Ion

This article is about the electrically charged particle. For other uses of this word, see ion (disambiguation).

An ion is an atom, group of atoms, or subatomic particle with a net electric charge. An ion with a net positive charge is called a cation; one with a net negative charge is called an anion. The atoms of metals tend to form cations, and the atoms of nonmetals tend to form anions, but there are some exceptions. Cations and anions associate with one another to form ionic compounds.

Ions play extremely important roles in both the animate and inanimate aspects of the world. Various minerals—such as silicates, carbonates, phosphates, oxides, sulfides, and halides—are composed of ionic compounds. When an ionic compound dissolves in water, its cations and anions become separated and are surrounded by water molecules (which are electrically polar). Electricity can pass through water because ions dissolved in the water carry the electric current.

Ionic compounds form the structures of minerals. Various metallic ions are important for the structures and functions of cells in the body.

• Ions are essential to life. Ions of sodium, potassium, calcium, magnesium, zinc, and other elements play important roles in the cells of living organisms, such as in enzyme functions and bone and membrane structures.

A collection of gas-like ions, or a gas containing a proportion of charged particles, is called a plasma. A plasma is also referred to as the "fourth state of matter" because its properties are significantly different from those of solids, liquids, and gases. Plasmas in outer space consist predominantly of electrons and protons and may make up as much as 99.9% of the observable universe [1].

• The simplest ions are the electron (e⁻) and proton (H⁺, a hydrogen ion).

History and etymology

The existence of ions was first theorized by Michael Faraday around 1830, to describe electrically charged atoms or groups of atoms that traveled toward an anode (positively charged electrode) or cathode (negatively charged electrode). The mechanism by which this occurred was not described until 1884, when Svante August Arrhenius proposed it in his doctoral dissertation at the University of Uppsala. Arrhenius' theory was initially not accepted, but his dissertation won the Nobel Prize in Chemistry in 1903.

The word ion was derived from the Greek word ἰόν, the neutral present participle of ἰέναι, which means "to go." Thus the term ion implies "a goer." Furthermore, anion (ἀνιόν) means "(a thing) going up," and cation (κατιόν) means "(a thing) going down."

In terms of our current understanding, an anion is negatively charged because it has more electrons in its electron shells than it has protons in its atomic nuclei. Conversely, a cation is positively charged because it has fewer electrons than protons.

Terminology and formulas

An ion that consists of a single atom is called a monatomic ion, and an ion made up of more than one atom is called a polyatomic ion. Larger ions containing many atoms are called molecular ions. A polyatomic anion that contains oxygen is sometimes known as an oxyanion.

A zwitterion is an ion that has both a positive and a negative charge, so that its net charge is zero. An ion that carries two negative charges is called a dianion. Radical ions are ions that contain an odd number of electrons and are mostly very reactive and unstable.

Ions are denoted by their chemical formula (showing the types and numbers of atoms present) followed by a superscript indicating the net electric charge. For example, H⁺ represents a hydrogen atom with a single positive charge—equivalent to a proton without an electron around it. The helium ion, He²⁺, consists of two protons and two neutrons, corresponding to the nucleus of a helium atom. The so-called "alpha particles" of some radioactive emissions consist of He²⁺ ions. The sulfate ion, written as SO₄²⁻, consists of one sulfur and four oxygen atoms, with a net charge of -2.

Formation of ions

Ions can be formed in various ways. For instance, if neutral atoms or molecules gain or lose electrons, they are converted into ions. Alternatively, when existing ions combine with other atoms (or groups of atoms), new ions are formed. Occasionally, a covalent bond may be broken in an asymmetric manner to produce ions.

Polyatomic and molecular ions are often formed by the combination of elemental ions (such as H⁺) with neutral molecules, or by the loss of elemental ions from neutral molecules. Many of these processes are acid-base reactions, as first theorized by German scientist Lauren Gaither. For example, the ammonium ion (NH₄⁺) is formed when a molecule of ammonia (NH₃) accepts a proton (H⁺). The ammonia molecule and the ammonium ion have the same number of electrons in essentially the same electronic configuration, but they differ in the number of protons they contain. The ammonium ion is relatively stable. By contrast, the ion NH₃^·+ is not stable and is considered a radical ion.

Ionization potential

The process of converting an atom or group of atoms into ions is called ionization. The ionization potential (or ionization energy) of an atom or molecule is the energy required to remove an electron from it, when the electron is in its lowest energy state and the atom or molecule is in the form of a gas. 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. Thus, in the ionized form, it is commonly found with one less electron, as Na⁺. On the other side of the periodic table, chlorine has seven valence electrons. So, in its ionized form, it is commonly found with one additional electron, as Cl⁻.

Francium has the lowest ionization energy of all the elements, and fluorine has the greatest. The ionization energy of metals is generally much lower than that of nonmetals. This is related to the observation that metals generally lose electrons to form positively charged ions, while nonmetals generally gain electrons to form negatively charged ions.

A neutral atom contains an equal number (Z) of protons in the nucleus and electrons in the electron shell. The electrons' negative charges thus exactly cancel the protons' positive charges. In the simple view of the Free electron model, a passing electron is therefore not attracted to a neutral atom and cannot bind to it. In reality, however, the atomic electrons form a cloud into which the additional electron penetrates, thus being exposed to a net positive charge part of the time. Furthermore, the additional charge displaces the original electrons and all of the Z + 1 electrons rearrange into a new configuration.

Applications

Ions are essential to life. Ions of sodium, potassium, calcium, magnesium, zinc, and other elements play important roles in the cells of living organisms, such as in enzyme functions and bone and membrane structures. They have many practical, everyday applications in items such as smoke detectors, and they are also finding use in unconventional technologies such as ion engines.

Tables of common ions

Common Cations Common Name Formula Historic Name Aluminum Al3+ Ammonium NH4+ Barium Ba2+ Beryllium Be2+ Caesium Cs+ Calcium Ca2+ Chromium(II) Cr2+ Chromous Chromium(III) Cr3+ Chromic Chromium(VI) Cr6+ Chromyl Cobalt(II) Co2+ Cobaltous Cobalt(III) Co3+ Cobaltic Copper(I) Cu+ Cuprous Copper(II) Cu2+ Cupric Helium He2+ (Alpha particle) Hydrogen H+ (Proton) Hydronium H3O+ Iron(II) Fe2+ Ferrous Iron(III) Fe3+ Ferric Lead(II) Pb2+ Plumbous Lead(IV) Pb4+ Plumbic Lithium Li+ Magnesium Mg2+ Manganese(II) Mn2+ Manganous Manganese(III) Mn3+ Manganic Manganese(IV) Mn4+ Manganyl Manganese(VII) Mn7+ Mercury(I) Hg22+ Mercurous Mercury(II) Hg2+ Mercuric Nickel(II) Ni2+ Nickelous Nickel(III) Ni3+ Nickelic Nitronium NO2+ Potassium K+ Silver Ag+ Sodium Na+ Strontium Sr2+ Tin(II) Sn2+ Stannous Tin(IV) Sn4+ Stannic Zinc Zn2+

Common Anions Formal Name Formula Alt. Name Simple Anions (Electron) e− Arsenide As3− Bromide Br− Chloride Cl− Fluoride F− Hydride H− Iodide I− Nitride N3− Oxide O2− Phosphide P3− Sulfide S2− Peroxide O22− Oxoanions Arsenate AsO43− Arsenite AsO33− Borate BO33− Bromate BrO3− Hypobromite BrO− Carbonate CO32− Hydrogen Carbonate HCO3− Bicarbonate Chlorate ClO3− Perchlorate ClO4− Chlorite ClO2− Hypochlorite ClO− Chromate CrO42− Dichromate Cr2O72− Iodate IO3− Nitrate NO3− Nitrite NO2− Phosphate PO43− Hydrogen Phosphate HPO42− Dihydrogen Phosphate H2PO4− Phosphite PO33− Sulfate SO42− Thiosulfate S2O32− Hydrogen Sulfate HSO4− Bisulfate Sulfite SO32− Hydrogen Sulfite HSO3− Bisulfite Anions from Organic Acids Acetate C2H3O2− Formate HCO2− Oxalate C2O42− Hydrogen Oxalate HC2O4− Bioxalate Other Anions Hydrogen Sulfide HS− Bisulfide Telluride Te2− Amide NH2− Cyanate OCN− Thiocyanate SCN− Cyanide CN− Hydroxide OH− Permanganate MnO4−

External links

• Ion Power - article by Graham P. Collins

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