An ion () is a particle
with a net electrical charge
The charge of the electron
is considered negative by convention. The negative charge of an ion is equal and opposite to charged proton(s) considered positive by convention. The net charge of an ion is non-zero due to its total number of electrons being unequal to its total number of proton
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 compound
Ions consisting of only a single atom are termed atomic or monatomic ion
s, while two or more atoms form molecular ions or polyatomic ion
s. In the case of physical ionization in a fluid (gas or liquid), "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
History of discovery
The word ''ion'' comes from the Greek word ἰόν, ''ion'', "going", the present participle of ἰέναι, ''ienai'', "to go". This term was introduced (after a suggestion by the English polymath William Whewell
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. Faraday did not know the nature of these species, but he knew that since metals dissolved into and entered a solution at one electrode and new metal came forth from a solution at the other electrode; that some kind of substance has moved through the solution in a current. This conveys matter from one place to the other. In correspondence with Faraday, Whewell also coined the words ''anode
'' and ''cathode
'', as well as ''anion'' and ''cation'' as ions that are attracted to the respective electrodes.
put forth, in his 1884 dissertation, the explanation of the fact that solid crystalline salts dissociate
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, he proposed that ions formed even in the absence of an electric current.
Ions in their gas-like state are highly reactive and will 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
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 dissolved salts.
As charged objects, ions are attracted to opposite electric charges (positive to negative, and vice versa) and repelled by like charges. When they move, their trajectories can be 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
. Cations are smaller than the corresponding parent atom or molecule due to the smaller size of the electron cloud. One particular cation (that of hydrogen) contains no electrons, and thus consists of a single proton - ''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 is pronounced ("anyin") (−) (), 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 is pronounced ("cash-in") (+) (), 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
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."
[Frank Press & Raymond Siever (1986) ''Earth'', 14th edition, p. 63, W.H. Freeman and Company ]
The terms ''anion'' and ''cation'' (for ions that respectively travel to the anode and cathode during electrolysis) were introduced by Michael Faraday in 1834
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 colour from neutral atoms, and thus light absorption by metal ions gives the colour of gemstone
s. In both inorganic and organic chemistry (including biochemistry), the interaction of water and ions is extremely important; an example is energy that drives the 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.
Ions can be non-chemically prepared using various ion source
s, usually involving high voltage
or temperature. These are used in a multitude of devices such as mass spectrometers
, optical emission spectrometer
s, particle accelerators
, ion 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 detector
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
Inorganic dissolved ions are a component of total dissolved solids
, a widely known indicator of water quality
Detection of ionizing radiation
The ionizing effect of radiation on a gas is extensively used for the detection of radiation such as alpha
, and X-rays
. The original ionization 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 ionizing 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.
Denoting the charged state
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. However, 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 sometimes circle the sign; this is merely ornamental and does not alter the chemical meaning. All three representations of , Fe, and Fe shown in the figure, are thus equivalent.
Monatomic ions are sometimes also denoted with Roman numerals, particularly in spectroscopy
; for example, the example seen above is referred to as Fe() or Fe
. The Roman numeral designates the ''formal oxidation state
'' of an element, whereas the superscripted Indo-Arabic numerals denote the net charge. The two notations are, therefore, exchangeable for monatomic ions, but the Roman numerals ''cannot'' be applied to polyatomic ions. However, it is possible to mix the notations for the individual metal centre with a polyatomic complex, as shown by the uranyl ion example.
If an ion contains unpaired electron
s, it is called a ''radical
'' ion. Just like uncharged radicals, radical ions are very reactive. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called ''oxyanion
s''. 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 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 do not 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 ionized by bombardment with radiation
, but the more usual process of ionization 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 very stable, a sodium atom tends to lose its extra electron and attain this stable configuration, becoming a sodium cation in the process
:Na → +
On the other 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 extra electron and attain a stable 8-electron configuration, becoming a chloride anion in the process:
: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 bond
s and combine to form sodium chloride
, NaCl, more commonly known as table salt.
: + → NaCl
Formation of polyatomic and molecular ions
Polyatomic and molecular ions are often formed by the gaining or losing of elemental ions such as a proton, , in neutral molecules. For example, when ammonia
, , accepts a proton, —a process called protonation
—it forms the ammonium
ion, . Ammonia and ammonium have the same number of electrons in essentially the same electronic configuration
, but ammonium has an extra proton that gives it a net positive charge.
Ammonia can also lose an electron to gain a positive charge, forming the ion . However, this ion is unstable, because it has an incomplete valence shell
around the nitrogen atom, making it a very reactive radical
Due to the instability of radical ions, polyatomic and molecular ions are usually formed by gaining or losing elemental ions such as , rather than gaining or losing electrons. This allows the molecule to preserve its stable electronic configuration while acquiring an electrical charge.
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 ''n''th ionization energy of an atom is the energy required to detach its ''n''th electron after the first ''n − 1'' electrons have already been detached.
Each successive ionization energy is markedly greater than the last. Particularly great increases occur after any given block of atomic orbital
s 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 ionized form it is commonly found with one lost electron, as . On the other side of the periodic table, chlorine has seven valence electrons, so in ionized form it is commonly found with one gained electron, as . Caesium has the lowest measured ionization energy of all the elements and helium has the greatest.
[Chemical elements listed by ionization energy](_blank)
In general, the ionization energy of metals
is much lower than the ionization 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'' is a kind of chemical bond
ing 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 do not 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 may be derived.
The most common type of ionic bonding is seen in compounds of metals and nonmetals (except noble gas
es, which rarely form chemical compounds). Metals are characterized 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 extra electrons in order to attain a stable configuration. This property is known as ''electropositivity
''. Non-metals, on the other hand, are characterized 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 extra 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 other to form a ''salt
* Air ionizer
* Gaseous ionization detectors
* Ion beam
* Ion exchange
* Ionizing radiation
* Stopping power of radiation particles