An ion (/ˈaɪən, -ɒn/) is an atom or molecule that has a non-zero net electrical charge (its total number of electrons is not equal to its total number of protons). A cation is a positively-charged ion, while an anion is negatively charged. Because of their opposite electric charges, cations and anions attract each other and readily form ionic compounds, such as salts. Ions can be created by chemical means, such as the dissolution of a salt into water, or by physical means, such as passing a direct current through a conducting solution, which will dissolve the anode via ionization. Ions consisting of only a single atom are atomic or monatomic ions. If they consist of two or more atoms, then they are called molecular ions or polyatomic ions.
In the case of physical ionization of a medium such as a gas, what are known as "ion pairs" are created by ion impact, and each pair consists of a free electron and a positive ion.
1 History of discovery 2 Characteristics
2.1 Anions and cations 2.2 Natural occurrences
3 Related technology
3.1 Detection of ionizing radiation
4.1.1 Denoting the charged state 4.1.2 Sub-classes
4.2.1 Formation of monatomic ions
4.2.2 Formation of polyatomic and molecular ions
4.3 Ionic bonding 4.4 Common ions
5 See also 6 References
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 by English physicist and chemist
attracted to opposite electric charges (positive to negative, and vice versa), repelled by like charges when moving, 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. 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
"Cation" and "Anion" redirect here. For the particle physics/quantum computing concept, see Anyon. For other uses, see Ion (other). 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.aɪ.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.aɪ.ən/), 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 m (10−8 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." A cation has radius less than 0.8 × 10−10 m (0.8 Å) while an anion has radius greater than 1.3 × 10−10 m (1.3 Å). Natural occurrences Further information: List of oxidation states of the elements 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 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. Related technology 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 are also 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, a widely-known indicator of water quality. Detection of ionizing 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 ionization event liberates one electron, and each subsequent collision liberates a further electron, so two electrons emerge from each collision: the ionizing electron and the liberated electron.
The ionizing effect of radiation on a gas is extensively used for the
detection of radiation such as alpha, beta, gamma 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.
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. 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 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 sometimes also 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. However, 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 very 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 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 → Na+ + e−
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 + 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+.
Common name Formula Historic name
Copper(I) Cu+ cuprous
Copper(II) Cu2+ cupric
Iron(II) Fe2+ ferrous
Iron(III) Fe3+ ferric
Lead(II) Pb2+ plumbous
Lead(IV) Pb4+ plumbic
Mercury(II) Hg2+ mercuric
Potassium K+ kalic
Silver Ag+ argentous
Sodium Na+ natric
Tin(II) Sn2+ stannous
Tin(IV) Sn4+ stannic
Ammonium NH+ 4
Mercury(I) Hg2+ 2 mercurous
Formal name Formula Alt. name
Azide N− 3
Carbonate CO2− 3
Chlorate ClO− 3
Chromate CrO2− 4
Dichromate Cr 2O2− 7
Dihydrogen phosphate H 2PO− 4
Monohydrogen phosphate HPO2− 4
Nitrate NO− 3
Nitrite NO− 2
Perchlorate ClO− 4
Permanganate MnO− 4
Peroxide O2− 2
Phosphate PO3− 4
Sulfate SO2− 4
Sulfite SO2− 3
Superoxide O− 2
Thiosulfate S 2O2− 3
Silicate SiO4− 4
Metasilicate SiO2− 3
Anions from organic acids
Acetate CH 3COO− ethanoate
Formate HCOO− methanoate
Oxalate C 2O2− 4 ethanedioate
^ "Ion" entry in Collins English Dictionary.
^ a b c Knoll, Glenn F (1999).
v t e
Voltaic pile Battery
Flow battery Trough battery
Concentration cell Fuel cell Thermogalvanic cell
Primary cell (non-rechargeable)
Secondary cell (rechargeable)
gel / VRLA
Molten salt Nanopore Nanowire Nickel–cadmium Nickel–hydrogen Nickel–iron Nickel–lithium Nickel–metal hydride Nickel–zinc Polysulfide bromide Potassium-ion Rechargeable alkaline Sodium-ion Sodium–sulfur Vanadium redox Zinc–bromine Zinc–cerium
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