The dichromate ion, ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$, has a structure where two chromium atoms are bridged by an oxygen atom (Cr-O-Cr linkage). Each chromium atom is tetrahedrally coordinated to four oxygen atoms.

The structure of ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$ is:

In this structure, the two Cr-O bonds in the Cr-O-Cr bridge are generally longer and weaker than the terminal Cr=O bonds. However, due to resonance, all terminal Cr-O bonds become identical in length and bond order. There are six such terminal Cr-O bonds (three on each chromium atom).

Option Analysis:

• A) 4: This would imply only two terminal bonds per chromium, which is incorrect.
• B) 6: This is correct. There are six terminal Cr-O bonds, which are identical due to resonance. The two bridging Cr-O bonds are different from these terminal bonds.
• C) 7: This would include all Cr-O bonds, including the two bridging ones, but they are not identical to the terminal bonds.
• D) 8: This is incorrect, as there are only 8 Cr-O bonds in total (6 terminal + 2 bridging).

Correct Answer: (B)