Ion

2008/9 Schools Wikipedia Selection. Related subjects: Chemistry

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

An ion is an atom or molecule which has lost or gained one or more valence electrons, making it positively or negatively charged.

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). Conversely, a positively-charged ion, which has fewer electrons than protons, is known as a cation (pronounced /ˈkætaɪən/; cat-eye-on).

An ion consisting of a single atom is called a monatomic ion, but if it consists of two or more atoms, it is a polyatomic ion. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanions.

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⁺ and SO₄²⁻.

Etymology

The name ion was given by Michael Faraday. It is derived from the Greek word ἰόν, participle of ἰέναι, "to go", or έἰμι , "I go"; thus "a goer". Anion, ἀνιόν, and cation, κατιόν, mean "(a thing) going up" and "(a thing) going down", respectively; and anode, ἄνοδος, and cathnic|άθοδος}}, mean "a way up" and "a way down", respectively, from ὁδός, "way," or "road".

Formation

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 gain of such elemental ions from neutral molecules. 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

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

• Anions (see pronunciation above) are negatively charged ions, formed when an atom gains electrons in a reaction. Anions are negatively charged because there are more electrons associated with them than there are protons in their nuclei.

• Cations (see pronunciation above) are positively charged ions, formed when an atom loses electrons in a reaction. Cations are the opposite of anions, since cations have fewer electrons than protons.

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

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

Plasma

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 visible matter in the universe.

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. Inorganic dissolved ions are a component of total dissolved solids, an indicator of water quality in the world.

Negative 'Ions' and Air Ionisers

Many manufacturers sell devices that release 'negative ions' into the air, claiming that a higher concentration of negative ions will make a room feel less 'stuffy'. Some also claim health benefits such as relieving asthma and depression.

The 'ions' referred to are in fact charged oxygen or nitrogen molecules surrounded by a cluster of water molecules, rather than ions. Scientific studies have shown no particular benefit from a greater concentration of negative ions.

Negative air ionization can reduce the concentration of bioaerosols and dust particles in the air by causing them to bond, forming larger particles and thus falling out of the air onto horizontal surfaces. This may help reduce infection due to airborne contamination. Ionization was shown to reduce transmission of the Newcastle Disease Virus in an experiment with chickens.

Common ions