- This article is about the electrically charged particle. For other uses of this word, see ion (disambiguation).
An ion is an atom or group of atoms with a net electric charge. A negatively-charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion, for it is attracted to anodes; a positively-charged ion, which has fewer electrons than protons, is known as a cation (pronounced cat-eye-on), for it is attracted to cathodes. An ion with a single atom is a monatomic ion, and an ion with more than one is a polyatomic ion. Larger ions containing many atoms are referred to as molecular ions. The process of converting into ions and the state of being ionized is called ionization. The recombining of ions and electrons to form neutral atoms is called recombination. A polyatomic anion that contains oxygen is sometimes known as an oxyanion.
Atomic and polyatomic ions are denoted by a superscript with the sign of the net electric charge and the number of electrons lost or gained, if more than one. For example: H+, SO32−.
A collection of non-aqueous gaseous ions, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its properties are quite different from solids, liquids, and gases.
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.
Formation of polyatomic and molecular 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 such elemental ions from neutral molecules. Many of these processes are acid-bases reactions. 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.
Other ions
A dianion is a species which has two negative charges on it. For example, the dianion of pentalene is aromatic. A zwitterion is an ion with a net charge of zero, but has both a positive and negative charge on it. Radical ions are ions that contain an odd number of electrons and are mostly very reactive and unstable.
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 ἰόν, neutral present participle of ἰέναι, "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, κάθοδος, mean "a going up" and "a going down", respectively, from ὁδός, "way," or "road."
Applications
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 and ion cannons.
Common Cations: (ions grouped by charge) Name Formula (Other name(s))
- Aluminum Al+3
- Ammonium NH4+
- Barium Ba+2
- Calcium Ca+2
- Chromium(II) Cr+2 (Chromous)
- Chromium(III) Cr+3 (Chromic)
- Copper(I) Cu+ (Cuprous)
- Copper(II) Cu+2 (Cupric)
- Iron(II) Fe+2 (Ferrous)
- Iron(III) Fe+3 (Ferric)
- Hydrogen H+
- Hydronium H3O+
- Lead(II) Pb+2
- Lithium Li+
- Magnesium Mg+2
- Manganese(II) Mn+2 (Manganous)
- Manganese(III) Mn+3 (Manganic)
- Mercury(I) Hg2+2 (Mercurous)
- Mercury(II) Hg+2 (Mercuric)
- Nitronium NO2+
- Potassium K+
- Silver Ag+
- Sodium Na+
- Strontium Sr+2
- Tin(II) Sn+2 (Stannous)
- Tin(IV) Sn+4 (Stannic)
- Zinc Zn+2
Common Anions: (ions grouped by charge) Simple ions:
- Hydride H-
- Oxide O2-
- Fluoride F-
- Sulfide S2-
- Chloride Cl-
- Nitride N3-
- Bromide Br-
- Iodide I-
Oxoanions:
- Arsenate AsO43-
- Phosphate PO43-
- Arsenite AsO33-
- Hydrogen phosphate HPO42-
- Dihydrogen phosphate H2PO4-
- Sulfate SO42-
- Nitrate NO3-
- Hydrogen sulfate HSO4-
- Nitrite NO2-
- Thiosulfate S2O32-
- Sulfite SO32-
- Perchlorate ClO4-
- Iodate IO3-
- Chlorate ClO3-
- Bromate BrO3-
- Chlorite ClO2-
- Hypochlorite OCl-
- Hypobromite OBr-
- Carbonate CO32-
- Chromate CrO42-
- Hydrogen carbonate or Bicarbonate HCO3-
- Dichromate Cr2O72-
Anions from Organic Acids:
- Acetate CH3COO-
- Formate HCOO-
Others:
