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Oxidation Number

Elements gain and lose electrons during redox reactions, with the result that their oxidation numbers change. The valency of an element is determined by the number of electrons it controls.

Each element in any chemical species may be assigned an oxidation number. A more precise term for valency is oxidation number. Oxidation numbers indicate where the electrons are and it is assigned to elements on the basis of how readily they react in redox reactions.

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What is Oxidation Number?

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"The oxidation number of an element is the number of electrons that need to be added to the element to make a neutral atom."
The oxidation number of any element can never be in fraction. If oxidation number of any element is in fraction, it is resultant oxidation number of that element. Oxidation number of an element in a simple binary ionic compound is the number of electrons gained or lost by an atom of that element when it forms the compound.

Oxidation Number Definition

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Define oxidation number

Oxidation number (O.N) of an element is defined as "the residual charge which its atom has or appears to have when all other atoms from the molecules are removed as ions."

The oxidation number of an atom indicates the number of electrons that this atom would have either gained or lost in the formation of the molecule of which it is part if the molecule were assumed to be entirely composed of ions. Positive oxidation numbers indicate a loss of electrons. Negative oxidation numbers indicate a gain of electrons.

Oxidation Number Rules

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The oxidation number of an element is the number of electrons that need to be added to the element to make a neutral atom. A reaction is classed as a redox reaction if it involves a change in the oxidation number of any element taking part.

The basic rules for calculating oxidation number are listed below.
  1. The oxidation number of a mono atomic ion is the same as the charge on the ion. Therefore in NaCl the oxidation number assigned to sodium is +1, and the oxidation number of chlorine is -1.
  2. The oxidation number of any element is zero when the atoms of that element are in the elemental state. An atom is in its elemental state when its atoms are not chemically combined with atoms of other elements.
  3. In a covalent compound of two elements, the electronegative element has a negative oxidation number and the other element has a positive oxidation number. For example, in ammonia NH3 the nitrogen is more electronegative. Each of the valence electrons from the three hydrogen atoms is drawn nearer to the nitrogen atom. The oxidation number of the nitrogen in ammonia is therefore -3. Since each hydrogen atom is partially losing one electron, the oxidation number of hydrogen is +1.
  4. The oxidation number of oxygen in most of its compounds is -2. In peroxides there is an exception and it is assigned -1.
  5. The oxidation number of hydrogen in most compounds is +1. In hydride there is an exception and it is assigned -1.
  6. In neutral compounds, for all the atoms represented by the formula the quantitative sum of the positive oxidation numbers must equal the quantitative sum of the negative oxidation numbers and their sum must equal 0.

Oxidation Number Chart

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Most elements have different oxidation numbers that can vary depending on the molecule that they are a part of it. The following chart shows the oxidation states of elements.

S.No
Compound
Oxidation number
Oxidation number of carbon
+4
2
Oxidation number of oxygen  -2
3
Oxidation number of nitrogen  -3
4
Oxidation number of sulfur
-2
5
MnO4 oxidation number -2
6Oxidation number of hydrogen +1
7
Oxidation number of Cl-1
8
NH3 oxidation number +3
9
NO3 oxidation number -1
10
H2O2 oxidation number -1
11
Zinc oxidation number +2
12
Aluminum oxidation number +3
13
Oxidation number of phosphorus +3
14
Oxidation number of Mn+7 to +2
15
Oxidation number of S in SO2
+4
16
CO2 oxidation number +4
17
NH4 oxidation number +1
18
Oxidation number of fluorine
-1
19
Silver oxidation number +1
20
Oxidation number of copper 0
21
Oxidation number of CO +2
22
Oxidation number of Fe 0
23
Oxidation number of sodium +1
24
Oxidation number of H2+1
25
SO4 oxidation number -2
26
Oxidation number of OH -1
27
Oxidation number of Na
+1
28
Oxidation number of Cr
+2, +3
29
Ag oxidation number0
30
Oxidation number of MnO4-
-1
31
Oxidation number of NO 0
32
Oxidation number of iodine -1
33
Oxidation number of HNO3 -1
34
Oxidation number of As in As0
35
Oxidation number of Br -1, 0, +1, +3, +4, +5, +7
36
Oxidation number of SO4
+6
37
Oxidation number of n in N2H4 +1, -2
38
Potassium oxidation number+1
39
Lithium oxidation number+1
40
Oxidation number of Sn in SnCl3--1
41
Nickel oxidation number +2
42
Zn oxidation number +2
43
Oxidation number of N in NO2
+4
45
Oxidation number of chromium+3, +6
46
KMnO4 oxidation number +7
47
Oxidation number of Ti in TiO2-2, +4
48
Oxidation number of o in K2O2 +1, -1
49
CuSO4 oxidation number +2
50
Oxidation number for NO3- -2
51
KClO3 oxidation number +1
52 Cobalt oxidation number +2

Oxidation Number Periodic Table

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The characteristic of elements can be addressed in terms of trends and periodicity is oxidation number. Since there is apparent relationship between the number of electrons that can be lost or gained and an elements position in the periodic table relative to the noble gases, it would seem that a trend in oxidation numbers might also be present.

The trend in oxidation numbers of elements are shown below.

Group IA Group IIA
Group IIIA
Group IVA
Group VA
Group VIA
Group VIIA
Group VIIIA
+1 +2
+3
+4, -4
-3
-2
-1

  1. On the left side of the periodic table elements lose the electrons in their outermost level by creating ions with positive charges equal in magnitude to the number of electrons that are lost.
  2. On the right side of the table, elements gain electrons in their outermost level creating ions with negative charges equal in magnitude to the number of electrons gained.
  3. In the middle of the table there would be a transition from a positive oxidation number to a negative one as the tendency switches from one in which electrons are lost to one in which electrons are gained.

Oxidation Number Problems

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Example problems based on the concept oxidation number is given below.

Solved Examples

Question 1: In Na2SO4, what is the oxidation number of sulfur?
Solution:
 
The oxidation number of one atom of Na is +1. There are 2 atoms
So, 2(+1) = +2
One atom of oxygen is assigned -2. There are 4 atoms
So, 4(-2) = -8
Since the positive sum plus the negative sum must equal 0
(+2) + x + (-8) = 0
x = +6
The sulfur must be using a +6 oxidation state.
 

Question 2: What is the oxidation number of chromium in K2Cr2O7?
Solution:
 
K = 2(+1) = +2
Cr = 2(x) = 2x
O = 7(-2) = -14
(+2) + (2x) + (-14) = 0
2x = +12
x = +6
So Cr is using a +6 oxidation number.
 

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