Oxidation number of elements

A+ A-

An oxidizing and reducing agent and oxidation number

The study oxidation number of an element used to specify some oxidation and reduction reactions and definitions of oxidation and reduction based on loss or gain of electrons limited to the scope. This definition holds goods for ionic compounds.

The formation of water from hydrogen and oxygen, can not be covered by the electronic concept since water is not an ionic compound.

2H₂ + O₂ → 2H₂O

Classically we could still say that hydrogen oxidized to water. In the same seance burning of magnesium in oxygen considered oxidation. Similarly, hydrogen and chlorine react to form a covalent molecule hydrogen chloride.

2H₂ + Cl₂ → 2HCl

Formation of hydrochloric acid, hydrogen oxidized or chlorine reduced but the resulting compound covalent one, the reaction cannot be covered by the electronic concept. To cover such reactions also under oxidation and reduction, the concept of oxidation number developed.

What is the oxidation number of an element?

How to calculate oxidation number of an element in a compound
Oxidation number of an element
The oxidation number of an element in a compound is the formal charge (positive or negative) which would be assigned to the element if all the bonds in the compounds were ionic bonds.

All the compounds are treated as though they were ionic merely because of the case of counting oxidation numbers. The oxidation number, therefore arbitrary.

The electronegativity concept utilized in adducing a formal charge to an atom. The less electronegative of the partners of a binary compound arbitrarily assigned a positive oxidation number and more electronegative one a negative oxidation number.

Florine, being the most electronegative, has always a negative oxidation number. The alkali metals are highly electropositive, have low electronegativity, hence they have been assigned positive oxidation numbers.

How to calculate the oxidation number of an element?

The following general rules are to be observed for the assignment of oxidation numbers.
  1. Atoms of diatomic molecules like hydrogen, chlorine, oxygen, etc or of metallic elements like zinc, copper, sodium, etc are assigned zero oxidation numbers since the same elements of similar electronegativity are involved in the bonding.
  2. Except for metal hydrides, the oxidation number of hydrogen +1. In alkali metal hydrides, lithium hydride, sodium hydride, cesium hydride, etc, the oxidation number of hydrogen -1.
  3. The oxidation number of metal positive.
  4. Oxygen has normally an oxidation number -2. In peroxide and superoxides the oxidation number of oxygen -1 and -1/2 respectively.
  5. The oxidation number of an ion equal to its charge.
  6. The algebraic sum of the oxidation numbers of all the atoms in a compound must be zero and the oxidation number of many atomic ions equal to its charge.

Hydrogen, alkali metal hydride, hydrochloric acid

H ➖H

In hydrogen molecules, two hydrogen atom of the same electronegativity involved for bonding. Thus the oxidation number of the hydrogen molecule zero.

NaH → Na⁺ + H⁻

Alkali metal always possesses a positive oxidation number. In sodium hydride, the oxidation number of sodium and hydrogen +1 and -1 respectively.

HCl → H⁺ + Cl⁻

In hydrochloric acid oxidation number of hydrogen and chlorine +1 and -1 respectively.

Metal oxide, peroxide, monoxide, superoxide of oxygen

CuO → Cu⁺² + O⁻²
In cupric oxide oxidation number of copper and oxygen +2 and -2 respectively.

Hydrogen peroxide (H₂O₂) sodium peroxide (Na₂O₂), the oxidation number of oxygen -1 since hydrogen has to be assigned +1.

The oxidation number of oxygen in water -2, but in hydrogen peroxide, hydrogen +1 state, and oxygen -1 state.

Sodium +1 state in sodium peroxide and the oxidation number of oxygen -1.

Barium +2 state in barium peroxide, the oxidation number of oxygen -1.

Fluorine is more electronegative than oxygen and the oxidation number of fluorine and oxygen in fluorine monoxide -1 and +1 respectively.

Potassium superoxides (KO₂), the oxidation number of oxygen -1/2.

Sodium and magnesium halide

NaCl → Na⁺ + Cl⁻

Sodium and chloride ion charge and oxidation number +1 and -1 respectively.

MgBr₂ → Mg⁺² + 2Br⁻

Here the charge and oxidation number of magnesium and bromide ion are +2 and -1 respectively.
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The oxidation number of compound and ion

In hydrochloric acid oxidation number of hydrogen +1 and the oxidation number of chlorine -1. And the sum of these = (+1) + (-1) = 0.

In the MnO₄⁻ ion sum of the oxidation number of manganese and oxygen equal to -1.

Question
Oxidation number of barium in Ba(H₂PO₂)₂ is - (a)+3, (b)+2, (c) +1, (d) -1.

Answer
The oxidation number of barium +2, the oxidation number of hydrogen +1 and the oxidation number of oxygen -2.
Let the oxidation number of P x.
∴ (+ 2) + 2{2(+1) + x +2(-2)} = 0
or, 2x - 2 = 0
or, x = +1

Manganese in potassium permanganate

Let the oxidation number of manganese in permanganate x. Thus according to the above rule,
(+1) + x + 4(-2) = 0
or, x = +7
The oxidation number of manganese in permanganate +7

Manganese in manganate ion

Let the oxidation number of manganese in manganate ion x and the oxidation number of oxygen -2(according to the above rule).

Thus the sum of the oxidation number of MnO₄⁻² = Charge of the MnO₄⁻².
∴ x +4 (-2) = -2
or, x = +6
The oxidation number of manganese in manganate ion +6

Chromium in dichromate ion

Let the oxidation number of chromium in Cr₂O₇⁻² x
∴ 2x + 7(-2) = -2
or, x = +6
The oxidation number of chromium in dichromate +6

Sulfur in sulphuric acid

Let the oxidation number of sulfur in sulfuric acid x. According to the rule oxidation number of hydrogen +1 and oxygen -2.

∴ 2(+1) + x + 4(-2) = 0
or, x = +6
The oxidation number of sulfur in sulfuric acid +6

The oxidation number of carbon

Let the oxidation number of carbon in CH₃COCH₃ x. The oxidation number of hydrogen and oxygen +1 and -2 respectively.

∴ 3x + 6(+1) + (-2) = 0
or, x = -(4/3)
The oxidation number of carbon in CH₃COCH₃ 4/3

Phosphorus in H₄P₂O₇

Let the oxidation number of phosphorus in H₄P₂O₇ x.
∴ 4(+1) + 2x + 7(-2) = 0
or, x = +5
The oxidation number of phosphorus in H₄P₂O₇ +5

The oxidation state of iron in Fe(CO)₅

The oxidation number CO zero.
The oxidation number of Fe also zero.

Question
Calculate the oxidation number of iron in [Fe(H₂O)₅(NO)⁺]SO₄.

Answer
Water is neutral thus the oxidation number zero, the oxidation number of (NO)⁺ +1 and the oxidation number of sulfate ion -2.

Let the oxidation number of iron in [Fe(H₂O)₅(NO)⁺]SO₄ x.
or, x - 1 = 0
or, x = +1
The oxidation number of iron in [Fe(H₂O)₅(NO)⁺]SO₄ +1.

Chromium in [Cr(NH₃)₆]Cl₃ complex

Let the oxidation number of Cr in [Cr(NH₃)₆]Cl₃ x. Ammonia neutral thus the oxidation number zero and the oxidation number of chlorine -1.

∴ x + 0 +3(-1) = 0
or, x = +3
The oxidation number of chromium in [Cr(NH₃)₆]Cl₃ +3

Question
What is the oxidation number of chromium in CrO₅?

Answer
structure and oxidation number of CrO5
Structure of CrO5
Due to the peroxy linkage oxidation number of chromium in CrO₅ +6.

Organic compounds oxidation number zero

Some organic compounds where the oxidation number of carbon on this compound zero.
Let, the oxidation number of carbon in glucose (C₁₂H₂₂O₁₁) x.
∴ 6x + 12(+1) + 6(-2) = 0
or, x = 0

Compound Formula Oxidation Number
Sugar C₁₂H₂₂O₁₁ 0
Glucose C₆H₁₂O₆ 0
Formaldehyde HCHO 0

Study of oxidation number of elements, hydrogen, oxygen, carbon, chromium, iron, oxidation and reduction, an oxidizing and reducing agent

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