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Atoms, or groups of atoms, that are either positively charged (cations) or negatively charged (anions), as a result of the losses or gains of electrons during chemical reactions or exposure to certain forms of radiation.

In solutions or in the molten states, ionic compounds; such as, salts, acids, alkalis, and metal oxides conduct electricity known as electrolytes.

In pairs or other multiples they make up the substance of many crystalline materials, including table salt.

When such an ionic substance is dissolved in water, the ions are freed—to a considerable extent—from the restraints that hold them within the rigid array of the crystal, and they move around in the solution with relative freedom.

Certain insoluble materials bearing positive or negative charges on their surfaces react with ionic solutions to remove various ions selectively, replacing them with ions of other kinds. Such processes are called ion-exchange reactions.

They are used in a variety of ways to remove ions from a solution and to separate ions of various kinds from one another. Such separations are widely utilized in the scientific laboratory to effect purifications and to aid in the analysis of unknown mixtures.

Ion-exchange materials such as zeolites (group of crystalline, hydrated alkali-aluminum silicates) are also employed commercially to purify water, among other uses, and medically to serve as artificial kidneys and for other purposes.

This entry is located in the following unit: ion, ion- + (page 9)
Word Entries containing the term: “ions
positive and negative ions; cation, cations; anion, anions
Any atom or group of atoms that bears one or more positive or negative electrical charges.

Positively charged ions are called cations; negatively charged ions are labeled, anions.

Ions are formed by the addition of electrons to, or the removal of electrons from, neutral atoms or molecules or other ions; by combination of ions with other particles; or by rupture of a covalent bond between two atoms in such a way that both of the electrons of the bond are left in association with one of the formerly bonded atoms.

Examples of these processes include the reaction of a sodium atom with a chlorine atom to form a sodium cation and a chloride anion; the addition of a hydrogen cation to an ammonia molecule to form an ammonium cation; and the dissociation of a water molecule to form a hydrogen cation and a hydroxide anion.

Many crystalline substances are composed of ions held in regular geometric patterns by the attraction of the oppositely charged particles for each other.

Ions migrate under the influence of an electrical field and are the conductors of electric current in electrolytic cells.

—Compiled from "ions, positive and negative", Encyclopædia Britannica; 2010;
Encyclopædia Britannica Online; May 22, 2010.
This entry is located in the following unit: ion, ion- + (page 10)