Oxides

Oxides are binary compounds containing oxygen (typically with oxidation state -2) combined with another element. They are fundamental in inorganic chemistry and have important industrial applications.

Nomenclature using prefixes

Multiplicative prefixes indicate the number of atoms of each element: mono-, di-, tri-, tetra-, penta-, hexa-, etc.

Examples:

• CO₂: carbon dioxide
• Fe₂O₃: diiron trioxide (iron(III) oxide)
• N₂O₅: dinitrogen pentoxide

Important notes:

• The prefix "mono-" is often omitted in modern names, but it can be used to disambiguate compounds (e.g., carbon monoxide vs carbon dioxide).
• Prefix-based names are sometimes traditional; oxidation-state names (using Roman numerals) are preferred for metals with variable oxidation states.

Nomenclature by oxidation state

Oxygen is usually assigned oxidation state -2. The total negative charge from oxygen is compensated by the other element, whose oxidation state is indicated with Roman numerals or numbers in parentheses.

To determine the oxidation state of the other element, multiply -2 by the number of oxygens to get the total negative charge, then divide the positive charge among the atoms of the other element.

For elements that typically have only one oxidation state (alkali metals +1, alkaline earth metals +2, aluminum +3), the oxidation state is often omitted from the name.

Examples:

• Fe₂O₃: iron(III) oxide (total O charge -6, Fe each +3)
• FeO: iron(II) oxide (total O charge -2, Fe +2)
• CuO: copper(II) oxide (Cu +2)
• Ag₂O: silver(I) oxide (Ag +1)
• CaO: calcium oxide (calcium typically shown without oxidation state)

Peroxides

In peroxides the oxygen has oxidation state -1 and contains an -O-O- bond. They are named as "peroxide" of the element, e.g. sodium peroxide.

Peroxides commonly form with group 1 and group 2 metals and are often unstable; they decompose releasing oxygen or oxidize other substances.

General formula for simple metal peroxides: M₂O₂ (for metals with +1 oxidation state).

Examples:

• H₂O₂: hydrogen peroxide (not HO)
• Na₂O₂: sodium peroxide
• BaO₂: barium peroxide
• CaO₂: calcium peroxide