Rules for assigning oxidation numbers

Oxidation number or oxidation state is defined as the charge an atom has in a molecule or compound. It is an apparent charge on an atom of an element in a molecule or compound. It may be positive, negative, or zero.

Before understanding the rules for assigning oxidation numbers, we must be acquainted with these basics:

  • Reactions in which change of charges take place from one or all of the atoms are called oxidation-reduction or redox reactions.
  • During chemical reactions, some atoms are oxidized while some are reduced. The elements that show an increase in oxidation states are oxidized whereas the elements whose oxidation number decreases are reduced.

For example, a reaction between zinc and sulfur.

Zn(s) + S(s) → ZnS(s)

In this reaction, zinc and sulfur combine in free states i.e charges of reactants are zero. When zinc sulfide is formed, the charge on zinc is increased i.e. it oxidizes, and the charge on sulfur is decreased i.e. gets reduced.

Zn0→Zn+2+2e (Oxidized)

S0+2e→S2- (Reduced)

The above reactions are called half-reactions. In the zinc half-reaction, oxidation takes place whereas reduction takes place in the sulfur half-reaction. The overall reaction is called a redox reaction.

Oxidation vs Reduction

Oxidation Reduction
Loss of electrons Gain of electrons
Loss of hydrogen Gain of hydrogen
Gain of oxygen Loss of oxygen
Increase in oxidation number Decrease in oxidation number

Rules for Assigning oxidation numbers

The basics rules for assigning oxidation numbers are as follow:

1. Elements in a free state

The oxidation number of elements in a free or uncombined state is always zero.

For example

The oxidation state of all free state elements like Cl2, H2, Zn, N2, Na, and Mg, etc are zero.

The oxidation number of sulfur in S8 is also zero because it has the same atoms forming bonds and exists under normal conditions as an octatomic molecule.

2. Ions

The oxidation number of a single ion is the same as the charge on ions.

For example

The oxidation states of Na+,Ca+2,Cl, Al3+, and S2- are (+1),(+2),(-1),(+3), and (-2) respectively.

3. Binary ionic compounds

The oxidation state of an ionic binary compound is the charge per atom.

For example

In the case of CdCl2, the oxidation number of Cd is (+2) and Cl is (-1) as there are two chlorides per cadmium.

Similarly, in Hg2Cl2 the oxidation number of Hg and Cl are (+1) and (-1) respectively.

4. Complex ions

In the case of complex ions, the total sum of oxidation states of atoms is equal to the charge of their ions.

For example

In carbonate ion (CO32-) the sum of oxidation states of carbon and three oxygen is equal to the charge on ion i.e (-2).

Similarly, in ammonium (NH4+), the sum of oxidation states of nitrogen and three hydrogens should be +1.

5. Hydrogen

The oxidation number of hydrogen in all of its compounds is (+1), but if hydrogen is linked with metals, which are more electropositive than hydrogen, then the oxidation state of hydrogen is seen as (-1).

For example

In HCl, the oxidation number of hydrogen is (+1) but in the cases of NaH and CaH2, the oxidation number of hydrogen is (-1).

6. Oxygen

The oxidation number of oxygen is usually (-2) in all its compounds except peroxides, OF2, and superoxides.

For example

In H2O, the oxidation number of oxygen is (-2). But in peroxide (O22-) the oxidation number of oxygen is (-1) and in OF2 it is (+2).

The positive oxidation number on the oxygen atom indicates that it is attached to a more electronegative atom.

7. Neutral molecules

The oxidation number of each of the atoms in a molecule is counted individually and their algebraic sum is zero.

For example

In HCl, the oxidation states on hydrogen and chlorine are (+1) and (-1) respectively but their algebraic sum is zero.

Similarly, in CO2 the overall oxidation state due to one C and two oxygen atoms is zero.

8. Different elemental Groups

  1. The oxidation number of group IA elements is +1.
  2. The oxidation number of group IIA elements is +2.
  3. The oxidation number of group IIIA elements is +3.
  4. The oxidation number of group VA elements is -3.
  5. The oxidation number of group VIA elements is -2.
  6. The oxidation number of group VIIA elements is -1.

Oxidation number vs Formal charge

Oxidation number Formal Charge
An apparent charge on an atom in a molecule that depends on the number of electrons lost or gained by an atom. The formal charge of an atom is the hypothetical charge the atom would have if we redistribute the electrons in the bond equally between two atoms.
Bonding electrons are assigned to more electronegative atoms. Bonding electrons are assigned equally to the bonded atoms.
Oxidation numbers on an atom remain the same for all resonance structures. The formal charge on an atom is not the same for all resonance structures.
It can help to predict the most reasonable resonance structures of molecules. It can be used to draw the correct lewis structures of molecules.
For example The oxidation number of Br in BrCl3 is (+3). For example The formal charge on Br in BrCl3 is (0).

Oxidation number vs Valency

Oxidation number Valency
It is an apparent charge on an atom. It is the combining capacity of an atom.
It is the number of electrons that an atom can lose, gain or share to form a bond with another atom. It is the maximum number of valence electrons that an atom can lose, gain or share with another atom to become stable.
It is not fixed for a particular element. It is fixed for particular elements with some exceptions.
It may be positive, negative, or may even be zero. It is always a positive number.
Example The oxidation number of hydrogen is either (+1) or (-1). Example The valency of hydrogen is always (+1).

Ket takeaway(s)

Oxidation state vs oxidation number

Oxidation state is a particular number of electrons that an atom can lose, share or gain whereas oxidation number is the charge on central metal ion in a coordination complex when all of the ligands have ionic bonds with the central metal atom.

Oxidation states can be represented by Arabic numbers while oxidation numbers are represented using roman integers.

Oxidation state provides the detail about the type of charge on an atom e.g. positive or negative whereas oxidation number is not much descriptive.

Oxidation vs reduction

table of oxidation and reduction

Concepts Berg

What are the rules for assigning oxidation numbers?

Some basics rules for assigning oxidation numbers are

  1. The oxidation number of elements in a free state is zero.
  2. The oxidation number of ions is the same as the charge on ions.
  3. The oxidation number of a neutral molecule is zero.
  4. The oxidation number of hydrogen is (+1) but in metal hydrides, it is (-1).
  5. The oxidation number of oxygen is (-2) except in peroxide and OF2.

What is an oxidation number?

The oxidation number is an apparent charge on an atom of an element in a molecule or a compound. It is the number of electrons lost or gained by an atom.

For example, in table salt (NaCl), the oxidation number of Na+ is (+1), and Cl is (-1) because one electron is lost by sodium and the same one is gained by chlorine.

Is there any way to assign the oxidation number for transition elements?

Since transition elements have variable oxidation states, the oxidation number of transition elements in a complex can be written in roman numerals.

For example in complex hexaaquachromium Cr(H2O)6,

The oxidation state of chromium is (+3) which can be written as chromium(III). So, the name of given compound becomes hexaaquachromium(III).

What is the need for an oxidation number?

The oxidation number is important in various ways.

a) It tells us which atom is oxidized and which one is reduced.

b) It is helpful to solve redox equations.

c) It tells us about the valency of the element.

For example, the oxidation number of oxygen in H2O is

(1×2) + O = 0

2 + O = 0

O = -2

It shows that oxygen can make two bonds (ignoring the -ve sign).

What is the oxidation number change method?

The oxidation number change method is a method of balancing the redox equations. Some basics steps of the oxidation number method are:

  1. Write the skeleton of the equation and identify which elements change their oxidation number.
  2. Equate the number of electrons lost or gain on both sides of the equation by multiplying with a suitable number.
  3. Balance the equation by inserting the coefficient.

What is the oxidation number of Cl in Cl2O3?

Since Dichlorine trioxide (Cl2O3) is a neutral molecule so the sum of the oxidation number of all elements must be zero.

The oxidation number of oxygen is (-2) so,

2xCl + 3(-2) = 0

2xCl-6 = 0

2Cl = 6

Cl = +3

Thus, the oxidation number of chlorine in Cl2O3 is (+3).

What are the oxidation numbers of non-metals?

Oxidation numbers of non-metals depend on the group number to which non-metals belong. Non-metals usually gain electrons to complete their octet.

For example,

In the case of oxygen, six valence electrons are present. It requires 2 electrons to complete its octet. So its oxidation number is usually (-2).

In the case of halogens, they need one electron to complete the octet so their oxidation state is (-1).

What is the reduction of hydrogen?

Reduction is either the addition of hydrogen or the removal of oxygen. It is also defined as the gain of electrons.

The reaction in which hydrogen gains an electron is called the reduction of hydrogen.

For example,

In the case of a metal hydride, NaH the oxidation number of sodium is (+1) and hydrogen is (-1) because sodium loses one electron and hydrogen gains that electron.

What are the compounds with exceptional oxidation numbers?

There are some examples of compounds that show exceptional behaviors.

  1. The oxidation number of oxygen in OF2 is (+2). But in the case of hypochlorous acid HOCl, it has an oxidation number of (0).
  2. The oxidation number of nitrogen in hydroxylamine (NH2OH) is (-1) and in hydrazoic acid (HN3) one of the nitrogens has an oxidation number of (-1) and another one has (0).

What are the oxidation numbers of nitrogen atoms in NH4NO2?

In NH4NO2,

NH4NO2 ⇌ NH4+ + NO2

In the case of NH4+ (Overall oxidation state = +1)

Let the oxidation number of nitrogen be x

(N)+4(H) = 1

x + 4(1) = 1

x = 1 – 4

x = -3

In the case of NO2(Overall oxidation state = -1)

Let the oxidation number of nitrogen is x

(N)+2(O) = -1

x+2(-2) = -1

x = -1+ 4

x = 3

The oxidation number of nitrogen in NH4NO2 is (-3) and (+3) respectively.

What’s the oxidation number of EDTA?

Ethylenediaminetetraacetic acid (EDTA) is weakly acidic, so, upon ionization, it loses the four hydrogens and leaves with four acetate ions, and shows an oxidation number of (+4).

How to find the oxidation number of oxygen in SO3?

Let the oxidation number of oxygen in SO3 be x.

(S)+3(O) = 0

+6+3x = 0

3x = -6

x = -2

So the oxidation number of oxygen is (-2).

What is the oxidation number of P in the molecule PO4 with a -3 as the total charge?

We can calculate the oxidation number of PO43- as;

Let oxidation number of phosphorus is x.

(P)+4(O) = -3

x+4(-2) = -3

x-8 = -3

x = -3+8

x = 5

So the oxidation number of phosphorus is (+5).

What is the oxidation number of Sb in SbOCl?

Let the oxidation number of Sb be x.

(Sb)+(O)+(Cl) = 0

x+(-2)-1 = 0

x-3 = 0

x = 3

So the oxidation number of Sb is (+3).

What determines the oxidation number, valence electrons, or valency?

Valence electrons are defined as the number of electrons present in the outermost shell of an atom whereas valency is the number of electrons lost or gained by an atom. An element can lose, gain electrons to become stable, and reach an oxidation state, so, both valency and valence electrons can determine oxidation number.

What is the oxidation number of Cl in sodium chlorate (NaClO3)?

Let the oxidation number of Cl be x. Since it is a neutral molecule, the sum of the oxidation number of all atoms must be zero.

(Na)+(Cl)+3(O) = 0

+1+x+3(-2) = 0

1+x-6 = 0

x-5 = 0

x = 5

The oxidation number of Cl in NaClO3 is (+5).

What is the general oxidation number for halogen?

Since halogens have seven electrons in their outermost shell and they need one electron to complete their octet. They gain electron(s) and usually show (-1) oxidation state.

What is the oxidation number of P in P2O3?

Let the oxidation number of P be x. Since it is a neutral molecule, the sum of the oxidation number of all atoms must be zero.

2(P)+3(O) = 0

2x+3(-2) = 0

2x+3(-2) = 0

2x-6 = 0

2x = 6

x = 3

The oxidation number of P in P2O3 is (+3).

How do I find the oxidation number from the Lewis structure?

We can find the oxidation number from lewis structures by the following steps:

  1. If both atoms of a bond have the same electronegativity, then write the lewis structure and assign half of the electrons to one atom and half to the other atoms.
  2. If the atoms of a bond have some electronegativity difference, assign both electrons (of a single bond) to the atom having more electronegativity.
  3. Assign lone pairs to the atom to which they belong.
  4. Compare the valence electrons present in a neutral atom with the number of electrons assigned in the structure.
  5. If there are some electrons leftover, the element has a negative oxidation number and if the element has a deficiency of electrons, it has a positive oxidation number.

What is the oxidation number of C in CH3Cl?

Let the oxidation number of C be x. Since it is a neutral molecule, the sum of the oxidation number of all atoms must be zero.

(C)+3(H)+(Cl) = 0

x+3(1)-1 = 0

x+2 = 0

x = -2

The oxidation number of carbon in CH3Cl is (-2).

What is the oxidation state of nitrogen in the compound N2O3?

Let the oxidation number of N be x. Since it is a neutral molecule, the sum of the oxidation number of all atoms must be zero.

2(N)+3(O) = 0

2x+3(-2) = 0

2x-6 = 0

2x = 6

x = 3

The oxidation number of nitrogen in N2O3 is (+3).

What is the difference between oxidation number and ionic charge?

Ionic charge is formed when an atom loses or gains electrons. If an atom loses an electron, it develops a positive charge and if it gains, it develops a negative charge.

The oxidation number is an apparent charge on an atom and develops if all bonds present are ionic. It can be positive, negative, or even zero.

Both these terms are not the same because the oxidation number of elements in the free state is zero but the oxidation number of pure ion is equal to the charge on the ion.

What is the oxidation number of nitrogen in the reaction N2 +3H2→ 2NH3?

Since the oxidation number of elements in the free state is zero, the oxidation number of nitrogen in the reactant side is zero. The oxidation number of hydrogen on the product side is (+1). Three hydrogen atoms are present so it has oxidation number +3. Since ammonia is a neutral molecule, nitrogen must have a (-3) oxidation state so that the overall oxidation state of ammonia could be zero.

What is the oxidation number of Cl in HClO4?

Let the oxidation number of Cl be x. Since it is a neutral molecule, the sum of the oxidation number of all atoms must be zero.

(H)+(Cl)+4(O) = 0

1+x+4(-2) = 0

1+x-8 = 0

x = 8 – 1

x = 7

The oxidation number of chlorine in HClO4 is (+7).

What is the oxidation number of Br in NaBrO?

Let the oxidation number of Br is x. Since it is a neutral molecule, the sum of the oxidation number of all atoms is zero.

(Na)+(Br)+(O) = 0

1+x-2 = 0

1+x = 2

x = 2-1

x = 1

The oxidation number of bromine in NaBrO is (+1).

In H2+ CI2→2HCI which substance is oxidized and reduced?

The oxidation state of hydrogen and chlorine on the reactant side is zero as they are in a free state.

The oxidation state of hydrogen on the product side is (+1) and chlorine is (-1). It means that hydrogen loses one electron i.e. it gets oxidized. On the other hand, chlorine gains one electron i.e. gets reduced.

What is the oxidation number of oxygen in this compound Fe2( CO3 )3?

Let oxidation number of oxygen is x. Since it is a neutral molecule, the sum of the oxidation number of all elements is zero.

2(Fe)+3((C)+3(O)) = 0

2(3)+3(4+3x) = 0

2(3)+3(4+3x) = 0

6+12+9x = 0

18+9x = 0

9x = -18

x = -2

The oxidation number of oxygen in Fe2[(CO3 )3] is (-2).

Which is the oxidizer and reducer in S+H2→H2S?

The oxidation state of hydrogen and sulfur on the reactant side is zero since they are in a free state.

The oxidation state of hydrogen on the product side is (+1) and sulfur is (-2). It means that hydrogen loses one electron i.e. it gets oxidized. On the other hand, sulfur gains one electron i.e. gets reduced.

What is the change of the oxidation number of Cr in Cr2O7 in an acidic medium?

The oxidation state of Cr in Cr2O72- is (+6). In an acidic medium, it is converted into 2Cr3+ so it changes its oxidation number from (+6) to (+3). It means one Cr atom decreases oxidation number 3 but the overall decrease in oxidation number is 6.

What is the oxidant and reductant of CH4+2O2→CO2+2H2O?

From the above reaction, the oxidation state of carbon on the reactant side is (-4) and on the product, side is (+4). So carbon has been oxidized.

Whereas, oxygen is an oxidizing agent so it oxidizes both products.

What is the oxidation state of barium in BaO2?

Let the oxidation state of barium is x. Since it is a neutral molecule, the sum of oxidation states is zero.

(Ba)+2(O) = 0

x+2(-2) = 0

x-4 = 0

x = 4

The oxidation state of barium in BaO2 is (+4).

What does it mean to assign oxidation numbers?

The assigning of oxidation number means that giving charge to species in a chemical reaction to indicate which species is being oxidized or reduced. It shows the degree of oxidation or reduction in a redox reaction as well.

What is the most common oxidation number of oxygen?

The common oxidation number of oxygen is (-2) except in OF2 and peroxide where it has oxidation numbers (+2) and (-1) respectively.

What is the oxidation number of H in NaH?

The oxidation number of hydrogen in metal hydrides e.g. NaH is -1.

What is the difference between oxidation number and oxidation state?

Oxidation state is the number of electrons that an atom can lose or gain to form a bond with other atoms. It can be fractional. It is applied in all atoms, ions, neutral molecules, etc. Moreover, it is the average charge on all atoms present in a molecule.

The oxidation number is applied in coordination chemistry. It is the charge that central atoms have if all of the ligands are removed from the central metal atom. It can never be fractional. It is the charge on individual atoms when all bonds break off.

What is the sum of oxidation numbers in a compound?

The sum of the oxidation numbers of all atoms in a neutral compound must be zero. The oxidation number of a polyatomic ion is equal to the charge on that ion or radical.