Reduction-Oxidation Notes
(Redox)
Oxidation – a loss of electron(s). Oxidation number increases from left to right in a reaction.
Oxidizing Agent (Oxidizer) – the species that is reduced.
Reduction – a gain of electron(s). Oxidation number decreases from left to right in a reaction.
Reducing Agent – the species that is oxidized.
Cation – net positively charged ion, atom or group of atoms
Anion – net negatively charged ion, atom or group of atoms
Oxidation number – of simple ions is equal to the charge on the ion.
of an atom is zero in a neutral substance that contains atoms of only one
element. Thus, the atoms in O2, O3, P4, S8, and aluminum metal all have an
oxidation number of 0.
Some Simple Rules:
1. Atoms in elemental form, solid metals and diatomic gases have an oxidation number of
zero. For example, Mg, H2, Ar and Fe(s) are all examples of atoms in their elemental
states. In the case of Mg, if no charge and no state are shown, we have to assume it is
metallic Mg.
2. Group 1A and 2A elements have the same oxidation number that the ion of the metal
would, unless it's in its elemental form. Alkali metals (column #1) always have an
oxidation number of +1.
3. Halogens, following a metal, have an ox. # of -1.
Reduction-Oxidation Notes
(Redox)
4. Hydrogen almost always has an oxidation number of +1. A rarely-encountered exception
is when H is bound to a metal in a hydride compound.
a. Hydrogen, acting like a metal, will have a charge of +1 and oxygen will have a
charge of -2.
b. Metal hydrides, made with a metal first and a hydrogen last, for example, NaH,
the H will have a charge of -1.
5. Oxygen almost always has an oxidation number of -2. In rare exceptions, when oxygen is
in a peroxide (O2-, like H2O2), its oxidation number is -1.
a. Peroxides made with a column 1 metal and oxygen for ex: K2O2, or H2O2, the
oxygen will have a -1 charge.
b. There are peroxides with column 2 metals, for example MgO2, the Oxygen will
still have a charge of -1.
6. Fluorine always has a -1 oxidation number, and Cl, Br and I almost always do.
7. This may be the most important rule: The oxidation numbers of a molecule have to add
up to the total charge on the molecule. If the molecule is neutral, that's zero. For example,
the sums of the oxidation numbers of CO2 and CO32- are0 and -2, respectively.
Reduction-Oxidation Notes
(Redox)
1. Assign oxidation numbers to each atom in the molecule.
2. For each reaction below, (1) balance the reaction if necessary, (2) assign oxidation numbers to
each atom, (3) identify which atoms are oxidized and reduced, if any, and (4) identify the
oxidizing and reducing agents, if any.