Oxidation Number or Oxidation State

When an element is oxidised it must be acting as a reducing agent and it, therefore, loses electrons; when reduced, it gains electrons. The oxidation state or oxidation number of an element is the number of electrons it might be considered to have lost or gained.

• All elements in the elementary, uncombined state are given oxidation numbers of zero. When sodium, for example, is oxidised it loses one electron, and the Na⁺ ion is said to have an oxidation number of +1. Similarly, the Cu²⁺ and Al³⁺ ions have oxidation numbers of +2 and +3, whilst F^- and O^2- have oxidation numbers of - 1 and - 2. For simple ions, the oxidation number is equal to the ionic charge, e.g.

• 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.

• The sum of the oxidation numbers of all the atoms in an uncharged compound is zero. In case of an ion,  the algebraic sum of the oxidation numbers of all the atoms is equal to the charge on the ion,

• Oxidation number of any element in its elementary state is zero.

• Fluorine is the most electronegative element. Its oxidation number is always  -1 .

• Oxygen after fluorine is the second most electronegative element. It  shows an oxidation state of -2 in almost all the compounds excepts seroxides and superoxides,

• In peroxides (O₂^–), oxygen has oxidation number –1; in superoxides (O₂^2–), oxygen has oxidation number –1/2; and in OF₂, the oxygen has an oxidation number +2.

the oxidation numbers in the peroxide ion being calculated by splitting the shared pair equally between the two oxygen atoms

• In all compounds, except ionic metallic hydrides, the oxidation number of hydrogen is +1.

• In any compounds has more than two elements, the oxidation number of any one of them may have to be obtained by first assigning reasonable oxidation numbers to the other elements.

• Some elements like manganese, chromium, nitrogen and chlorine show variable oxidation states. 

• When an element is oxidised its oxidation number gets increased while reduction on any element decreases its oxidation number. Change in oxidation number can be used to decide whether an oxidation or a reduction has taken place. In the change from chloromethane to dichloromethane, for example,
  C  H₃  Cl    C  H₂ Cl₂
  -2 +1 -1     0 +1 -1
  The oxidation number of carbon is increased from -2 to 0. The carbon is therefore being oxidised.

Common Oxidising and Reducing Agents

• The functioning of some common oxidising and reducing agents is summarised below

Oxidation State as Periodic Property

Oxidation state of an atom depends upon the electronic configuration of atom, a periodic property.

• IA group or alkali metals shows +1 oxidation state.

• II A group or alkali earth metals show +2 oxidation state

• The maximum normal oxidation state, shown by III A group elements is +3. These elements also show +2 to +1 oxidation states.

• Elements of IV A group show their max. and min. oxidation states +4 and -4 respectively.

• Non-metals show number of oxidation states, the relation between max. and mini. oxidation states for non metals is equal to maximum oxidation state - minimum oxidation state =8
  For example S has maximum oxidation number +6 as being VI Agroup element.

• Maximum oxidation number of an element is equal to group number – 8

• I Group elements always show +1 oxidation number.

• II Group elements always show +2 oxidation number.

• III Group elements always show +3 oxidation number.

• IV Group elements show -4 to +4 oxidation number.

• V Group elements show -3 to +5 oxidation number.

• VI Group elements show -2 to +6 oxidation number.

• VII Group elements show -1 to +7 oxidation number.

• Inert gases show zero oxidation number.

Application of Oxidation State

To compare the strength of acid and base

• Strength of acids increases with increase in oxidation number.

• Strength of base decreases with increase in oxidation number.

To determine the oxidising and reducing nature

• If any compound is in maximum oxidation state , then it will act as oxidant only.

• If any compound is in minimum oxidation state, then it will act as reductant only.

• If the oxidation state is intermediate, then compound can act as both reductant as well as oxidant.

To determine molecular formula of any compound

Suppose that there are three atoms A, B, C and their oxidation number are 6, -1, -2 respectively. Then the molecular formula of compound formed by them will be AB₄C  because

+6 = (-14)+(-2)

or +6 = -6

Watch this Video for more reference
 

Oxidation State V/S Valency

Question 1: Oxidation state of Mg in MgO is

a. -1
b. -2
c. +2
d. +1

Question 2: Which of the following oxidation states are not possible for O?

a. -1

b. -2

c. -1/2

d. +3/2

Question 3: Oxidation state of O in OF₂ is..

a. +2

b. -2

c. -1

d. +1

Question 4: In the case of neutral molecules, the algebraic sum of the oxidation number of all the atoms present in the molecule is.

a. zero

b. one

c. two

d. three

Related Resources

• Click here to know the syllabus of chemistry for  IIT JEE

• Have a look at past year papers of IIT JEE

• You can refer to oxidation and reduction

To read more, Buy study materials of Redox Reactions comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.