Oxidation numbers in compounds and polyatomic ions

Use the guidelines below to deduce the oxidation number of a particular atom in a polyatomic species.

The sum of oxidation numbers of atoms in a species equals its overall charge (zero if the species is a compound).

The most electronegative (Electronegativity is the tendency of a bonded atom to attract the bonding electrons to itself. In the bond H-X where X has a higher electronegativity than H, there is unequal sharing of the bonding electrons, and the H is slightly positive and the X is slightly negative. The range of electronegativity values for main group elements is given in a table in the Glossary (see Options).) atom in a species has a negative oxidation number. Thus

• oxygen usually has an oxidation number of -2, and fluorine, chlorine, bromine, iodine are usually -1
• metals have positive oxidation numbers in their common compounds
• other nonmetals have negative oxidation numbers in their compounds with metals but may have positive oxidation numbers in their compounds with other nonmetals.
  
  
  For example hydrogen is assigned an oxidation number of +1 in its compounds with other nonmetals because it is less electronegative than any of the other nonmetals.