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 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. Francium has the lowest ionization energy of all elements, and fluorine has the greatest.

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 orbital blocks that are filled with electrons. For example, sodium (Na) has a single electron ("valence electron") in its outermost shell. In its common ionized form, sodium loses this electron to form Na⁺, leaving the next (lower) block of orbitals filled with electrons. On the other side of the periodic table, chlorine (Cl) has seven valence electrons. Its common ionized form is Cl⁻, which has one additional electron that fills up an orbital block.

Usefulness of ions

Ions are essential for all living organisms. For instance, in our own bodies, calcium and phosphate ions are involved in the formation of bones and teeth, the contraction of muscles, and the transmission of nerve impulses. Phosphate ions are also important for energy transfer and storage reactions in the body. Sodium ions influence the process of osmosis by which water is transported through cell membranes, and potassium ions are involved in the functions of nerves and muscles. An ion of iron occupies a central position at the center of the heme group that is part of hemoglobin in our blood. Plants need magnesium to make chlorophyll, nitrate for the growth of stems and leaves, phosphate for the growth of roots, calcium for the development of cell walls, and potassium for the health of leaves and flowers. [2]

Many ions are put to use in everyday applications, 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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