Deducing oxidation numbers

Many compounds and polyatomic ions contain one or more of the following elements.  For these elements the oxidation number (state) N_ox can be predicted because
 

• fluorine is always -1 (most electronegative element)
• oxygen is nearly always is -2 (next most electronegative element)
   
  except in peroxides (these have an O-O bond like HOOH) where it is -1
  AND when it is bonded to a more electronegative element as in OF₂
   
• chlorine, bromine, iodine are usually -1
   
  unless the halogen is bonded to a more electronegative element (as in HOCl or ClF₃)
  
  
• Group 1 metals are always +1 (for example Na⁺, K⁺)
• Group 2 metals are always +2  (for example Mg²⁺, Ca²⁺ and Ba²⁺)
• hydrogen is nearly always +1
  
  
  except when it is bonded to a less electronegative element (for example NaH)
• sulfur is -2 in binary compounds with metals and hydrogen

For polyatomic ions and compounds the oxidation number (state) of elements other than those in the list can be deduced from
 

• knowledge of the above oxidation numbers (states) and
• the fact that the sum of oxidation numbers of atoms in a polyatomic species equals its overall charge
  
  
  Example polyatomic ion:  For NO₃^– the overall charge is -1
  Example compounds:  For NH₃ the overall charge is 0

 

2 × Nox(H) + Nox(S) + 4 × Nox(O) = 0	Principle: The sum of the oxidation states equals the overall charge (0 if a compound).
2 × (+1) + Nox(S) + 4 × (-2) = 0	Principle: Oxidation state of H nearly always +1.  Oxidation state of O nearly always -2.
Nox(S) = +6	Solve