How can you find an oxidation number?

1 Answer
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 #OF_2#