Ion
Encyclopedia
An ion is an atom
or molecule
in which the total number of electron
s is not equal to the total number of proton
s, giving it a net positive or negative electrical charge
. The name was given by physicist Michael Faraday
for the substances that allow a current to pass ("go") between electrodes in a solution, when an electric field
is applied. It is from Greek ιον, meaning "going".
An ion consisting of a single atom is an atom
ic or monatomic ion
; if it consists of two or more atoms, it is a molecular or polyatomic ion
.
Conversely, a cation (+) , from the Greek word κατά (katá), meaning "down", is an ion with fewer electrons than protons, giving it a positive charge. Since the charge on a proton is equal in magnitude to the charge on an electron, the net charge on an ion is equal to the number of protons in the ion minus the number of electrons.
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 solution at one electrode, and new metal came forth from solution at the other electrode, that some kind of substance moved through the solution in a current, conveying matter from one place to the other.
Faraday also 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
.
s. These gas-like ions 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
and 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 the dissolved salts there.
All ions are charged, which means that like all charged objects they are-
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. Conversely, cations are generally smaller than the corresponding parent atom or molecule, for the same reason. One particular cation (that of hydrogen) contains no electrons, and thus is very much smaller than the parent hydrogen atom.
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 color from neutral atoms, and thus light absorption by metal ions gives the color of gemstone
s. 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 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.
A collection of non-aqueous
gas-like ions, or even a gas containing a proportion of charged particles, is called a plasma. >99.9% of visible matter in the Universe may be in the form of plasma
s. These include our Sun
and other stars, the space between planets
, as well as the space in between stars
. Plasmas are often called the fourth state of matter because its properties are substantially different from solid
s, liquid
s, and gas
es. Astrophysical plasmas predominantly contain a mixture of electrons and protons (ionized hydrogen).
s, 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 detector
s.
As signaling and metabolism in organisms are controlled by a precise ionic gradient across membrane
s, 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.
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. Conventionally the magnitude of the charge is omitted for singly charged molecules/atoms; for example, the sodium
cation is indicated as and not .
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 shown in the figure are thus equivalent.
Monatomic ions are sometimes also denoted with Roman numerals; for example, the 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 denotes the net charge. The two notations are therefore exchangeable for monatomic ions, but the Roman numerals cannot be applied to polyatomic ions. It is however possible to mix the notations for the individual metal center with a polyatomic complex, as shown by the uranyl ion example.
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 centered on a carbon, it is termed a carbocation
(if positively charged) or carbanion
(if negatively charged).
of the atom, which is the outer-most electron shell in an atom, or the losing of electrons from this shell. 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
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:
This driving force is what causes sodium and chlorine to undergo a chemical reaction, where 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 together to form sodium chloride
, NaCl, more commonly known as rock salt.
Polyatomic and molecular ions are often formed by the gaining or losing of elemental ions such as in neutral molecules. For example, when ammonia
, , accepts a proton, , 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
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.
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 ionization 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 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. The ionization energy of metals is generally much lower than the ionization energy of nonmetals, which is why metals will generally lose electrons to form positively charged ions while nonmetals will generally gain electrons to form negatively charged ions.
ing that arises from the mutual attraction of oppositely charged ions. Since ions of like charge repel each other, they do not usually exist on their own. Instead, many of them may form a crystal lattice, in which ions of opposite charge are bound to each other. 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 neighbors 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.
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
or molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
in which the total number of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s is not equal to the total number of proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s, giving it a net positive or negative electrical charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
. The name was given by physicist Michael Faraday
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
for the substances that allow a current to pass ("go") between electrodes in a solution, when an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
is applied. It is from Greek ιον, meaning "going".
An ion consisting of a single atom is an atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
ic or monatomic ion
Monatomic ion
A monatomic ion is an ion consisting of one or more atoms of a single element . For example calcium carbonate consists of the monatomic ion Ca2+ and the polyatomic ion CO32-....
; if it consists of two or more atoms, it is a molecular or polyatomic ion
Polyatomic ion
A polyatomic ion, also known as a molecular ion, is a charged species composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix "poly-" means "many," in...
.
Anions and cations
An anion (−) , 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).Conversely, a cation (+) , from the Greek word κατά (katá), meaning "down", is an ion with fewer electrons than protons, giving it a positive charge. Since the charge on a proton is equal in magnitude to the charge on an electron, the net charge on an ion is equal to the number of protons in the ion minus the number of electrons.
History and discovery
Etymologically the word ion is the Greek ιον (going), the present participle of ιεναι, ienai, "to go". This term was introduced by English physicist and chemist Michael FaradayMichael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
in 1834 for the (then unknown) species that goes from one electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
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 solution at one electrode, and new metal came forth from solution at the other electrode, that some kind of substance moved through the solution in a current, conveying matter from one place to the other.
Faraday also 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
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
in a galvanic device and anions were named due to their attraction to the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
.
Characteristics
Ions in their gas-like state are highly reactive, and do not occur in large amounts on Earth, except in flames, lightning, electrical sparks, and other plasmaPlasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
s. These gas-like ions 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
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
and entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
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 the dissolved salts there.
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, and
- when moving, travel in trajectories that are deflected by a magnetic fieldMagnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
.
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. Conversely, cations are generally smaller than the corresponding parent atom or molecule, for the same reason. One particular cation (that of hydrogen) contains no electrons, and thus is very much smaller than the parent hydrogen atom.
Natural occurrences
Ions are ubiquitous in natureNature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
and are responsible for diverse phenomena from the luminescence of the Sun, to the existence of the Earth's ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
. 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
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
s. 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 ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
. 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
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
gas-like ions, or even a gas containing a proportion of charged particles, is called a plasma. >99.9% of visible matter in the Universe may be in the form of plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
s. These include our Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and other stars, the space between planets
Interplanetary medium
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.-Composition and physical characteristics:...
, as well as the space in between stars
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
. Plasmas are often called the fourth state of matter because its properties are substantially different from solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
s, liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s, and gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es. Astrophysical plasmas predominantly contain a mixture of electrons and protons (ionized hydrogen).
Related technology
Ions can be non-chemically prepared using various ion sourceIon source
An ion source is an electro-magnetic device that is used to create charged particles. These are used primarily to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.- Electron ionization :...
s, usually involving high voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
or temperature. These are used in a multitude of devices such as mass spectrometers
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
, optical emission spectrometers, particle accelerators, ion implanters
Ion implantation
Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
and ion engines
Ion thruster
An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...
.
As reactive charged particles, they are also used in air purification
Air ioniser
An air ioniser is a device that uses high voltage to ionise air molecules. Negative ions, or anions, are particles with one or more extra electrons, conferring a net negative charge to the particle. Cations are positive ions missing one or more electrons, resulting in a net positive charge...
by disrupting microbes, and in household items such as smoke detector
Smoke detector
A smoke detector is a device that detects smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector...
s.
As signaling and metabolism in organisms are controlled by a precise ionic gradient across membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
s, 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
Gramicidin
Gramicidin is a heterogeneous mixture of six antibiotic compounds, gramicidins A, B and C, making up 80%, 6%,and 14% respectively, all of which are obtained from the soil bacterial species Bacillus brevis and called collectively gramicidin D. Gramicidin D are linear pentadecapeptides; that is...
and amphotericin (a fungicide
Fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
).
Inorganic dissolved ions are a component of total dissolved solids
Total dissolved solids
Total Dissolved Solids is a measure of the combined content of all inorganic and organic substances contained in a liquid in: molecular, ionized or micro-granular suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve...
, an indicator of water quality
Water quality
Water quality is the physical, chemical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which...
in the world.
Denoting the charged state
When writing the chemical formulaChemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
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. Conventionally the magnitude of the charge is omitted for singly charged molecules/atoms; for example, the sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
cation is indicated as and not .
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 shown in the figure are thus equivalent.
Monatomic ions are sometimes also denoted with Roman numerals; for example, the example seen above is occasionally referred to as Fe(II) or FeII. The Roman numeral designates the formal oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
of an element, whereas the superscripted numerals denotes the net charge. The two notations are therefore exchangeable for monatomic ions, but the Roman numerals cannot be applied to polyatomic ions. It is however possible to mix the notations for the individual metal center with a polyatomic complex, as shown by the uranyl ion example.
Sub-classes
If an ion contains unpaired electronUnpaired electron
In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair. As the formation of electron pairs is often energetically favourable, either in the form of a chemical bond or as a lone pair, unpaired electrons are relatively...
s, it is called a radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
ion. Just like uncharged radicals, radical ions are very reactive. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanion
Oxyanion
An oxyanion or oxoanion is a chemical compound with the generic formula AxOyz− . Oxoanions are formed by a large majority of the chemical elements. The formulae of simple oxoanions are determined by the octet rule...
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 centered on a carbon, it is termed a carbocation
Carbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
(if positively charged) or carbanion
Carbanion
A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. The carbanion exists in a trigonal pyramidal geometry. Formally a carbanion is the conjugate base of a carbon acid.where B...
(if negatively charged).
Formation of monatomic ions
Monatomic ions are formed by the addition of electrons to the valence shellValence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
of the atom, which is the outer-most electron shell in an atom, or the losing of electrons from this shell. The inner shells of an atom are filled with electrons that are tightly bound to the positively charged atomic nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
, 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
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
, but the more usual process of ionization encountered in chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
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
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
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
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
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, where 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 bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
s and combine together to form sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
, NaCl, more commonly known as rock salt.
- + → NaCl
Formation of polyatomic and molecular ions
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, , accepts a proton, , 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
Valence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
around the nitrogen atom, making it a very reactive radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
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 energyEnergy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
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 ionization 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 ionization energy is markedly greater than the last. Particularly great increases occur after any given block of atomic orbital
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
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
Valence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
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. The ionization energy of metals is generally much lower than the ionization energy of nonmetals, which is why metals will generally lose electrons to form positively charged ions while nonmetals will generally gain electrons to form negatively charged ions.
Ionic bonding
Ionic bonding is a kind of chemical bondChemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
ing that arises from the mutual attraction of oppositely charged ions. Since ions of like charge repel each other, they do not usually exist on their own. Instead, many of them may form a crystal lattice, in which ions of opposite charge are bound to each other. 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 neighbors from which the spatial extension and the ionic radius
Ionic radius
Ionic radius, rion, is the radius of an atom's ion. Although neither atoms nor ions have sharp boundaries, it is important to treat them as if they are hard spheres with radii such that the sum of ionic radii of the cation and anion gives the distance between the ions in a crystal lattice...
of individual ions may be derived.
The most common type of ionic bonding is seen in compounds of metals and nonmetals (except noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
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
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
. 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.
Chemical applications
Gas-like ions and solvated ions both have tremendous impact on chemical analysis and synthesis.Common ions
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See also
- Air ionizer
- AnodeAnodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
- CathodeCathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
- Ion beamIon beamAn ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing and other industries. A variety of ion beam sources exist, some derived from the mercury vapor thrusters developed by NASA in the 1960s.-Ion beam etching or sputtering:One type...
- Ion sourceIon sourceAn ion source is an electro-magnetic device that is used to create charged particles. These are used primarily to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.- Electron ionization :...
- Ionic radiusIonic radiusIonic radius, rion, is the radius of an atom's ion. Although neither atoms nor ions have sharp boundaries, it is important to treat them as if they are hard spheres with radii such that the sum of ionic radii of the cation and anion gives the distance between the ions in a crystal lattice...
- Auroras