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An ion is an atom or group of atoms which have lost or gained one or more electrons, making them negatively or positively charged. An ion consisting of a single atom is called a monatomic ion, and an ion consisting a negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion (; an-eye-on) due to its attraction to anodes. A positively-charged ion, which has fewer electrons than protons, is known as a cation (; cat-eye-on) due to its attraction to cathodes. 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<sup>+</sup>, SO<sub>4</sub><sup>2−</sup>. An alternate way of denoting charge is like this:
The word ion is a name given by Michael Faraday , neutral present participle of , "to go", thus "a goer". So; anion, , and cation, κ, mean "(a thing) going up" and "(a thing) going down", respectively; and anode, , and cathode, κ, mean "a going up" and "a going down", respectively, from , "way," or "road."
Polyatomic and molecular ions are often formed by the combination of elemental ions such as H<sup>+</sup> 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 NH<sub>4</sub><sup>+</sup> which can be formed by ammonia NH<sub>3</sub> accepting a proton, H<sup>+</sup>. 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<sup>+</sup>) 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 NH<sub>3</sub>·<sup>+</sup> is not stable because of an incomplete valence shell around nitrogen and is in fact a radical ion.
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<sup>+</sup>. 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<sup>−</sup>. 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 ddd 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.
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.
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.[1]
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.
Furthermore, negative ions are used in ion therapy which utilizes 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.
Ions are found in what has quickly become one of the most prevalent sources for long-lasting, hand-held energy: Lithium-Ion batteries.