[arvind1990]

Hi,

      As we all know that config. of Cr is [noblegas]3d5 4s1.
If Cr+ exists which one woll be more stable ,is it Cr3+ or Cr+.I think Cr+ will be more stable due to half filled stability.
But from teachers and others I came to know that Cr3+ is more stable than Cr+,and the most common oxidation state of Cr is +3 and not +1 and Cr+ exists very rarely.
Why is it so??.

[tamim83]

I think it is because when Cr bonds with ligands to form coordination compounds, the d orbitals are split and the dx^2-y^2 and dz^2 orbitals are higher in energy than the other three d orbitals.  Cr3+ has an E.C. of [Ar] 3d^3 so these three electrons go in the three lower energy d orbitals.  Cr+ would have the extra two electrons in the higher energy d orbitals which would be less favorable energetically. 

[Borek]

Are we talking about Cr in general, or Cr in water solution? These are completely different cases. Look at the configuration of copper - one may expect Cu⁺ to be the stable ion, yet it is Cu²⁺ that dominates, thanks to ligand field of water molecules.