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An ion is an atom or group of atoms in which the number of electron s is different from the number of proton s. If the number of electrons is less than the number of protons, the particle is a positive ion, also called a cation. If the number of electrons is greater than the number of protons, the particle is a negative ion, also called an anion.

When an atom of an element is short an electron, a plus sign is placed after its chemical symbol as a superscript to indicate that fact. For example, a carbon atom with 5 electrons (the nucleus has 6 protons) is symbolized C + . If the element is short 2 or more electrons, a numeral is also placed in the superscript, directly before the plus sign, to indicate the extent of the electron deficiency. A carbon atom with 4 electrons is therefore symbolized C 2+ , and a carbon atom with 3 electrons is symbolized C 3+ .

If an atom of an element has an excess of an electron, a minus sign is placed after its chemical symbol as a superscript. If there are 2 or more extra electrons, a numeral is included to indicate the extent of the electron surplus. An oxygen atom with 9 electrons (the nucleus has 8 protons) is symbolized O - . An oxygen atom with 10 electrons is symbolized O 2- , and an oxygen atom with 11 electrons is symbolized O 3- .

A compound , as well as individual atom, can be ionized. A common example is nitrate, which consists of a nitrogen atom and 3 oxygen atoms (NO 3 ) in the form of an anion; this is symbolized NO 3 - because it normally has a surplus of a single electron. Another example is sulfate, which consists of a sulfur atom and 4 oxygen atoms (SO 4 ), which occurs with an excess of 2 electrons and is symbolized SO 4 2- .

Ionized substances often behave differently than when they are not ionized. A common phenomenon is for an electrical insulator (non-conductor) to become electrically conductive when it is ionized. In the Earth's upper atmosphere, ultraviolet radiation from the Sun causes ionization of certain gases. As a result, electromagnetic waves can refracted, and their polarization shifted, at certain frequencies when the waves pass through the gases. This makes long-distance radio communication possible, without the aid of satellites, at some frequencies. The ionization occurs in layers which, taken together, form the Earth's ionosphere .

This was last updated in September 2005
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