Oxidation number of an atom is an important section in chemistry because according to the oxidation number, properties of compounds are changed widely. Some elements in the periodic table have only one oxidation number or two or more oxidation numbers. Some elements such as Manganese have several oxidation numbers. Oxidation number of an atom in a compound can be positive or negative or may be zero.
In this chapter, we discuss very important facts about oxidation numbers with examples with different compounds.
Some elements can show more than one oxidation states. Specially d block elements show this behavior in chemistry.
When an element has not combined, it's oxidation number is 0 because no electron transfer is occurred.
Example: Gold (Ag) is found as a pure metal in the nature because Gold is very low reactive metal. In such cases, oxidation number of gold is 0.
When an element has combined with same kind element, it's oxidation number becomes 0.
Ex: oxidation number of Bromine in Br2 molecule is 0.
Here we are going to summarize oxidation numbers of all elements in the periodic table into one table.
NOTE: * is for rare oxidation number
|Atomic Number||Element||Oxidation numbers|
|1||Hydrogen||-1 , 0 , +1|
|6||Carbon||-4 , -3 , -2 , -1 , 0 , +1 , +2 , +3 , +4|
|7||Nitrogen||-5 , -4 , -3 , -2 , -1 , 0 , +1 , +2 , +3|
|8||Oxygen||-2 , -1 , 0 , +1 , +2|
|9||Fluorine||-1 , 0|
|14||Silicon||0 , +4|
|15||Phosphorous||0 , +1 , +3 , +5|
|16||Sulfur||-2 , 0 , +2 , +4 , +6|
|17||Chlorine||-1 , 0 , +1 , +3 , +5 , +7|
Oxidation numbers of group 1 group 2 elements are studied here.
Hydrogen forms three oxidation states, -1 , 0 , +1. Oxidation number 0 occurs only in hydrogen molecule.
When hydrogen forms compounds with metals, hydrogen's oxidation number is -1. Sodium hydride (NaH) and Calcium hydride (CaH2) are some examples.
When carbon form compounds with non-metal elements, oxidation number of hydrogen become +1 in most occasions.
Alkali metals (except hydrogen) remove one electron to form +1 ion. So only oxidation number of alkali metals is +1. So Lithium, Sodium, Potassium, Rubidium, Cesium have only single oxidation number is +1.
As alkali metals, alkali earth metals only form +2 oxidation state. Beryllium, Magnesium, Calcium, Strontium, Barium have +2 oxidation number.
Group three elements are Boron, Aluminium, Galleum, Indium and Thallium.
Boron shows +3 oxidation state.
Aluminium is a metal. So it removes it's three electrons of the last shell to show +3 oxidation number. Some examples for +3 oxidation states of aluminium are AlCl3 and Al2O3.
Oxidation numbers of carbon changes from -4 to +4.
Group 5 includes Nitrogen, Phosphorous, Arsenic, Antimony and Bismuth. Oxidation number variation change when going down the group.
Nitrogen has various oxidation numbers from -3 to +5.
Phosphorous also have various oxidation numbers from -3 to +5.
We are going to discuss about oxidation numbers of oxygen and sulfur under group 6 elements.
Oxygen has the second highest electronegative value in periodic table. So in most occasions, oxidation number of oxygen is negative. Oxygen only forms positive oxidation numbers when it combine with fluorine.
Sulfur is another p block element which has different oxidation numbers.
Group 7 has Fluorine, Chlorine, Bromine, Iodine and Astatine as elements. These metals show variable oxidation numbers.
FLuorine is the most electronegative element in the periodic table. Therefore. fluorine atom does not make positive oxidation numbers at all.
Chlorine also have lot of oxidation numbers. (from -1 to +7)
Most of the 3d metals have variable oxidation numbers. As an example, iron have two oxidation numbers, +2 and +3.
Zinc has only one oxidation number, +2.
|3d Metal||Oxidation state|
|Ti||+2 , +3 ,+4|
|Va||+2 , +3 , +4 , +5|
|Cr||+2 , +3 , +6|
|Mn||+2 , +4 , +6 , +7|
|Fe||+2 , +3|
|Co||2 , +3|
|Cu||+1 , +2|
Let's take an example.
Sulfur and chlorine and are positioned at 6th and 7th groups of the periodic table. So there are six and seven electrons at their final energy levels respectively. These electrons can be combined with oxygen to form different compounds. Some examples are discussed below.
Sulfur gives its all last six electrons to make sulfuric acid molecule (+6 oxidation state). Chlorine can give seven electrons to make chloric acid to show +7 oxidation number.
Chlorine can take one electron to form chloride anion.(-1 oxidation state). Sulfur can take two electrons to form sulfide anion. (-2 oxidation state).
|Common Oxidation State||Example Compound|
|0||Elemental sulfur (S8)|
|+6||SO3 , H2SO4|
|Common Oxidation State||Example Compound|
Charge of hypochlorite ion OCl- is -1. So we can calculate oxidation number of chlorine.
-2 + x = -1
x = +1
oxidation number of chlorine in Ca(OCl)2 is +1.
Oxidation number and charge of ion is not equal at every time.
Silicon sulfide (SiS2) contain silicon and sulfur atoms. Because you want to find the negative oxidation number, find which element's electronegativity is greater?
Elecronegativity of sulfur is 2.5 and silicon's 1.8 . Therefore, sulfur is more electronegative than silicon. Thus, sulfur should have the negative oxidation number (-2).
As a metal, manganese have +7 oxidation number. Some example compounds for +7 are Potassium permanganate ( KMnO4 ), Mn2O7
As a non-metal, chlorine forms +7 oxidation number. HClO4, Cl2O7 are examples for +7 oxidation number of chlorine.
Fluorine has the zero or negative oxidation number always because fluorine is the most electronegative element in the periodic table. So it always gains an electron when fluorine combines with different element to form -1 oxidation number. Fluorine form zero oxidation number in F2 molecule.
In NaF and HF, oxidation number of fluorine is -1.
Chlorine gas (oxidation number 0) reacts with cold dilute NaOH or hot concentrated NaOH to give +1 and +5 oxidation states of chlorine atom respectively.
Only hydrogen shows variable oxidation numbers. All other elements show single oxidation numbers. Group 1 elements show +1 oxidation state and group 2 elements show +2 oxidation state.
Group VII elements form highest oxidation numbers. From group VII, chlorine form +7 oxidation number.
Both manganese and chlorine have +7 oxidation number. In potassium permanganate ( KMnO4) and HMnO4, manganese oxidation number is +7. Chlorine forms chloric acid (HClO4) to show +7 oxidation number.
When oxidation number increases, acidity strength also increases. Consider acidity of different oxides of nitrogen.
N2O and NO are neutral acidic gases and other oxides of nitrogen ( N2O3, NO2, N2O4 and N2O5 ) are acidic. N2O5 is a strong acidic compound.