Correct Answer - Option 2 : 6
Concept:
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The oxidation number/ oxidation state of an element in a compound is the number of charges (positive or negative) assigned to an atom in a molecule or ion according to a set of some arbitrary rules.
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Rules for assigning Oxidation number:
- The oxidation number of all elements in the free state is zero.
- The most electronegative element, Fluorine has a -1 oxidation number in all compounds.
- The oxidation number of monoatomic ions is the same as its charge. For example:
- Oxidation state of Li+ , Ca2+ and Al3+ are +1, +2 and +3 respectively.
- The oxidation state of Cl- and SO42- are -1 and -2 respectively.
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Hydrogen has a +1 oxidation state in all compounds except ionic hydrides (-1 oxidation state).
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The oxidation state of oxygen is -2 in a compound except in peroxides which are -1.
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Alkali metals (Li, Na, K, etc.) always have +1, and alkaline earth metals (Be, Mg, etc.) have +2 oxidation states respectively.
- In transition metal complexes, the charge of ligands is taken into consideration.
- The ligands can be categorized into anionic, cationic, and neutral. The neutral ligands do not carry any charge.
- Some examples of ligands are:
| Ligands |
Category |
| Cl-, Br-, I-, NO3-, N3-, S2- etc |
Anionic ligands |
|
Ethylenediamine(en), water, ammonia, CO, PPh3, etc |
Neutral ligands |
| NO+, NO2+
|
Positive ligands |
Calculation:
- Let the oxidation state be 'x' of Chromium in K2Cr2O4.
- Four oxygen atoms give us - 2 × 4 = -8 charge.
- The metal ion is in a unipositive state, so two K+ ions will provide a +2 charge.
- The molecule as a whole has no charge, it is neutral.
- Hence, the equation for charges is:
x + 2 + ( -8 ) = 0,
or, x - 6 = 0, or x = 6
Hence, the charge on each chromium ion in K2CrO4 is +6.
