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Oxidation States

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The idea of oxidation state has been devised in order to keep track of the movement of electrons. In other words an oxidation state is defined for each atom in a molecule. Each atom in a homo nuclear neutral diatomic molecule is defined to be in the oxidation state of zero.


In hetero atomic molecules oxidation states are defined based on the electronegativity of atoms. The oxidation states of metallic elements are not conceptual as in the case of the oxidation state of carbon atoms in organic compounds. The oxidation state of an atom does not necessarily represent its actual electric charge, though it is close to the real electric change in simple ionic compounds.

Oxidation State Definition

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"The oxidation state is the number of electrons lost or gained by total or partial electron transfer when a compound is formed from its elements. If electron loss is involved the oxidation state is positive, if electron gain is involved the oxidation state is negative."
The oxidation state of an atom in a  compound is defined as the charge which it would carry in the most probable ionic formulation of the compound. This is decided on the basis of the electronegativities of the elements in the compound.

The oxidation state of a metal is defined as the charge left on the metal atom after all ligands have been removed in their normal, closed shell configuration that is with their electron pairs.

Determining Oxidation States

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The following are the rules for assigning the oxidation states of elements. The Oxidation States Rules are listed below.
  1. The oxidation state of an element in its elemental or free state is zero.
  2. The oxidation state of a simple ion is equal to the charge on the ion.
  3. Elements of Group I always have an oxidation state of +1 in their compounds. The elements include Li, Na and K.
  4. Elements of Group II always have an oxidation state of +2 in their compounds. The elements include Be, Mg, Ca and Ba.
  5. The oxidation number of hydrogen is usually +1 except in ionic hydride when it is -1.
  6. The oxidation state of oxygen is usually -2 except in hydrogen peroxide, peroxide and super oxides.
  7. Fluorine always have the oxidation state of -1. Other halogens also shows -1 oxidation state in halides. Cl, Br and I have also positive oxidation state.
  8. When a compound is formed between two non metals the more electronegative element has the negative oxidation state. For example, in HCl the oxidation state of H is +1 while that of Cl is -1.
  9. The sum of oxidation states of all the atoms in neutral compound is always zero.
  10. The sum of oxidation states of all the atoms in polyatomic ion is equals the charge on the ion.

Periodic Table with Oxidation States

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The maximum oxidation state of an element can be worked out from a knowledge of its group number. Magnesium id Group II has a positive oxidation state of +2. This value corresponds to the number of electrons in the outer shell of magnesium. Aluminum in Group III has a positive oxidation state of +3. In similar fashion it can be worked out that the maximum possible oxidation state for phosphorus is +5.

The Oxidation States of Elements are listed below.

S.No
Element
Oxidation state
Iron Oxidation States
+2 and +3
2
Oxidation State of Chromium
+3 and +6
3
Oxidation State of Silver  +1 
4
Manganese Oxidation States
+2, +3, +4, +6 and +7 
Oxidation States of Carbon
-4, -3, -2, -1, 0, +1, +2, +3, +4
6
Zinc Oxidation States  +2

Periodic table with corresponding oxidation states is shown below.

Periodic Table with Oxidation States

Oxidation State of Transition Metals

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Transition metals have electrons of similar energy in both the 3d and 4s levels. This means that one particular element can form ions of roughly the same stability by losing different numbers of electrons. Thus all the transition metals from titanium to copper exhibit two or more oxidation states in their compounds.

The following generalizations emerges from a study of the oxidation states.
  1. The common oxidation states for each element include +2 or +3 or both. +3 states are relatively more common at the beginning of the series. +2 states are more common towards the end.
  2. The highest oxidation states from scandium to manganese correspond to the involvement of all the electrons outside the argon core. One of the most important oxidation states is that which involves the weakly held electrons in the outer 4s shell only.
  3. The transition metals usually exhibit their highest oxidation states in compounds with oxygen or fluorine. 
  4. Ti, V, Cr and Mn never form simple ions in their highest oxidation state since this would result in ions of extremely high charge density.

Calculating Oxidation States

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Solved examples are given below based on oxidation state.

Solved Examples

Question 1: What is the oxidation number of Au in KAuCl4?
Solution:
 
K has a charge of +1. Cl has a charge of -1.
1(+1) + 1(x) + 4(-1) = 0
x = +3

 

Question 2: What is the oxidation number of I in H5IO6?
Solution:
 
H will have a +1 oxidation number since it is combined with nonmetals. Iodine will have a + charge because it is less electronegative than oxygen.
5H + I + 6O = 0
5(+1) + I) + 6(-2) = 0
(+5) + (I) + (-12) = 0
I + (-7) = 0
I = +7