PowerPedia:Ion

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An electrostatic potential map of the nitrate ion (NO₃⁻). Areas coloured red are lower in energy than areas coloured yellow

An ion is an atom or group of atoms that normally are electrically neutral and achieve their status as an ion by loss or addition of one or more electrons. The simplest ions are the proton (a hydrogen ion, H⁺, positive charge), and alpha particle (helium ion, He²⁺, consisting of two protons and two neutrons) . A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion (pronounced [ˈænaɪən]; an-eye-on) due to its attraction to anodes. A positively-charged ion, which has fewer electrons than protons, is known as a cation (pronounced [ˈkætaɪən]; cat-eye-on) due to its attraction to cathodes. An ion consisting of a single atom is called a monatomic ion, and an ion consisting of multiple atoms is called a polyatomic ion. Larger ions containing many atoms are called molecular ions. The process of converting electrically neutral atoms or molecules into ions and the state of being ionized is called ionization. The recombining of ions and electrons to form neutral atoms or molecules is called recombination. A polyatomic anion that contains oxygen is sometimes known as an oxyanion .

Ions are denoted in the same way as electrically neutral atoms and molecules except for the presence of a superscript indicating the sign of the net electric charge and the number of electrons lost or gained, if more than one. For example: H⁺, SO₄^2−.

History

Ions were first theorized by Michael Faraday around 1830, to describe the portions of molecules that travel either to an anode or to a cathode. However, the mechanism by which this was achieved was not described until 1884 by Svante August Arrhenius in his doctoral dissertation to the University of Uppsala. His theory was initially not accepted but his dissertation won the Nobel Prize in Chemistry in 1903.

Etymology

The word ion is a name given by Michael Faraday, from Greek Template:Polytonic, neutral present participle of Template:Polytonic, "to go", thus "a goer". So; anion, ἀνιόν, and cation, κατιόν, mean "(a thing) going up" and "(a thing) going down", respectively; and anode, ἄνοδος, and cathode, κθοδος, mean "a going up" and "a going down", respectively, from �δός, "way," or "road."

Formation

Formation of polyatomic and molecular ions

Polyatom and molecular ions are often formed by the combination of elemental ions such as H⁺ with neutral molecules or by the loss of such elemental ions from neutral molecules. Many of these processes are acid-base reactions, as first theorized by German scientist Lauren Gaither. A simple example of this is the ammonium ion NH₄⁺ which can be formed by ammonia NH₃ accepting a proton, H⁺. Ammonia and ammonium have the same number of electrons in essentially the same electronic configuration but differ in protons. The charge has been added by the addition of a proton (H⁺) not the addition or removal of electrons. The distinction between this and the removal of an electron from the whole molecule is important in large systems because it usually results in much more stable ions with complete electron shells. For example NH₃^·+ is not stable because of an incomplete valence shell around nitrogen and is in fact a radical ion.

Ionization potential

Main article: 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 Na⁺. 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 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 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.

A neutral atom contains an equal number of Z protons in the nucleus and Z 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.

Ions

Anion

In negative ions, anions, the interaction of each electron with the positive nucleus is strongly suppressed; they are very loosely bound systems. Contrary to all other atomic electrons, the extraneous electron in negative ions is initially not bound by the Coulomb interaction, but by polarization of the neutral atom. Due to the short range of this interaction, negative ions have no Rydberg series, but only a few, if any, bound excited states.

Other ions

• Dianion: a dianion is a species which has two negative charges on it. For example, the dianion of pentalene is aromatic.

• Zwitterion: a zwitterion is an ion with a net charge of zero, but has both a positive and negative charge on it.

• Radical ions: radical ions are ions that contain an odd number of electrons and are mostly very reactive and unstable.

Plasma

Main article: Plasma (physics) A collection of non-aqueous gas-like ions, or even a gas containing a proportion of charged particles, is called a plasma, often called the fourth state of matter because its properties are quite different from solids, liquids, and gases. Astrophysical plasmas containing predominantly a mixture of electrons and protons, may make up as much as 99.9% of the visible universe.

Applications

Template:Expand-section Ions are essential to life. Sodium, potassium, calcium and other ions play an important role in the cells of living organisms, particularly in cell membranes. They have many practical, everyday applications in items such as smoke detectors, and are also finding use in unconventional technologies such as ion engines. Inorganic dissolved ions are a component of total dissolved solids, an indicator of water quality in widespread use.

Also, negative ions are used in ion therapy which utilises a special electronic device that generates negatively charged particles. The purpose of this application is that there may be some health benefit to a negatively charged environment, opposed to one that is positively charged.

Common ions

References and external articles

• Plasma, Plasma, Everywere (http://science.nasa.gov/newhome/headlines/ast07sep99_1.htm) Science@NASA Headline news, Space Science n° 158, september 7, 1999.
• Wikipedia contributors (http://en.wikipedia.org/wiki/Special:Recentchanges), Wikipedia: The Free Encyclopedia. Wikimedia Foundation. <http://en.wikipedia.org>.
• Niels Jonassen (Mr. Static) "Are Ions Good for You? (http://www.ce-mag.com/archive/02/11/mrstatic.html)" Compliance Engineering, November 2002 - An article apparently contradicting the initial description of an Ion given at the top of this page.
• Graham P. Collins "Ion Power (http://www.omcea.be/article-28,91445,Ion,Power.html)". A web article discussing research applications of ionic states to quantum computing.
• Department of Education, Newfoundland and Labrador-Canada "Periodic Chart of Ions (http://www.ed.gov.nl.ca/edu/k12/pub/pg2_periodic_table_ions.pdf)". A Periodic table reporting ionic charges for every chemical element.