Oxidation Numbers

1) Elements by itself =

0

Ex: 2Na + Cl₂ → 2NaCl

• Na = 0 ← element by itself

• Cl₂ = 0 ← element by itself

2) Monatomic ions =

The charge

Ex. Mg²⁺ + 2Cl⁻ → MgCl₂

• Mg²⁺ → +2 oxidation number

• Cl⁻ → −1 oxidation number

3) O =

-2 (except peroxides = -1)

Ex. 2Mg + O₂ → 2MgO

In MgO:

• Mg = +2

• O = −2 ← Rule 3

Peroxide = O–O → O is −1

• H₂O₂ → peroxide

• Na₂O₂ → peroxide

• BaO₂ → peroxide

Ex. 2H₂O₂ → 2H₂O + O₂

In H₂O₂ (hydrogen peroxide):

• H = +1

• O = −1 ← peroxide exception

4) H =

+1 (except when bonded with metal aka hydrides = -1)

Ex. 2Na + 2HCl → 2NaCl + H₂

In HCl:

• H = +1 ← Rule 4

• Cl = −1

Ex. 2Na + H₂ → 2NaH

Here:

• Na = metal

• H = hydrogen

• NaH = metal hydride

• H has an oxidation number of −1 instead of its usual +1.

5) Halogens =

-1

Ex. 2Na + Cl₂ → 2NaCl

In NaCl:

• Na = +1

• Cl = −1 ← Rule 5

6) The Sum of all oxidation numbers =

0

Ex. H₂O

* H = +1

* O = −2

2(+1) + (−2) = 0 ✔