Equivalent notations for an iron atom (Fe) that lost two electrons, referred to as ferrous.

When writing the chemical formula for an ion, its net charge is written in superscript immediately after the chemical structure for the molecule/atom. The net charge is written with the magnitude before the sign; that is, a doubly charged cation is indicated as 2+ instead of +2. Notwithstanding the magnitude of the charge is omitted for singly charged molecules/atoms; for example, the sodium cation is indicated as Na+ and not Na1+.

An alternative (and acceptable) way of showing a molecule/atom with multiple charges is by drawing out the signs multiple times; this is often seen with transition metals. Chemists at times circle the sign; this is merely ornamental and doesn't alter the chemical meaning. All three representations of Fe2+
shown in the figure are, thus, equivalent.

Mixed Roman numerals and charge notations for the uranyl ion. The oxidation state of the metal is shown as superscripted Roman numerals, whereas the charge of the entire complex is shown by the angle symbol together with the magnitude and sign of the net charge.

Monatomic ions are at times additionally denoted with Roman numerals; for example, the Fe2+
example seen above is occasionally referred to as Fe(II) or FeII. The Roman numeral designates the formal oxidation state of an element, whereas the superscripted numerals denote the net charge. The two notations are, therefore, exchangeable for monatomic ions, but the Roman numerals cannot be applied to polyatomic ions. Notwithstanding it is possible to mix the notations for the individual metal centre with a polyatomic complex, as shown by the uranyl ion example.

Sub-classes

If an ion contains unpaired electrons, it is called a radical ion. Just like uncharged radicals, radical ions are quite reactive. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanions. Molecular ions that contain at least one carbon to hydrogen bond are called organic ions. If the charge in an organic ion is formally centred on a carbon, it is termed a carbocation (if positively charged) or carbanion (if negatively charged).

Formation

Formation of monatomic ions

Monatomic ions are formed by the gain or loss of electrons to the valence shell (the outer-most electron shell) in an atom. The inner shells of an atom are filled with electrons that are tightly bound to the positively charged atomic nucleus, and so don't participate in this kind of chemical interaction. The process of gaining or losing electrons from a neutral atom or molecule is called ionization.

Atoms can be ionised by bombardment with radiation, but the more usual process of ionisation encountered in chemistry is the transfer of electrons between atoms or molecules. This transfer is usually driven by the attaining of stable ("closed shell") electronic configurations. Atoms will gain or lose electrons depending on which action takes the least energy.

For example, a sodium atom, Na, has a single electron in its valence shell, surrounding 2 stable, filled inner shells of 2 and 8 electrons. Since these filled shells are quite stable, a sodium atom tends to lose its additional electron and attain this stable configuration, fitting a sodium cation in the process

Na → Na+
+
e

On the additional hand, a chlorine atom, Cl, has 7 electrons in its valence shell, which is one short of the stable, filled shell with 8 electrons. Thus, a chlorine atom tends to gain an additional electron and attain a stable 8-electron configuration, fitting a chloride anion in the process:

Cl +
e
Cl

This driving force is what causes sodium and chlorine to undergo a chemical reaction, wherein the "extra" electron is transferred from sodium to chlorine, forming sodium cations and chloride anions. Being oppositely charged, these cations and anions form ionic bonds and combine to form sodium chloride, NaCl, more commonly known as table salt.

Na+
+ Cl
→ NaCl

Formation of polyatomic and molecular ions

An electrostatic potential map of the nitrate ion (NO3). The 3-dimensional shell represents a single arbitrary isopotential.

Polyatomic and molecular ions are often formed by the gaining or losing of elemental ions such as a proton, H+, in neutral molecules. For example, when ammonia, NH3, accepts a proton, H+—a process called protonation—it forms the ammonium ion, NH4+. Ammonia and ammonium have the same number of electrons in essentially the same electronic configuration, but ammonium has an additional proton that gives it a net positive charge.

Ammonia can additionally lose an electron to gain a positive charge, forming the ion ·NH+
3
. Notwithstanding this ion is unstable, because it has an incomplete valence shell around the nitrogen atom, making it a quite reactive radical ion.

Due to the instability of radical ions, polyatomic and molecular ions are usually formed by gaining or losing elemental ions such as H+
, rather than gaining or losing electrons. This allows the molecule to preserve its stable electronic configuration while acquiring an electrical charge.

Ionization potential

The energy required to detach an electron in its lowest energy state from an atom or molecule of a gas with less net electric charge is called the ionization potential, or ionization energy. The nth ionisation energy of an atom is the energy required to detach its nth electron after the first n − 1 electrons have already been detached.

Each successive ionisation energy is markedly greater than the last. Particularly great increases occur after any given block of atomic orbitals is exhausted of electrons. For this reason, ions tend to form in ways that leave them with full orbital blocks. For example, sodium has one valence electron in its outermost shell, so in ionised form it is commonly found with one lost electron, as Na+
. On the additional side of the periodic table, chlorine has seven valence electrons, so in ionised form it is commonly found with one gained electron, as Cl
. Caesium has the lowest measured ionisation energy of all the elements and helium has the greatest. In general, the ionisation energy of metals is much lower than the ionisation energy of nonmetals, which is why, in general, metals will lose electrons to form positively charged ions and nonmetals will gain electrons to form negatively charged ions.

Ionic bonding

Ionic bonding is a kind of chemical bonding that arises from the mutual attraction of oppositely charged ions. Ions of like charge repel each other, and ions of opposite charge attract each other. Therefore, ions don't usually exist on their own, but will bind with ions of opposite charge to form a crystal lattice. The resulting compound is called an ionic compound, and is said to be held together by ionic bonding. In ionic compounds there arise characteristic distances between ion neighbours from which the spatial extension and the ionic radius of individual ions might be derived.

The most common type of ionic bonding is seen in compounds of metals and nonmetals (except noble gases, which rarely form chemical compounds). Metals are characterised by having a small number of electrons in excess of a stable, closed-shell electronic configuration. As such, they have the tendency to lose these additional electrons in order to attain a stable configuration. This property is known as electropositivity. Non-metals, on the additional hand, are characterised by having an electron configuration just a few electrons short of a stable configuration. As such, they have the tendency to gain more electrons in order to achieve a stable configuration. This tendency is known as electronegativity. When a highly electropositive metal is combined with a highly electronegative nonmetal, the additional electrons from the metal atoms are transferred to the electron-deficient nonmetal atoms. This reaction produces metal cations and nonmetal anions, which are attracted to each additional to form a salt.

Common ions

Common cations
Common nameFormulaHistoric name
Simple cations
AluminiumAl3+
BariumBa2+
BerylliumBe2+
CalciumCa2+
Chromium(III)Cr3+
Copper(I)Cu+cuprous
Copper(II)Cu2+cupric
HydrogenH+
Iron(II)Fe2+ferrous
Iron(III)Fe3+ferric
Lead(II)Pb2+plumbous
Lead(IV)Pb4+plumbic
LithiumLi+
MagnesiumMg2+
Manganese(II)Mn2+
Mercury(II)Hg2+mercuric
PotassiumK+kalic
SilverAg+argentous
SodiumNa+natric
StrontiumSr2+
Tin(II)Sn2+stannous
Tin(IV)Sn4+stannic
ZincZn2+
Polyatomic cations
AmmoniumNH+
4
HydroniumH3O+
Mercury(I)Hg2+
2
mercurous
Common anions
Formal nameFormulaAlt. name
Simple anions
AzideN
3
BromideBr
ChlorideCl
FluorideF
HydrideH
IodideI
NitrideN3−
OxideO2−
SulfideS2−
Oxoanions
CarbonateCO2−
3
ChlorateClO
3
ChromateCrO2−
4
DichromateCr
2
O2−
7
Dihydrogen phosphateH
2
PO
4
Hydrogen carbonateHCO
3
bicarbonate
Hydrogen sulfateHSO
4
bisulfate
Hydrogen sulfiteHSO
3
bisulfite
HydroxideOH
HypochloriteClO
Monohydrogen phosphateHPO2−
4
NitrateNO
3
NitriteNO
2
PerchlorateClO
4
PermanganateMnO
4
PeroxideO2−
2
PhosphatePO3−
4
SulfateSO2−
4
SulfiteSO2−
3
SuperoxideO
2
ThiosulfateS
2
O2−
3
SilicateSiO4−
4
MetasilicateSiO2−
3
Aluminium silicateAlSiO
4
Anions from organic acids
AcetateCH
3
COO
ethanoate
FormateHCOO
methanoate
OxalateC
2
O2−
4
ethanedioate
CyanideCN