In
chemistry, an ionic compound is a
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
composed of
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s held together by
electrostatic forces
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is convention ...
termed
ionic bonding
Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds ...
. The compound is neutral overall, but consists of positively charged ions called
cations and negatively charged ions called
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s. These can be
simple ion
A monatomic ion (also called simple ion) is an ion consisting of exactly one atom. If an ion contains more than one atom, even if these are of the same element, it is called a polyatomic ion. For example, calcium carbonate consists of the monatomi ...
s such as the
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
(Na
+) and
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
(Cl
−) in
sodium chloride, or
polyatomic
A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a complex (chemistry), metal complex, that can be considered to behave as a single unit and that has a net electrical charge, charge that is no ...
species such as the
ammonium () and
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
() ions in
ammonium carbonate
Ammonium carbonate is a salt with the chemical formula (NH4)2CO3. Since it readily degrades to gaseous ammonia and carbon dioxide upon heating, it is used as a leavening agent and also as smelling salt. It is also known as baker's ammonia and is ...
. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network. Ionic compounds usually form
crystalline structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns t ...
s when solid.
Ionic compounds containing basic ions
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...
(OH
−) or
oxide (O
2−) are classified as bases. Ionic compounds without these ions are also known as
salts
In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
and can be formed by
acid–base reactions. Ionic compounds can also be produced from their constituent ions by
evaporation of their
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
,
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
,
freezing
Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...
, a
solid-state reaction
A dry media reaction or solid-state reaction or solventless reaction is a chemical reaction system in the absence of a solvent. The drive for the development of dry media reactions in chemistry is
* economics (save money on solvents)
* ease of pur ...
, or the
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons.
Electrochemical processes ar ...
reaction of
reactive
Reactive may refer to:
*Generally, capable of having a reaction (disambiguation)
*An adjective abbreviation denoting a bowling ball coverstock made of reactive resin
*Reactivity (chemistry)
*Reactive mind
*Reactive programming
See also
*Reactanc ...
metals with reactive non-metals, such as
halogen gases.
Ionic compounds typically have high
melting
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...
and
boiling points, and are
hard
Hard may refer to:
* Hardness, resistance of physical materials to deformation or fracture
* Hard water, water with high mineral content
Arts and entertainment
* ''Hard'' (TV series), a French TV series
* Hard (band), a Hungarian hard rock supe ...
and
brittle
A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Br ...
. As solids they are almost always
electrically insulating, but when
melted or
dissolved they become highly
conductive
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gene ...
, because the ions are mobilized.
History of discovery
The word ''ion'' is the Greek , ''ion'', "going", the present participle of , ''ienai'', "to go". This term was introduced by physicist and chemist
Michael Faraday
Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic inducti ...
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, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
to the other through an aqueous medium.
In 1913 the crystal structure of sodium chloride was determined by
William Henry Bragg
Sir William Henry Bragg (2 July 1862 – 12 March 1942) was an English physicist, chemist, mathematician, and active sportsman who uniquelyThis is still a unique accomplishment, because no other parent-child combination has yet shared a Nob ...
and
William Lawrence Bragg
Sir William Lawrence Bragg, (31 March 1890 – 1 July 1971) was an Australian-born British physicist and X-ray crystallographer, discoverer (1912) of Bragg's law of X-ray diffraction, which is basic for the determination of crystal structu ...
.
[ This revealed that there were six equidistant nearest-neighbours for each atom, demonstrating that the constituents were not arranged in molecules or finite aggregates, but instead as a network with long-range ]crystalline
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, macros ...
order.[ Many other inorganic compounds were also found to have similar structural features.][ These compounds were soon described as being constituted of ions rather than neutral ]atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, ...
s, but proof of this hypothesis was not found until the mid-1920s, when X-ray reflection experiments (which detect the density of electrons), were performed.
Principal contributors to the development of a theoretical treatment of ionic crystal structures were Max Born, Fritz Haber
Fritz Haber (; 9 December 186829 January 1934) was a German chemist who received the Nobel Prize in Chemistry in 1918 for his invention of the Haber–Bosch process, a method used in industry to synthesize ammonia from nitrogen gas and hydroge ...
, Alfred Landé
Alfred Landé (13 December 1888 – 30 October 1976) was a German-American physicist known for his contributions to quantum theory. He is responsible for the Landé g-factor and an explanation of the Zeeman effect.
Life and achievements
Alf ...
, Erwin Madelung
Erwin Madelung (18 May 1881 – 1 August 1972) was a German physicist.
He was born in 1881 in Bonn. His father was the surgeon Otto Wilhelm Madelung. He earned a doctorate in 1905 from the University of Göttingen, specializing in crystal structu ...
, Paul Peter Ewald
Paul Peter Ewald, FRS (January 23, 1888 in Berlin, Germany – August 22, 1985 in Ithaca, New York) was a German crystallographer and physicist, a pioneer of X-ray diffraction methods.
Education
Ewald received his early education in the clas ...
, and Kazimierz Fajans
Kazimierz Fajans (Kasimir Fajans in many American publications; 27 May 1887 – 18 May 1975) was a Polish American physical chemist of Polish-Jewish origin, a pioneer in the science of radioactivity and the discoverer of chemical element protac ...
. Born predicted crystal energies based on the assumption of ionic constituents, which showed good correspondence to thermochemical
Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or phase changes such as melting and boiling. A reaction may release or absorb energy, and a phase change may do the same. Thermochemistry focuses on ...
measurements, further supporting the assumption.[
]
Formation
Ionic compounds can be produced from their constituent ions by evaporation, precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, or freezing
Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...
. Reactive metals such as the alkali metals can react directly with the highly electronegative
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
halogen gases to form an ionic product. They can also be synthesized as the product of a high temperature reaction between solids.
If the ionic compound is soluble in a solvent, it can be obtained as a solid compound by evaporating the solvent from this electrolyte solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Soluti ...
. As the solvent is evaporated, the ions do not go into the vapor, but stay in the remaining solution, and when they become sufficiently concentrated, nucleation occurs, and they crystallize into an ionic compound. This process occurs widely in nature and is the means of formation of the evaporite
An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
minerals. Another method of recovering the compound from solution involves saturating a solution at high temperature and then reducing the solubility by reducing the temperature until the solution is supersaturated and the solid compound nucleates.
Insoluble ionic compounds can be precipitated by mixing two solutions, one with the cation and one with the anion in it. Because all solutions are electrically neutral, the two solutions mixed must also contain counterions of the opposite charges. To ensure that these do not contaminate the precipitated ionic compound, it is important to ensure they do not also precipitate. If the two solutions have hydrogen ions and hydroxide ions as the counterions, they will react with one another in what is called an acid–base reaction
An acid–base reaction is a chemical reaction that occurs between an acid and a base. It can be used to determine pH via titration. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their applica ...
or a neutralization reaction
In chemistry, neutralization or neutralisation (see spelling differences) is a chemical reaction in which acid and a base react quantitatively with each other. In a reaction in water, neutralization results in there being no excess of hydrogen o ...
to form water. Alternately the counterions can be chosen to ensure that even when combined into a single solution they will remain soluble as spectator ions
A spectator ion is an ion that exists as a reactant and a product in a chemical equation. A spectator ion can, therefore, be observed in the reaction of aqueous solutions of sodium carbonate and copper(II) sulfate but does not affect the equilibr ...
.
If the solvent is water in either the evaporation or precipitation method of formation, in many cases the ionic crystal
In chemistry, an ionic crystal is a crystalline form of an ionic compound. They are solids consisting of ions bound together by their electrostatic attraction into a regular lattice. Examples of such crystals are the alkali halides, including ...
formed also includes water of crystallization
In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions. In some contexts, water of crystallization i ...
, so the product is known as a hydrate, and can have very different chemical properties.
Molten salts will solidify on cooling to below their freezing point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depend ...
. This is sometimes used for the solid-state synthesis of complex ionic compounds from solid reactants, which are first melted together. In other cases, the solid reactants do not need to be melted, but instead can react through a solid-state reaction route The solid-state reaction route is the most widely used method for the preparation of polycrystalline solids from a mixture of solid starting materials. Solids do not react together at room temperature over normal time scales and it is necessary to h ...
. In this method, the reactants are repeatedly finely ground into a paste and then heated to a temperature where the ions in neighboring reactants can diffuse together during the time the reactant mixture remains in the oven. Other synthetic routes use a solid precursor with the correct stoichiometric ratio of non-volatile ions, which is heated to drive off other species.
In some reactions between highly reactive metals (usually from Group 1 Group 1 may refer to:
* Alkali metal, a chemical element classification for Alkali metal
* Group 1 (racing), a historic (until 1981) classification for Touring car racing, applied to standard touring cars. Comparable to modern FIA Group N
* Group On ...
or Group 2 The term Group 2 may refer to:
* Alkaline earth metal, a chemical element classification
* Astronaut Group 2, also known as The New Nine, the second group of astronauts selected by NASA in 1962
* Group 2 (racing), an FIA classification for cars in ...
) and highly electronegative halogen gases, or water, the atoms can be ionized by electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons.
Electrochemical processes ar ...
, a process thermodynamically understood using the Born–Haber cycle.
Bonding
Ions in ionic compounds are primarily held together by the electrostatic force
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is convention ...
s between the charge distribution of these bodies, and in particular, the ionic bond resulting from the long-ranged Coulomb attraction between the net negative charge of the anions and net positive charge of the cations. There is also a small additional attractive force from van der Waals interactions which contributes only around 1–2% of the cohesive energy for small ions. When a pair of ions comes close enough for their outer
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