Oxidation Number

Oxidation Number OR 

 Oxidation State (Oxi. No.)

Definition of Oxidation Number

In covalent bond formation the electrons are not transferred as in ionic bond formation, but partial transfer of electronic charge takes place, known as electron shift. The oxidation number method always assumes that there is a complete transfer of electrons from a less electronegative atom to a more electronegative atom.

Oxidation Number is a fictitious charges assigned to the atom of an element in a covalently bonded molecule by arbitrary conventions that results when the electrons in a covalent bond are assigned to the more electronegative atom (assuming the bonding were ionic) making certain the law of charge conservation is strictly obeyed. Oxidation Number is a purely a hypothetical number without any theoretical justification and it does not correspond (coincide with) to the real (actual) charge on the atoms, except in the special case of simple ionic compounds. Oxidation number of an atom in a molecule or ion is the hypothetical or real charge present on an atom due to electronegativity difference.

the number of charges an atom would have in a molecule of a compound or polyatomic ion if bonding electrons were transferred completely in the direction indicated by the difference in electronegativity. Thus oxidation number reflects the number of electrons transferred in a covalent molecule or polyatomic ion.

OR

the apparent charge (i.e. not real), positive or negative or zero, on an atom of element in a molecule of a compound or in a polyatomic ion (radical) that results when the electrons in a covalent bond are assigned to the more electronegative atom is called oxidation number.

OR

It is the fictitious charge that an atom appears to have in a given species when the bonding electrons are counted towards more electronegative atom i.e. It is the hypothetical charge an atom would possess in a compound if the bonding were completely ionic.

It is the number of electrons lost or gained by an atom of an element during its change from free state into a particular compound.

The oxidation state is a “measure (or indicator) of the degree of oxidation” of an atom in a chemical compound and it is the fundamental key to understanding redox reactions, reaction mechanisms, catalysis, etc.

Oxidation number is the number with positive or negative sign which indicates the extent to which an element has been oxidized or reduced i.e. it shows the number of electrons which an atom has lost or gained as a result of bonding. (In writing oxidation numbers, we will write the sign before the number to distinguish them from actual electronic charges, which we write with number first).

The oxidation number for an element in a covalent compound is by taking the oxidation number to be equal to the charge that the element would carry, if all the bonds in the compound were regarded as ionic instead of covalent. In doing this, a shared pair or electrons between two atoms is assigned to the atom with the greater electronegativity. Or, if the two atoms are alike, the shared pair is split between the two, one electron being assigned to each atom. The resulting charges on the various atoms when the bonding electrons are so assigned are the oxidation numbers of the atoms.

1.   Oxidation number may be fractional.

2.   Oxidation number is positive in metallic elements

3. Oxidation number is positive or negative in non-metallic elements

4.  Oxidation number is represented in Roman numbers in parenthesis (brackets) after the symbol of the metal in compounds 

e.g. Au(III) Cl₃ (auric chloride), 

Sn(II) Cl₂ (stannous chloride), 

Hg(II)Cl₂ (mercuric chloride).

Covalency

It is the number of hydrogen atoms which can combine with a given atom. It is equal to the number of single bonds which an atom can form. It is also equal to the number of electrons an atom can share.

Oxidation State

It is the oxidation number per atom.

Examples of Oxidation Number

How to get Oxidation Number

1)   Given a compound, write its Lewis dot structure.

2)   For each separate bond decide which element is most electronegative (EN).

3)   Give the most electronegative element all the electrons of that bond. If the atoms in the bond are the same, give each element half of the electrons.

4)   When all electrons have been assigned subtract the number of electrons on each atom from the valence of each element to get the oxidation state (number).

Difference between Valency and Oxidation number

Valency is a different term than oxidation number though sometimes the valency and the oxidation number of an element are same in a compound.

1.   oxidation number is just the apparent charge (not necessarily actual) over the atom when the electrons are counted according to the arbitrary rules i.e. oxidation number is the number with positive or negative sign which indicates the extent to which an element has been oxidized or reduced. While valency is mere a number without positive or negative sign which expresses the combining or displacing tendency of an atom of an element and valency of an element is given by the number of electrons it actually loses or gains or shares during the formation of a compound.

2.   The oxidation no. of an atom may be in fraction, whereas the valency is always in whole number. The oxidation number of an atom in a compound may be zero but valency of an element cannot be zero (except noble gases).

3.   the oxidation state of an element may vary in its different compounds whereas in most of the cases, the valency of an element is constant.

4.   Valency and oxidation states of carbon in its different compounds give a good example to differentiate the two concepts. In CH₄, CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄, the valency of carbon is always four (due to sharing of four electrons) but its oxidation numbers is -4, -2, 0, +2 and +4 respectively.

 Significance of Oxidation number

Oxidation number provides a measure of whether the atom in a molecule is neutral, electron rich or electron-poor. It guides us to identify elements that are oxidized (oxidation number increases) and reduced (oxidation number decreases) at a glance by comparing its oxidation number before and after the reaction.

Oxidation numbers are used:

Important Points of Oxidation Number          

1. Range of Oxidation Number

2. Positive and Negative Oxidation Numbers

4. Maximum Oxidation Numbers

5. Oxidation Number measures covalent and ionic character

6. Fractional Oxidation Number

7. Oxidation number and group number

6. Fractional Oxidation Number

Elements as such do not have any fractional oxidation numbers. In reality no element can have a fractional oxidation state as electrons cannot be transferred in fraction. When two or more atoms of an element are present in different oxidation states, then calculated oxidation number in a compound or ion may come out as fractional due to average of all the different oxidation states. Fractional oxidation number is the average fractional number. Fractional oxidation number of a particular element can be calculated only if we know about the structure of the compound in which it is present.

e.g.

(i)   S₄O₆^2- tetrathionate ion. The oxidation number of end S atoms is +5 each and of the middle atoms is 0 each. The total oxidation number of 4 S atoms is 5+0+0+5=10 and the average oxidation number is  10 ÷ 4 = 2.5.

Stock Notation

Representation of oxidation state of element by Roman numerals within parenthesis is known as stock notation i.e. Expressing the oxidation state of a metal by Roman numerals like I, II, III etc. within parenthesis is called stock notation. e.g.

FeSO₄ = Iron(II) sulphates,       

FeCl₃  = iron(III) chloride,        

Na₂CrO₄  = sodium chromate(VI)

Some Important Points on Oxidation Number

1.  The oxidation number of metals in amalgams and metal carbonyls i.e. Ni(CO)₄, Fe(CO)₃, Cr(CO)₆ etc. is zero.

2.   If a compound contains two or more atoms of the same element, all of them may or may not have same oxidation number e.g.

   (i) In Na₂S₂O₃, one S-atom has oxidation number = -2 while the other has oxidation number = +6.

    (ii) In bleaching powder; CaOCl₂ or Ca(OCl)Cl, oxidation number of one Cl = -1 while oxidation number of other Cl = +1.

(iii) In Fe₃O₄ or FeO.Fe₂O₃, oxidation number of one Fe = +2 while that of each of the other two = +3.

 (iv) In NH₄NO₃, oxidation number of N of NH₄⁺ = -3 while that of N in NO₃^- = +5.

 (v) In sodium tetrathionate, Na₂S₄O₆, having the structure

3.   In case of coordinate bond, it gives +2 value of oxidation number to less electronegative atom and -2 values to more electronegative atom when coordinate bond is directed form less electronegative atom  to more electronegative atom.

4.   If coordinate bond is directed from more electronegative to less electronegative atom then its contribution be zero for both the atoms.

5.   For p-block elements (except F and O), the highest oxidation number is equal to their group number and lowest oxidation number is equal to the group number minus eight.

6.   In transition elements the lowest oxidation number is equal to the number of ns electrons and highest oxidation number is equal to number of ‘ns’ and (n – 1)d unpaired electrons.

7.   Oxidation number of O in compounds of fluorine is positive as F is the most electronegative  element.

8.   Os, Ru, Xe show maximum oxidation number i.e. +8

9.   In 3d-series of transition metals, Mn shows maximum oxidation number of +7.

10. Electronegativity values of no two elements are same

      P > H, C > H, S > C, Cl > N

11. Oxidation state of same element can be different in same or different compounds

      O.N of S in H₂S = - 2

      H₂SO₃ = +4

      H₂SO₄ = +6

 Summary of Rules for Finding Oxidation Number

Summary of Rules for Finding Oxidation

 Number