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An ion () is an
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

atom
or
molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion I ...

molecule
with a net
electrical charge Electricity is the set of physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' (album), a 1981 album by Olivia Newton-John **Physical (Olivia Newton-John song), "Physi ...
. The charge of an
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

electron
is considered negative by convention and this charge is equal and opposite to charge of a
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

proton
, which is considered positive by convention. The net charge of an ion is not zero because its total number of electrons is unequal to its total number of protons. A cation is a positively charged ion with fewer electrons than protons while an anion is a negatively charged ion with more electrons than protons. Opposite electric charges are pulled towards one another by
electrostatic force ''F'' between two point charges ''q''1 and ''q''2 is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them. Like charges repel each other, and opposite charges mut ...

electrostatic force
, so cations and anions attract each other and readily form
ionic compound structure of sodium chloride, NaCl, a typical ionic compound. The purple spheres represent sodium cations, Na+, and the green spheres represent chloride anions, Cl−. The yellow stipples show the electrostatic forces. In chemistry, an ionic compo ...
s. Ions consisting of only a single atom are termed atomic or
monatomic ion A monatomic ion is an ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objec ...
s, while two or more atoms form molecular ions or
polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The b ...
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 Dissolution may refer to: Arts and entertainment Books * Dissolution (Forgotten Realms novel), ''Dissolution'' (''Forgotten Realms'' novel), a 2002 fantasy novel by Richard Lee Byers * Dissolution (Sansom novel), ''Dissolution'' (Sansom novel), a 2 ...
of a
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline mineral is known as rock salt or halite. Sa ...
in liquids, or by other means, such as passing a
direct current Direct current (DC) is one-directional flow Flow may refer to: Science and technology * Flow (fluid) or fluid dynamics, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (mathe ...
through a conducting solution, dissolving an
anode An anode is an electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor A semiconductor material has an Electrical resistivity and conductivity, electrical conductivit ...

anode
via
ionization Ionization or ionisation is the process by which an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
.


History of discovery

The word ''ion'' was coined from Greek ion, neuter present participle of ienai (Greek ἰέναι) "to go" from PIE root *ei- "to go.", cf. a cation is something that moves down (Greek kato κάτω kat-ion) and an anion is something that moves up (Greek ano ἄνω, an-ion). So called because ions move toward the electrode of opposite charge. This term was introduced (after a suggestion by the English
polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known to draw on complex bodies of knowledge to solve specific prob ...

polymath
William Whewell William Whewell ( ; 24 May 17946 March 1866) was an English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval Eng ...

William Whewell
) by English physicist and chemist
Michael Faraday Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge ...

Michael Faraday
in 1834 for the then-unknown species that ''goes'' from one
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor A semiconductor material has an Electrical resistivity and conductivity, electrical conductivity value falling between tha ...

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 An anode is an electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor A semiconductor material has an Electrical resistivity and conductivity, electrical conductivit ...

anode
'' and ''
cathode A cathode is the from which a leaves a polarized electrical device. This definition can be recalled by using the ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in which positive charges move. Electrons ha ...
'', as well as ''anion'' and ''cation'' as ions that are attracted to the respective electrodes.
Svante Arrhenius Svante August Arrhenius ( , ; 19 February 1859 – 2 October 1927) was a . Originally a , but often referred to as a , Arrhenius was one of the founders of the science of . He received the in 1903, becoming the first Nobel laureate. In 1905, ...

Svante Arrhenius
put forth, in his 1884 dissertation, the explanation of the fact that solid crystalline salts
dissociate Dissociation in chemistry and biochemistry is a general process in which molecules (or ionic compounds such as salt (chemistry), salts, or coordination complex, complexes) separate or split into smaller particles such as atoms, ions, or radical (c ...
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.


Characteristics

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 In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the law of states that energy can be in form, but not created or destroyed. The unit of measurement in the (SI) of energy is the , which is the ...

energy
and
entropy Entropy is a scientific concept as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics ...

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 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 A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
. Electrons, due to their smaller mass and thus larger space-filling properties as
matter waves Matter waves are a central part of the theory of quantum mechanics Quantum mechanics is a fundamental theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The p ...
, 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 In atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and verified in accordance with the scientific method The sc ...
. 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 (−) ( , 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 (+) ( , 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 An ion () is an atom or molecule with a net electric charge, electrical charge. The charge of an electron is considered negative by convention and this charge is equal and opposite to charge of a proton, which is considered positive by conven ...
and an ion with a +2 charge is known as a
dication A dication is any cation, of general formula X2+, formed by the removal of two electrons from a neutral species. Diatomic dications corresponding to stable neutral species (e.g. formed by removal of two electrons from H2) often decay quickly into ...
. A
zwitterionIn chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo ...
is a neutral molecule with positive and negative charges at different locations within that molecule. Cations and anions are measured by their
ionic radius Ionic radius, ''r''ion, is the radius of a monatomic ion in an ionic crystal structure. Although neither atoms nor ions have sharp boundaries, they are sometimes treated as if they were hard spheres with radii such that the sum of ionic radii of th ...
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 Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
. From this fact it is apparent that most of the space of a
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosco ...

crystal
is occupied by the anion and that the cations fit into the spaces between them." The terms ''anion'' and ''cation'' (for ions that respectively travel to the anode and cathode during electrolysis) were introduced by Michael Faraday in 1834.


Natural occurrences

Ions are ubiquitous in
nature Nature, in the broadest sense, is the natural, physical, material world or universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter an ...

nature
and are responsible for diverse phenomena from the luminescence of the Sun to the existence of the Earth's
ionosphere The ionosphere () is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere The thermosphere is the layer in the directly above the and below the . Within this layer of the atmosphere, ...
. Atoms in their ionic state may have a different color from neutral atoms, and thus light absorption by metal ions gives the color of
gemstone A gemstone (also called a fine gem, jewel, precious stone, or semi-precious stone) is a piece of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemic ...

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 ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

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 source An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for Mass spectrometry, mass spectrometers, optical emission spectrometers, particle accelerators, Ion implantation, ion implanters and Ion thrust ...
s, usually involving high
voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

voltage
or temperature. These are used in a multitude of devices such as
mass spectrometers Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are typically presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is ...
, optical emission spectrometers,
particle accelerators , a synchrotron File:Synchrotron Solaris.jpg, SOLARIS synchrotron in Poland (electromagnets in storage ring) A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle bea ...
, ion implanters, and
ion engines 290px, NEXIS ion engine test (2005) An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellite ...
. As reactive charged particles, they are also used in air purification by disrupting microbes, and in household items such as
smoke detector A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial smoke detectors issue a signal to a fire alarm control panel as part of a fire alarm system. Household smoke detectors, also known as ''smoke alarms'', ...

smoke detector
s. As signalling and metabolism in organisms are controlled by a precise ionic gradient across
membranes Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membr ...

membranes
, the disruption of this gradient contributes to cell death. This is a common mechanism exploited by natural and artificial
biocides A biocide is defined in the European legislation as a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ...
, including the
ion channels s (typically four per channel), 2 - outer vestibule, 3 - selectivity filter, 4 - diameter of selectivity filter, 5 - phosphorylation site, 6 - cell membrane. Ion channels are pore-forming membrane protein Membrane proteins are common proteins ...

ion channels
gramicidin Gramicidin, also called gramicidin D, is a mix of ionophore, ionophoric antibiotics, gramicidin A, B and C, which make up about 80%, 5%, and 15% of the mix, respectively. Each has 2 isoforms, so the mix has 6 different types of gramicidin molecule ...
and amphotericin (a
fungicide Fungicides are biocidal chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by hav ...
). Inorganic dissolved ions are a component of
total dissolved solids Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular ( colloidal sol) suspended form. TDS concentrations are often repor ...
, a widely known indicator of
water quality Water quality refers to the Chemical property, chemical, Physical property, physical, and Biology, biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against whi ...

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 Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', modern pronunciation ''álfa'') is the first letter Letter, letters, or literature may refer to: Characters typeface * Letter (alphabet) A letter is a segmental symbol A s ...

alpha
,
beta Beta (, ; uppercase , lowercase , or ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the . In the system of Greek numerals it has a value of 2. In , beta represented the . In , it represents the (while in foreig ...
,
gamma Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter rep ...
, and
X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''ph ...

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 #REDIRECT Ionization chamber#REDIRECT Ionization chamber The ionization chamber is the simplest of all gas-filled radiation detectors, and is widely used for the detection and measurement of certain types of ionizing radiation; X-rays, gamma rays, ...

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 The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter A Geiger counter is an instrument used for detecting and measuring ionizing radiation. Also known as a Geiger–Müller counter (or Geiger–Muller counter), it ...
and the
proportional counterThe proportional counter is a type of gaseous ionization detector device used to measure particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, obj ...
both use a phenomenon known as a
Townsend avalanche The Townsend discharge or Townsend avalanche is a gas ionisation process where free electron The electron is a subatomic particle, symbol or , whose electric charge Electric charge is the physical property of matter that causes it to exper ...
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.


Chemistry


Denoting the charged state

When writing the
chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and ...
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 Sodium is a with the  Na (from Latin ''natrium'') and  11. It is a soft, silvery-white, highly reactive . Sodium is an , being in of the periodic table. Its only stable is 23Na. The free metal does not occur in nature, and must be ...

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 Spectroscopy is the study of the interaction Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way ...
; for example, the example seen above is referred to as Fe() or Fe. The Roman numeral designates the ''formal
oxidation state The oxidation state, or oxidation number, is the hypothetical of an atom if all of its to different atoms were fully . It describes the degree of (loss of s) of an in a . Conceptually, the oxidation state may be positive, negative or zero. Whil ...
'' 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.


Sub-classes

If an ion contains
unpaired electron Periodic table with elements that have unpaired electrons coloured In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their comp ...
s, it is called a ''
radical Radical may refer to: Arts and entertainment Music *Radical (mixtape), ''Radical'' (mixtape), by Odd Future, 2010 *Radical (Smack album), ''Radical'' (Smack album), 1988 *"Radicals", a song by Tyler, The Creator from the 2011 album ''Goblin (album ...
'' ion. Just like uncharged radicals, radical ions are very reactive. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called ''
oxyanionAn oxyanion, or oxoanion, is an ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyd ...
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 A carbocation () is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and Vinyl cation, vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are ...
'' (if positively charged) or ''
carbanionA carbanion is an anion An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and opposite to charged ...
'' (if negatively charged).


Formation


Formation of monatomic ions

Monatomic ions are formed by the gain or loss of electrons to the
valence shell In chemistry and physics, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bon ...
(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 The atomic nucleus is the small, dense region consisting of s and s at the center of an , discovered in 1911 by based on the 1909 . After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickl ...
, 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 upThe international symbol for types and levels of ionizing radiation (radioactivity) that are unsafe for unshielded humans. Radiation, in general, exists throughout nature, such as in light and sound. In physics Physics (from grc ...

radiation
, but the more usual process of ionization encountered in
chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a with other . ...

chemistry
is the transfer of electrons between atoms or molecules. This transfer is usually driven by the attaining of stable ("closed shell")
electronic configuration In atomic physics and quantum chemistry Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds ...
s. Atoms will gain or lose electrons depending on which action takes the least energy. For example, a
sodium Sodium is a with the  Na (from Latin ''natrium'') and  11. It is a soft, silvery-white, highly reactive . Sodium is an , being in of the periodic table. Its only stable is 23Na. The free metal does not occur in nature, and must be ...

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 Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betw ...

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 In atomic physics and quantum chemistry Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compoun ...
, 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 Ionic bonding is a type of chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of attraction be ...
s and combine to form
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salt (chemistry), salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With Molar mass, molar ...
, 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 Ammonia is a compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fort ...

ammonia
, , accepts a proton, —a process called
protonation In chemistry, protonation (or hydronation) is the addition of a proton (or hydron, or hydrogen cation), (H+) to an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance ...
—it forms the
ammonium The ammonium cation An ion () is an atom or molecule with a net electric charge, electrical charge. The charge of an electron is considered negative by convention and this charge is equal and opposite to charge of a proton, which is conside ...

ammonium
ion, . Ammonia and ammonium have the same number of electrons in essentially the same
electronic configuration In atomic physics and quantum chemistry Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds ...
, 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 In chemistry and physics, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bon ...
around the nitrogen atom, making it a very reactive
radical Radical may refer to: Arts and entertainment Music *Radical (mixtape), ''Radical'' (mixtape), by Odd Future, 2010 *Radical (Smack album), ''Radical'' (Smack album), 1988 *"Radicals", a song by Tyler, The Creator from the 2011 album ''Goblin (album ...
ion. 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.


Ionization potential

The
energy In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the law of states that energy can be in form, but not created or destroyed. The unit of measurement in the (SI) of energy is the , which is 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 ''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 In atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural world and universe that can be repeatedly tested and verified in accordance with the scientific method The sc ...
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 chemistry and physics, a valence electron is an electron in the outer shell Shell may refer to: Architecture and design * Shell (structure)A shell is a type of structural element which is characterized by its geometry, being a three-dimension ...
'' 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
Lenntech.com
In general, the ionization energy of
metals A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts Electrical resistivity and conductivity, e ...
is much lower than the ionization energy of , 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 bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
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 In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of Translation operator (quantum mechanics)#Discrete Translational Symmetry, discrete translation operations described in th ...
. 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 Ionic radius, ''r''ion, is the radius of a monatomic ion in an ionic crystal structure. Although neither atoms nor ions have sharp boundaries, they are sometimes treated as if they were hard spheres with radii such that the sum of ionic radii of th ...
of individual ions may be derived. The most common type of ionic bonding is seen in compounds of metals and nonmetals (except
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under Standard conditions for temperature and pressure, standard conditions, they are all odorl ...
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 In atomic physics and quantum chemistry Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds ...
. As such, they have the tendency to lose these extra electrons in order to attain a stable configuration. This property is known as ''
electropositivity Electronegativity, symbolized as '' χ'', is the tendency of an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. ...
''. Non-metals, on the other hand, are characterized by having an
electron configuration In atomic physics and quantum chemistry Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compoun ...
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 Electronegativity, symbolized as '' χ'', is the tendency for an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. ...

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 Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure fo ...
''.


Common ions


See also

* Air ionizer *
Aurora An aurora (plural: auroras or aurorae), sometimes referred to as polar lights (aurora polaris), northern lights (aurora borealis), or southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in high-l ...

Aurora
*
Electrolyte An electrolyte is a medium containing ions that is electrically conducting Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that quantifies how strongly it resist ...

Electrolyte
*
Gaseous ionization detectors Gaseous ionization detectors are radiation detection instruments used in particle physics to detect the presence of ionizing particles, and in radiation protection applications to measure ionizing radiation. They use the ionising effect of radia ...
* Ioliomics *
Ion beam An ion beam is a type of charged particle beam A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same position, kinetic energy (resulting in the same velocity), and direction. ...
*
Ion exchange Ion exchange is a reversible interchange of one kind of ion present on an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...

Ion exchange
*
Ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of s or s that have sufficient to s or s by detaching s from them. The particles generally travel at a speed that is greater than 1% of , and the electromagnetic w ...
* Stopping power of radiation particles


References

{{Authority control Ions Physical chemistry Charge carriers