in Chemistry

Oxidation Numbers

Oxidation numbers are used by chemists to track how many electrons an atom has.

They don’t always correspond to the actual numbers of electrons an atom has.


  1. Atoms in their elemental state have an oxidation number of 0
  2. Atoms in monatomic (consisting of one atom) ions have an oxidation number equal to their charge
  3. In compounds, Fluorine is assigned a -1 oxidation number;
    • Oxygen is usually assigned a -2 oxidation number (except in peroxide, where it is -1)
    • Hydrogen is usually assigned a +1 oxidation number, except when existing as a hydride ion H
  4. In compounds, all other atoms are assigned an oxidation number so that the sum of the oxidation numbers on all atoms in the species equals the charge of the species

Side Note: A peroxide is a compound that has an oxygen-oxygen single bond. They are uncommon due to O-O single bonds being weak.


Question: What is the oxidation state of the Hydrogen atom in 1) H2, and 2) H20?


1) 0 – Because it is in its elemental state (rule #1).

2) +1 – The total is 0 yes, but the question asked for the oxidation state of the Hydrogen (+1), as per rules 3, and 4.


Oxidation is loss of electrons.

Reduction is gain of electrons.


2H2 (g) + O2 (g) → 2H20 (l)

Oxidation numbers:

2H2 : 0
O2 : 0

2H2 : +1 and 0: -2

The Hydrogen loses electrons, and therefore the oxidation number increases.

Oxygen gained 2 electrons which is evident from the -2

Negative oxidation number = gain in electrons.

Chemical reactions that involve electron transfer are called oxidation-reduction, or redox reactions.

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