Describe how to assign a Oxidation number from the text:

1. The oxidation number of an atom in an elemental substance is zero.
2. The oxidation number of a monatomic ion is equal to the ion's charge.
3. Oxidation numbers for common nonmetals are usually assigned as follows:

• Hydrogen: +1 when combined with nonmetals, -1 when combined with metals
• Oxygen: -2 in most compounds, sometimes -1 (so-called peroxides,      O22-),O22-), very rarely   -12-12 (so-called superoxides,      O2-),O2-), positive values when combined with F (values vary)
• Halogens: -1 for F always, -1 for other halogens except when combined with oxygen or other halogens (positive oxidation numbers in these cases, varying values)

1. The sum of oxidation numbers for all atoms in a molecule or polyatomic ion equals the charge on the molecule or ion.