Anion: definition in chemistry
Anion: A negatively charged ion, formed when an atom or group of atoms gains one or more electrons.
With more electrons than protons, an anion carries a negative charge, and a simple anion is larger than the neutral atom it came from. Nonmetals tend to form anions because their outer shells are nearly full. Simple anions have names ending in -ide, such as chloride and oxide, while many polyatomic anions containing oxygen end in -ate or -ite. During electrolysis, anions move toward the positive electrode, the anode.
Examples
- Cl⁻ — chloride ion
- O²⁻ — oxide ion
- SO₄²⁻ — sulfate ion, a polyatomic anion
- NO₃⁻ — nitrate ion, found in fertilizers
On the periodic table
Nonmetals in groups 15 to 17 form anions whose charge equals the group number minus 18: −3 for nitrogen, −2 for oxygen and −1 for the halogens.