Ion: definition in chemistry
Ion: An atom or group of atoms that has gained or lost electrons and so carries an overall electric charge.
Positive ions, called cations, form when atoms lose electrons, which is typical of metals. Negative ions, called anions, form when atoms gain electrons, which is typical of nonmetals. Oppositely charged ions attract each other to form ionic compounds such as table salt, and dissolved ions allow solutions to conduct electricity.
Examples
- Na⁺ — sodium ion, found in table salt
- Ca²⁺ — calcium ion, found in milk and hard water
- OH⁻ — hydroxide ion, a polyatomic ion found in bases
- NH₄⁺ — ammonium ion, a polyatomic cation
On the periodic table
Metals on the left side of the table tend to lose electrons and form cations, while nonmetals on the right (except the noble gases) tend to gain electrons and form anions.