NEET Chemistry Notes Redox Reactions – Oxidation Number

NEET Chemistry Notes Redox Reactions – Oxidation Number

Oxidation Number

Oxidation Number

The real or imaginary charge, which an atom appears to have in its combined state is called oxidation number of that atom.

Rules for Assigning Oxidation Number

The oxidation number of an element or atom can be calculated with the help of following rules:

• The oxidation number of an element in its elementary state is zero, e.g. H in H₂, S in S₈, P in P₄
• Oxidation number of an ion is equal to the electrical charge present on it.
• Oxidation number of a compound is zero.
•  Oxidation number of fluorine is always -1 in all of its compounds.
• The oxidation number of alkali metals is always +1 and those of alkaline earth metals is +2.
•  Oxidation number of hydrogen is +1 except in ionic hydrides, where it is -1.
•  Two oxidation numbers of N are -3 and +3, when it is bonded with less electronegative and more electronegative atoms respectively.
•  Oxidation number of oxygen is -2 except in OF₂(+2),0₂F₂(+l), peroxides (-1) and superoxides
•  The oxidation number of halogens is always -1 in metal halides.
•  In interhalogen compounds, the more electronegative of the two halogens gets the oxidation number of-1.
• Oxidation number of metals in amalgams and carbonyls, e.g.  [Fe(CO)₅] is zero.

Fractional Oxidation States

These are often used to represent the average oxidation states of several atoms in a structure, e.g. in K02, the superoxide ion has a charge of -1 divided among two equivalent atoms, so each oxygen is assigned an oxidation state of  This ion is described as a resonance hybrid of two. Lewis structures, and each oxygen has oxidation state 0 in one structure and -1 in the other.

• For the cyclopentadienyl ion , the oxidation state of C is The -1 occurs because each C is bonded to one less electronegative H, and the  because the total ionic charge is divided among five equivalent C
•  If the average refers to atoms which are not equivalent, the average oxidation state may not be representative of each atom. This is true in magnetite Fe304, whose formula leads to an average oxidation state of . In fact one third of the iron ions are Fe3+ and two third Fe2+.
• Examples of fractional oxidation states for carbon
  

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