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Oxidation Number

Oxidation Number of Periodic Table Elements

The oxidation number of periodic table elements in the chemical compound is the formal charge (positive or negative) if all the bonds in the compounds were ionic bonds. Electronegativity concepts utilize to calculate the formal charge of an atom. Therefore, the less or more electronegative partner of a binary compounds arbitrary assigned positive or negative oxidation number of periodic table elements. For example, fluorine and alkali crystalline metals have assigned negative and positive oxidation respectively because fluorine is most electronegative and alkali metals are highly electropositive. Therefore, some general rules used to find the oxidation number of s, p, d, and f-block elements. To describes some chemical reactions, we use oxidation number rules because some of the reactions can not be explained by electronic or classical concepts of redox reactions.

The water molecule formed by the hydrogen bonding with oxygen and hydrochloric acid formed by bonding hydrogen with chlorine. These two chemical reactions can not be explained by classical concepts of redox reactions. To explain these types of reactions we use oxidation numbers rules. Therefore, these rules also easily predict the oxidizing agent reducing agents in the chemical reactions and also balancing equations in a more convenient way.

How to find the oxidation number of elements?

Calculate oxidation number or state of periodic table chemical elements

The following general rules are found in these online courses for calculating the oxidation number of elements in the compounds.

  • The atom of the diatomic molecules like chlorine, oxygen, the hydrogen atom, etc, and metallic elements like zinc, copper, sodium, etc are assigned zero oxidation numbers.
    Since the same elements of similar electronegativity are involved in the bonding.
  • Except for metal hydrides the oxidation number of hydrogen +1. Hence alkali metal hydrides like lithium hydride, sodium hydride, cesium hydride, etc, the oxidation state of hydrogen -1.
  • Metal always possess a positive oxidation state.
  • Oxygen has normally -2 but in peroxide and superoxides the oxidation number of oxygen -1 and -1/2 respectively.
  • The oxidation number of an ion in polar molecules equal to its charge.
  • The algebraic sum of the oxidation numbers of all the atoms in a compound must be zero but in many atomic ions equal to its charge.

Oxidation Numbers of Hydrogen

Like alkalines, hydrogen has a single electron particle in outer quantum shall, and like halogen, hydrogen has just one electron short of next noble gas helium. The electron configuration of hydrogen, 1s1. Therefore, hydrogen can easily losses this electron to show the +1 state and affinity to gaining one electron to show the -1 oxidation number.

H → H+ + e
H + e → H

Oxidation Numbers of Alkali and Alkaline Metals

Alkali and alkaline earth metals are highly electropositive with very low ionization energy. Therefore, like the halogens in alkali halides, halogen shows negative oxidation numbers but alkali and alkaline earth metals show the positive oxidation numbers. Hence the electrolysis of crystalline solid alkaline hydrides like LiH, CsH, and CaH2, etc liberated hydrogen at the anode. For example, NaH, CsH2 and CaH2 has oxidation number = -1. These hydrides are formed by the direct combination of hydrogen and the alkalies at 350°C to 600°C.

Alkali + H2 → Alkali Hydrides

Oxidation Number of Superoxides and Peroxides

When alkalies react with oxygen form the number of binary compounds like monoxides (M2O), peroxides (M2O2), and superoxides (MO2). Alkali and alkaline earth elements are the only known example for the formation of superoxides. But with the increasing size of the alkali ion, the stability of superoxides also increases. Lithium only forms monoxide, sodium up to peroxides, and another alkali up to superoxides. The oxidation number of oxygen in peroxide and superoxide are -1 and -½ respectively.

Fluorine is more electronegative than oxygen. Thus fluorine forms monoxides and peroxides with oxidation number = -1.

Oxidation Numbers of Atoms in Compounds

Manganese in potassium permanganate

Let the oxidation number of manganese in potassium permanganate (KMnO4) = x. Therefore, according to the above rule

(+1) + x + 4(-2) = 0
or, x = +7

Chromium in dichromate ion

Let the oxidation number of chromium in dichromate ion (Cr2O7-2) = x.

∴ 2x + 7(-2) = -2
or, x = +6

Both nitrogen in ammonium nitrate

Ammonium nitrate (NH4NO3) present as a cation NH4+ and NO3 ion and let  the oxidation number of nitrogen in NH4+ = x and NO3 = y.

∴ x+3(+1) = +1
or, x=0
y+3(-2) =-1
or, y= +5

Both chlorine atoms in bleaching powder

In bleaching powder Ca(OCl)Cl, one chlorine combine with oxygen to form OCl ion and another chlorne in present as Cl ion. Therefore, the oxidation number of one chlorine = +1 and other =-1.

Phosphorus in hypophosphorous acid

Let the phosphorus of phosphorus in hypophosphoric acid ( H4P2O7) = x.

∴ 4(+1) + 2x + 7(-2) = 0
or, x = +5

Question: What is the oxidation state of barium in Ba(H2PO2)2?

Answer: The oxidation state of hydrogen and oxygen in Ba(H2PO2)2 are +1 and -2 respectively and let for barium = x.

∴ (+2) + 2{2(+1)+x+2(-2)} = 0
or, x = +1

Sulfur in sulphuric acid

Let the oxidation state or number of sulfur in sulphuric acid (H2SO4) = x and hydrogen +1 and oxygen -2 respectively.

∴ 2(+1) + x + 4(-2) = 0
or, x = +6

Metals in coordination compounds

Metal ions ion in coordination compound possesses two kinds of valencies primary and secondary valency.

  1. Hence the primary valency satisfied by the requisite number of anions or negative groups being present inside and outside the coordination sphere.
  2. But secondary valency indicated the capacity of metal ion or groups around itself in the first sphere of coordination zone.

Now primary valency equated with the oxidation state and secondary valency coordination number. Therefore, the coordination number of chromium = +6 in [Cr(NH3)6]Cl3 and let the oxidation number of chromium in this compounds = x and ammonia and chlorine = 0 and -1 respectively.

Hence, x + 0 +3(-1) = 0
or, x = +3

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

Answer: Let the oxidation state of iron in [Fe(H2O)5NO+]SO4 = x and water, NO+ and sulfate ion = 0, +1and -2 respectively.

∴ x+5(0)+(+1)-2=0
or, x = +1

Iron in iron pentacarbonyl

Both the carbonyl and iron has zero oxidation state in  Fe(CO)5

Question: What is the oxidation state of chromium in CrO5?

Answer: Due to the peroxy linkage oxidation state or number of chromium in CrO5 = +6.

Carbon in acetone compounds

Let the oxidation numbers of carbon in acetone = x and hydrogen and oxygen +1 and -2 respectively.

∴ 3x + 6(+1) + (-2) = 0
or, x = -(4/3)

Oxidation Number of Carbon Compounds

Oxidation numbers of hydrocarbon and carbon compounds like methane, methyl chloride, dichloromethane, chloroform, and carbon tetrachloride are -4, -2, 0, +2, +4 respectively.  But Sugar, glucose, formaldehyde, etc are the examples of organic compounds where the oxidation number of carbon on these compounds is always zero.