How many \(Cr-O\) bonds in dichromate ions are of the same bond length and are in resonance?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 6
The dichromate ion, \( \ce{Cr2O7^2-} \), consists of two CrO4 tetrahedra sharing one corner (one bridging oxygen, Cr–O–Cr angle 126°). Each Cr carries three terminal oxygen atoms, so there are 3 + 3 = 6 terminal Cr–O bonds. Their double-bond character is shared by resonance, so all six have the same length (163 pm). The two bridging Cr–O bonds are longer (179 pm) and take no part in this resonance.
\( \displaystyle \text{Equal, resonating Cr--O bonds} = 3 + 3 = 6 \)
NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – “… The structures of chromate ion, CrO42− and the dichromate ion, Cr2O72− are shown below. The chromate ion is tetrahedral whereas the dichromate ion consists of two tetrahedra sharing one corner with Cr–O–Cr bond angle of 126°.”
