Oxidation Number

The oxidation number of an atom is the charge that atom would carry if the compound it is a part of were composed of ions.

Rules for Assigning Oxidation Numbers

Rule

Example

Compound

Oxidation numbers of atoms

If compound is neutral and contains only one element, oxidation number of atoms must be zero
S₈
H₂ S: 0
H: 0

Oxidation number of simple ions is equal to the charge on the ion
Na⁺
I^- Na: +1
I: -1

When H is bound to a non-metal, its oxidation number is +1
CH₄H₂O H: +1, C: -4
H: +1, O: -2

When H is bound to a metal, its oxidation number is -1
NaH
LiAlH₄ H: -1, Na: +1
H: -1, Li: +1, Al: +3

Elements toward the bottom left corner of the periodic table are more likely to form positive oxidation states than elements toward the upper right corner SO₂ XeF₄
NO₃- S: +4, O: -2
Xe: +4, F: -1
N: +5, O: -2

Rule	Example
Rule	Compound	Oxidation numbers of atoms
If compound is neutral and contains only one element, oxidation number of atoms must be zero	S₈ H₂	S: 0 H: 0
Oxidation number of simple ions is equal to the charge on the ion	Na⁺ I^-	Na: +1 I: -1
When H is bound to a non-metal, its oxidation number is +1	CH₄H₂O	H: +1, C: -4 H: +1, O: -2
When H is bound to a metal, its oxidation number is -1	NaH LiAlH₄	H: -1, Na: +1 H: -1, Li: +1, Al: +3
Elements toward the bottom left corner of the periodic table are more likely to form positive oxidation states than elements toward the upper right corner	SO₂ XeF₄ NO₃-	S: +4, O: -2 Xe: +4, F: -1 N: +5, O: -2