Oxidation Number

What is an Oxidation Number?Oxidation-reduction reactions (redox

reactions) are reactions in which electrons are lost by an atom or ion in one reactant and gained by an atom or ion in another

reactant. Although electrons are gained and lost in these reactions, the balanced equation

for a redox reaction does not show the electrons that are being transferred. In order to tell whether a redox reaction has occurred

or not, we need a way to keep track of electrons. The best way to do so is by

assigning oxidation numbers to the atoms or ions involved in a chemical reaction

What does it mean?

Oxidation numbers are hypothetical numbers assigned to an individual atom or ion present in a substance using a set of rules. Oxidation numbers (or oxidation states) can be positive numbers, negative numbers, or zero. It is VERY IMPORTANT to remember that oxidation numbers are always given for one individual atom or ion and not for groups of atoms or ions (molecules).

Oxidation can be defined as "an increase in oxidation number".

In other words, if a species starts out at one oxidation state and ends up at a

higher oxidation state it has undergone oxidation.

Reduction can be defined as "a decrease in oxidation number".

Any species whose oxidation number is lowered during the

course of a reaction has undergone reduction.

Oxidation is ALWAYS accompanied by reduction. Reactions in which oxidation and reduction are occurring are usually

called Redox reactions.

Rules for assigning Oxidation Numbers

There are several rules for assigning the oxidation number to an element. Learning

these rules will simplify the task of determining the oxidation state of an element, and thus,

whether it has undergone oxidation or reduction.

The oxidation number for an atom in its elemental form is always zero.

A substance is elemental if both of the following are true:only one kind of atom is present

charge = 0Examples:

S8: The oxidation number of S = 0Fe: The oxidation number of Fe = 0

The oxidation number of a monoatomic ion = charge of the monatomic ion.

Examples:Oxidation number of S2- is -2.

Oxidation number of Al3+ is +3.The sum of the oxidation numbers of all atoms in a polyatomic ion =

charge on the polyatomic ion.The oxidation number of all Group 1A metals = +1 (unless elemental).The oxidation number of all Group 2A metals = +2 (unless elemental).

Hydrogen (H) has two possible oxidation numbers:+1 when bonded to a nonmetal

-1 when bonded to a metalOxygen (O) has two possible oxidation numbers:

-1 in peroxides (O22-)....pretty uncommon-2 in all other compounds...most common

The oxidation number of fluorine (F) is always -1.The sum of the oxidation numbers of all atoms (or ions) in a neutral

compound = 0.

Example:

Na  +  Cl2 ----->  2NaCl

The Na starts out with an oxidation number of zero (0) and ends up having an oxidation number of 1+. It has

been oxidized from a sodium atom to a positive sodium ion.

The Cl2 also starts out with an oxidation number of zero (0), but it ends up with an oxidation number of 1-. It, therefore, has been reduced from chlorine atoms to

negative chloride ions. Over here note that the sum of the oxidation states of the Na and Cl ions is equal to 0.

This happens in all compounds.

The substance bringing about the oxidation of the sodium atoms is the chlorine, thus the

chlorine is called an oxidizing agent. In other words, the oxidizing agent is being reduced

(undergoing reduction).

The substance bringing about the reduction of the chlorine is the sodium, thus the

sodium is called a reducing agent. Or in other words, the reducing agent is being

oxidized (undergoing oxidation).

Thank You