Oxide ion
Charge: 2- · Type: Anion · Molar mass: 15.999 g/mol
Made of: oxygen
How it forms
Oxygen needs two more electrons to complete its outer shell, which it takes from electropositive metals. O2- exists as a free ion only in solid ionic compounds; in water it reacts immediately to form hydroxide ions.
Naming note
O2- is the oxide ion. In compounds it takes the -ide ending: sodium oxide, calcium oxide, iron(III) oxide.
Examples in compounds
Calcium oxide
Magnesium oxide
Iron(III) oxide