# Oxidation States (Oxidation Numbers)

### Oxidation States (Oxidation Numbers)

Oxidation state shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state.

### Oxidation State of Some Elements

1. The oxidation state of an element is zero.
Example
 Element Oxidation State Mg 0 H2 0 Br2 0

2. For a simple ion with single atom, the oxidation state is equal to the charge.
Example
 Ion Oxidation State Cu2+ +2 Br- -1 O2- -2 Al3+ +3

3. Some elements almost always have the same oxidation states in their compounds:
Example 1: The oxidation state of oxygen is always -2 except peroxide, which is -1.
 Compound Oxidation state of oxygen H2O -2 H2SO4 -2 ZnO -2 KClO3 -2 H2O2 -1

Example 2: The oxidation state of hydrogen is always +1 except hydride, which is -1.
 Compound Oxidation state of hydrogen NH3 +1 HCl +1 NaOH +1 MgH2 -1 NaH -1

4. The sum of the oxidation states of all the atoms or molecule in a neutral compound is zero.
Example:
 Ion Sum of Oxidation State H2O 0 CO2 0 NH3 0

5. The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.
Example:
 Ion Sum of Oxidation State NO3- -1 CO32- -2 PO43- -3 NH4+ +1