Oxidation Number Assignments
The determination of the oxidation number (or oxidation state) of chemical
compounds can be made by following a few simple rules.
- The oxidation numbers of an atom or the atoms in a neutral molecule must add up
- If an atom or molecule is ionic its oxidation number must add up to its overall
- Alkali metal atoms (Group I) have an oxidation number equal to +1 within
compounds. Alkali earth atoms (Group II) have an oxidation number of +2 within compounds.
- Fluorine always has a -1 oxidation number within compounds.
- All halogens (besides fluorine) have a -1 oxidation number in compounds, except
when with oxygen or other halogens where their oxidation numbers can be positive.
- Hydrogen is always assigned a +1 oxidation number in compounds, except in metal
hydrides (e.g. LiH) where the previous rules apply. In the case of LiH lithium is assigned
a +2 charge (rule C) leaving hydrogen to neutralize the compound with a -2 charge.
- Oxygen is assigned an oxidation number of -2 in compounds, with two exceptions...
||Fluorine's oxidation number always takes precedence.
||Oxygen oxygen bonds follow previous rules, meaning other
assignments take place first leaving oxygen to neutralize the charge.
||The oxygen has an oxidation number of -1 in peroxide compounds
Examples: (The blue
colours are a positive oxidation state, the red
colours are a negative oxidation state, relevant to the atoms underneath