Oxidation Numbers, or Oxidation States, Tutorial
- An oxidation number is also known as an oxidation state.
- An oxidation number (or oxidation state) is the charge an atom would carry if the molecule or ion were completely ionic.1
When calculating the oxidation state of an element in a compound, treat all the elements present as if they are present as ions, EVEN if they are clearly part of a covalent molecule.
- Some rules for assigning oxidation numbers (oxidation states) to atoms are:
Rule 1: The oxidation number (or oxidation state) of the atoms making up an element in the free state, including its standard state2, is 0. Rule 2: The oxidation number (or oxidation state) for the ion of an element is equal to the charge on the ion. Rule 3: When found in compounds, the oxidation numbers (oxidation states) of some elements are always the same (with a few exceptions).
These elements and their oxidation numbers are listed below:
Exceptions to this Rule Group 1 element +1 Group 2 element +2 hydrogen +1 hydrides (eg NaH),
oxidation state of hydrogen is -1
fluorine -1 oxygen -2 (a) peroxides (eg H2O2):
oxidation state of oxygen is -1
(b) superoxides (eg KO2)
oxidation state of oxygen is -½
(c) oxygen fluorides:
⚛ in OF2 the oxidation state of oxygen is +2
⚛ in O2F2 the oxidation state of oxygen is +1
Rule 4: The oxidation numbers (oxidation states) of other elements in compounds can be calculated using the overall charge on the compound:
(a) For an electrically neutral3 molecule or compound, the sum of the oxidation numbers (or oxidation states) for each atom of each element in the molecule equals 0.
(b) For a charged molecule (a polyatomic ion), the sum of the oxidation numbers (states) for each atom of each element in the ion equals the charge on the ion.
- An oxidation number, or oxidation state, is assigned to help us determine whether or not an element in a reaction has been oxidised or reduced.