Oxidation States

Jacaranda: "oxidation state: a number given to an atom to indicate (theoretically) the number of electrons it has lost or gained (that is, its state of oxidation); also called oxidation number"

Many elements already have an oxidation state, the oxidation state of an element (esp. transitional elements) are written with Roman numerals.

Shown below are the rules for assigning oxidation states:

Category Oxidation state (OS) Examples
elements (free) 0 Copper metal: OS(Cu) = 0, chlorine gas: OS(Cl) = 0
simple ions charge on the ion Cu2+ ion: OS(Cu2+) = +II, Cl- ion: OS(Cl-) = -I
polyatomic ions sum of the oxidation states of each element must sum to the charge on the ion sulfate ion (SO42-): OS(S) =+VI, OS(O)=-II, [(+VI)+4(-II)] = 2-
molecular compounds sum of the oxidation states of each element must sum to zero sulfur dioxide (SO2): OS(S) = +IV, OS(O) = -II, [(+IV)+2(-II) = O]

The table above shows all the uncombined elements have a zero oxidation state.

### Oxidation states of hydrogen and oxygen

Oxygen (in most oxide compounds) has an oxidation state of -II Hydrogen (in most hydrogen compounds) has an oxidation state of +I

### In general

Oxidation = increase in oxidation state
Reduction = decrease in oxidation state

Oxidant + electrons \$<=>\$
[[\$ K^+ + e^- <=> K_{(s)} 4]]
Ca2+ + 2e- <=> Ca(s)
Na+ + e- <=> Na(s)
Al3+ =3e- <=> Al(s)

### Quick Rules

The rules are summarised below:

• The oxidation state of an element alone is 0
• The oxidation state of an ion is equal to its charge
• The sum of oxidations states of all atoms in a neutral molecule must add to 0
• Oxygen has an oxidation state of -2 unless in the form of peroxides where it is -1 and in F2O where it equals +2
• Hydrogen has an oxidation state of +1 unless in the form of metal hydride (-1)
page revision: 11, last edited: 27 Jun 2011 11:26