Question

All the dichromate anion $C{r_2}{O_7}\,:$
A) All $Cr - O$ bonds are equivalent.
B) $6\,Cr - O$ bonds are equivalent.
C) $3\,Cr - O$ bonds are equivalent.
D) No bonds in $C{r_2}{O_7}\,$ are equivalent.

Answer 1

Dichromate ion ( $C{r_2}{O_7}^{2 - }$ ) is an anionic part of its salt. Dichromate ion is one of the divalent inorganic anions. These are oxoanions having +6 oxidation state of Chromium. These are very good oxidising agents due to the attachment of oxygen atoms. Both the chromium atoms are bounded with the single oxygen in a bridged fashion.

Complete step by step answer:
Dichromate ion ( $C{r_2}{O_7}^{2 - }$ ) is divalent in inorganic chemistry. This is also a conjugate base of ${H_2}C{r_2}{O_7}$ i.e., hydrogen dichromate. By removing both protons from ${H_2}C{r_2}{O_7}$ the dichromate ion is obtained. It is used as an oxidising agent. Because of its ionic properties, it forms many salts which can be used in oxidation and reduction reactions.
For example, ${K_2}C{r_2}{O_7}$ acts as an oxidising agent in the reaction it gets reduced.
On the $C{r_2}{O_7}^{2 - }$ i.e., dichromate ion is having $- 2$ charge and the oxidation state of chromium in $C{r_2}{O_7}^{2 - }$ (dichromate ion) is $+ 6$ . The $C{r^{ + 6}}$ ion has $4s$ and $3d$ vacant orbital. These orbitals combine to form a hybrid orbital i.e., $s{d^3}$ . So, hybridisation of dichromate ions is $s{d^3}$ hybridisation. Therefore, each $C{r^{ + 6}}$ of dichromate ion is $s{d^3}$ hybridised. The dichromate ion consists of two tetrahedra sharing oxygen at the common vertex. Therefore, each of the chromium at the centre of the tetrahedral bonds to four oxygen atoms. It is having six $Cr - O$ terminal bonds in dichromate ion which are equivalent. This is six $Cr - O$ terminal bonds are equivalent because of the resonance. The remaining two $Cr - O$ bonds which share the single oxygen atom.
Therefore, the answer is $6\,Cr - O$ bonds are equivalent.

So, the correct option is (B); $6\,Cr - O$ bonds are equivalent.

Note: The two $Cr - O$ bonds which share the single oxygen atom are longer than the six $Cr - O$ terminal bonds that are equivalent. The salt of the dichromate ion is used for oxidation and reduction reactions. Dichromate poses an orange colour. The $C{r_2}{O_7}^{2 - }$ consists of two tetrahedra sharing oxygen at the common corner.