Oxidation Numbers Explanation ( Complete Explanation )

Oxidation numbers, sometimes called oxidation states, are signed numbers that are assigned to atoms within molecules. They allow us to keep track of the electrons that are associated with each atom . Oxidation numbers are frequently used to write chemical formulas, to predict properties of compounds , and to help balance equations where electrons are transferred ( Oxidation-reduction reactions ).

Knowing the oxidation state of an atom gives us an idea about atom's positive or negative character. In themselves, oxidation numbers have no physical meaning;they are employed to simplify tasks that are more difficult to accomplish without them.

Oxidation numbers are assigned by following a set of general rules :

Rule 1 . All uncombined elements ( as they exist naturally ) are assigned the oxidation number of zero.

Rule 2. All monoatomic ions are assigned oxidation numbers equal to their charges.

Rule 3. Certain elements usually posses a fixed oxidation number in compounds. Those that are most important include :

1. Oxygen's oxidation number is usually -2
2. Hydrogen's oxidation number is usually +1
3. Halogens normally have a -1 oxidation number in binary compounds.
4. Alkali metals and alkaline earth metals are assigned +1 and +2, respectively , as their oxidation numbers.

Rule 4. The sum of all oxidation numbers in a compound equals zero, and the sum of oxidation numbers in a polyatomic ion equals its charge.

Lets now look at each rule individually, starting with rule 1. All uncombined elements are assigned the oxidation number of zero , reagardless of how they exist in nature-by themselves , diatomically , or in larger aggregates ( P4 and S8 ). It is common practice to write oxidation numbers above the symbols of the elements.



Rule 2 states that all monoatomic ions are assigned an oxidation number equal to their charge :



In rule 3, note that the oxidation numbers correspond to the number of electrons that an atom loses or gains within a binary ionic compound . Halogens gain one electron , chalcogens gain two electron. Alkali metals lose one electron , and alkaline earth metals lose two electrons.

There are some exceptions to rule 3. For instance, in peroxides , the oxidation number of oxygen is -1, and not -2. Peroxides are compound that contain an O-O single bond. two examples of peroxides are hydrogen peroixde, H2O2, and sodium peroxide. Na2O2 . Metallic hydrides, compounds containing hydrogen bonded to a metal with a smaller electronegativity , are another exception to rule 3. Hydrogen's oxidation number is -1 in these compounds. Examples of metallic hydrides include sodium hydride, NaH, and calcium hydride, CaH2.

By applying rule 4, Oxidation numbers of other elements are obtained . For example , what is the oxidation number of N in NO ? Oxygen's oxidation number is -2, and the sum of NO's oxidation numbers equals zero. Thus N's oxidation number is +2 :



What is the oxidation number of S in SO2 ? again following rule 4, we assign -4 to the two O atoms;hence the oxidation number of S must be +4 in order for the sum to equal zero.



In a polyatomic ion the reasoning is the same except that the sum of the oxidation numbers is equal to the ion's charge. What is the oxidation number of P in PO43-? Four oxygen would have a total of -8 so P's oxidation state is +5 to add up to -3.



Example :

Determination the oxidation state for each element in the following (a) CuF2 (b) HNO3 (c) SO42- (d) C12H22O11.

Answer :

a. CuF2, each flourine atom has an oxidation number equal to -1; two flourines have a total of -2; it follows that Cu's oxidation number is +2



b. HNO3 , H's oxidation number is +1 , and three O's have a total of -6; therefore , N's oxidation number is +5 to yield a total of zero.



c. SO42- , Each oxygen's oxidation number is -2, so four O's have a total of 8. To add up to -2, the charge on SO42-, the oxidation number of S is +6.


d. C12H22O11. Twenty-two H's give +22, and 11 O's give -22, so the 12 C's have a combined oxidation number of zero-each C has an oxidation state of zero.



With the exception of the metals in groups IA, IIA, and IIIB, metals generally exist in more than one oxidation state. Chromium , Cr , for example is found in the +6, +3, and +2 oxidation states. Gold is found in both +3 and +1 oxidation state. Nonmetals, except F, also exhibit a range of oxidation states. Sulfur's oxidation states include +6, +4, +2, and -2.

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