# How can you find an oxidation number?

Jun 18, 2016

You can work them out based on the following rules for oxidation states.

1. All uncombined elements have an oxidation number of zero:
eg. Na, ${O}_{2}$, ${F}_{2}$

2. Compounds are always neutral overall: so all oxidation numbers in it add to zero:
eg. NaCl is neutral.
Na is Na+ and Cl is Cl-

3. Some elements always have the same oxidation state:

• Group 1 are +1
• Group 2 are +2
• Aluminium is +3
• Fluorine is -1
4. Others can be tricksy based on the compounds they are in:

• Hydrogen always +1 except in metal hydrides (like NaH) where it is -1
• Oxygen always -2, except in peroxides, superoxides, and $O {F}_{2}$