Ion

(center) contains a single proton and a single electron. Removal of the electron gives a cation (left), whereas addition of an electron gives an anion (right). Anions, with its loosely held electron cloud, has larger radii than the neutral atom/molecule, which in turn is larger than the cation.]]

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. 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. An ion consisting of a single atom is an atomic or monatomic ion; if it consists of two or more atoms, it is a molecular or polyatomic ion.

General

History and Discovery

Etymologically the word ion is derived from the Greek root ienai, "to go". This term was introduced by English physicist and chemist Michael Faraday in 1834 for the (then unknown) species that goes from one electrode to the other through an aqueous medium.

Characteristics

Ions, in its gas-like state, is highly reactive and not present in large amounts in the daily environment. These gas-like ions rapidly interact with ions of opposite charge to give ionic compounds (i.e., salts); they can also interact with solvents—for example, water—to give solvated ions which are much more energetically stable. These stabilized species are what is commonly found in our environment.

Regardless of whether they are stabilized, all ions are:

attracted to opposite electric charges (positive to negative, and vice versa),

repelled by like charges, and

moving trajectories are deflected by a magnetic field.

Anions are generally larger than the parent molecule, as the nuclei is insufficiently positively charged to hold onto the excess electrons. Conversely, cations are generally much smaller than the parent molecule.

Natural Occurrences

Ions are ubiquitous in Nature and are responsible for diverse phenomena from the luminescence of the Sun and existence of ionosphere on Earth down to the color of gemstones and the synthesis of ATP. The following sections describe contexts in which ions feature prominently and is arranged in decreasing scale from the astronomical to the microscopic.

Astronomical

of "Tycho's Supernova", a huge ball of expanding plasma. The outer shell shown in blue is X-ray emission by high-speed electrons.]]

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 plasmas. 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 solids, liquids, and gases. Astrophysical plasmas containing predominantly a mixture of electrons and protons (ionized hydrogen).

Terrestrial

ionosphere, salinity and ionic composition of water

Biological

ion gradient across membrane, co-factors and catalysis

Related Technology

Ions can be non-chemically prepared using various ion sources, usually involving high voltage or temperature. These are used in a multitude of devices such as mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.

As reactive charged particles, they are also used in air purification by disrupting microbes, and in household items such as smoke detectors.

As signaling and metabolism in organisms are controlled by a precise ionic gradient across membranes, the disruption of this gradient contributes to cell death. This is a common mechanism exploited by natural and artificial biocides, including the ion channels gramicidin and amphotericin (a fungicide).

Inorganic dissolved ions are a component of total dissolved solids, an indicator of water quality in the world.

Portrayal in Literature and Media

Chemistry

Notation

Denoting the charged state

atom (Fe) that lost two electrons.]]

When writing the chemical formula for an ion, its net charge is written in superscript immediately after the chemical structure for the molecule/atom. The net charge is written with the magnitude before the sign; that is, a doubly charged cation is indicated as 2+ instead of +2. 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 Fe^II. 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.

Sub-classes

If an ion contains unpaired electrons, it is called a radical ion. Just like uncharged radicals, radical ions are very reactive. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanions. Molecular ions that contain at least one carbon to hydrogen bond are called organic ions. If the charge in an organic ion is formally centered on a carbon, it is termed a carbocation (if positively charged) or carboanion (if negatively charged).

Formation

Formation of monatomic ions

Monatomic ions are formed by the addition of electrons to the valence shell 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

:Na → +

On the other hand, a chlorine atom, Cl, has 7 electrons in its valence shell, which is one short of the stable, filled shell with 8 electrons. Thus, a chlorine atom tends to gain an extra electron and attain a stable 8-electron configuration, becoming a chloride anion in the process:

:Cl + →

This driving force is what causes sodium and chlorine to undergo a chemical reaction, 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 bonds and combine together to form sodium chloride, NaCl, more commonly known as rock salt.

: + → NaCl

Formation of polyatomic and molecular ions

map of the nitrate ion (). The 3-dimensional shell represents a single arbitrary isopotential.]]

Polyatomic and molecular ions are often formed by the gaining or losing of elemental ions such as 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.

Ionization potential

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 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 orbitals 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.

Ionic bonding

Ionic bonding is a kind of chemical bonding 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 gases, 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.

Chemical Applications

Gas-like ions and solvated ions both have tremendous impact on chemical analysis and synthesis.

Mass Spectroscopy

Catalysis

Transition Metal Ions Catalysis

Templated Synthesis of Organic Compounds

Common ions

{| |valign="top"| {|class="wikitable" |+Common Cations |- !style="text-align: left"|Common Name !style="text-align: left"|Formula !style="text-align: left"|Historic Name |- !colspan="3" style="background-color: aliceblue"|Simple Cations |- |Aluminium||Al³⁺|| |- |Calcium||Ca²⁺|| |- |Copper(II)||Cu²⁺||cupric |- |Hydrogen||H⁺|| |- |Iron(II)||Fe²⁺||ferrous |- |Iron(III)||Fe³⁺||ferric |- |Magnesium||Mg²⁺|| |- |Mercury(II)||Hg²⁺||mercuric |- |Potassium ||K⁺||kalic |- |Silver||Ag⁺|| |- |Sodium||Na⁺||natric |- !colspan="3" style="background-color: aliceblue"|Polyatomic Cations |- |Ammonium||NH|| |- |Oxonium||H₃O⁺||hydronium |- |Mercury(I)||Hg||mercurous |} |valign="top"| {|class="wikitable" |+Common Anions |- !style="text-align: left"|Formal Name !style="text-align: left"|Formula !style="text-align: left"|Alt. Name |- !colspan="3" style="background-color: aliceblue"|Simple Anions |- |Chloride||Cl⁻|| |- |Fluoride||F⁻|| |- |Oxide||O²⁻|| |- !colspan="3" style="background-color: aliceblue"|Oxoanions |- |Carbonate||CO|| |- |Hydrogen carbonate||HCO||bicarbonate |- |Hydroxide||OH⁻|| |- |Nitrate||NO|| |- |Phosphate||PO|| |- |Sulfate||SO|| |- !colspan="3" style="background-color: aliceblue"|Anions from Organic Acids |- |Acetate||||ethanoate |- |Formate||||methanoate |- |Oxalate||||ethandioate |- |Cyanide|||| |- |} |}

See also

Air ionizer

Anode

Cathode

Ion beam

Ion source

Ionic radius

Auroras

References

Category:Physical chemistry Category:Charge carriers