In chemistry, the oxidation number is used to keep track of the positive or negative character of atoms or ions. When electrons are removed or shifted away from an atom during a chemical reaction, the atom is given a more positive oxidation number.
When electrons are gained or shifted toward an atom during a reaction, the atom is given a more negative oxidation number. The actual numerical value of the oxidation number depends on the number of electrons shifted or transferred.
For simple ions the oxidation number is equal to the charge on the ion. In NaF, the sodium ion has an oxidation number of +1 because it lost one electron. The fluoride ion has an oxidation number of -1 because it gained one electron. The elements of Group I of the periodic table have oxidation numbers of +1 since they have one valence electron which they can lose. In compounds involving only two elements (binary compounds) the elements of Group VII of the periodic table usually have oxidation number of -1 since they have seven valence electrons and can gain one more electron to complete a stable octet.
In compounds the Group II elements have oxidation numbers of +2 and the elements of Group III often have oxidation numbers of +3. In binary compounds the elements of Group VI usually have oxidation number of -2 and the elements of Group V can have oxidation number of -3
In covalent substances, the more electronegative element is assigned a negative oxidation number, and the less electronegative element is assigned a positive oxidation number. In HCl, the hydrogen atom has an oxidation number of +1 due to a shift of the valence electron of the hydrogen atom towards the more electronegative chlorine atom. The chlorine atom has an oxidation number of -1.
In H2O, the oxygen atom is more electronegative than the hydrogen atoms. Each hydrogen atom has an oxidation number of +1 and the oxygen is -2. In NH3 the oxidation number of each hydrogen is +1 and the oxidation number is -3.
Many elements have more than one oxidation number depending on their various compounds. Iron has an oxidation number of +2 in FeCl2 and +3 in FeCl3.
Tin has an oxidation number of +2 and +4 in SnCl2 and SnCl4, respectively.
USUAL OXIDATION NUMBERS OF SOME COMMON ELEMENTS
|ELEMENTS||PERIODIC TABLE PLACEMENTS||OXIDATION NUMBER|
|H||+1 (except -1 with metals)|
|Cl||Group VII||-1 (with metals and many nonmetals)|
|Br||Group VII||-1(with metals and many nonmetals)|
|I||Group VII||-1 (with metals and many nonmetals)|
|O||Group VI||-2 (except -1 in peroxides)|
|S||Group VI||-2 (also +4 and +6)|
|Hg||Transition Metal||+1 and +2|
|Cu||Transition Metal||+1 and +2|
|Fe||Transition Metal||+2 and +3|
|Co||Transition Metal||+2 and +3|
|Sn||Group VI||+2 and +4|