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The oxidation state of Cr in K2CrO4 is:
1. 4
2. 6
3. 7
4. -6

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Correct Answer - Option 2 : 6

Concept: 

  • 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.
  •  

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.
  • Hydrogen has a +1 oxidation state in all compounds except ionic hydrides (-1 oxidation state).
  • The oxidation state of oxygen is -2 in a compound except in peroxides which are -1.
  • Alkali metals (LiNaK, etc.) always have +1, and alkaline earth metals (BeMg, 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.

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