Question

In ${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }$ ion, the maximum number of bonds having the same Cr-O bond length is:
A. ${ 5 }$
B. ${ 4 }$
C. ${ 6 }$
D. ${ 3 }$

Answer 1

Hint: Bond length is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule.
Bond lengths are expressed in terms of Angstrom or picometer.

Complete step-by-step answer:
The structure of the dichromate ion is:


As we can see in the structure that there are six identical Cr - O bonds in the dichromate ion ${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }$.
It means there are 6 identical Cr - O terminal bonds in dichromate ion. All these bonds are equivalent due to resonance.
These are the oxoanions of chromium in the oxidation state of
${ +6 }$.
Hence, In ${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }$ ion, the maximum number of bonds having the same Cr-O bond length is ${ 6 }$.

Therefore, the correct option is C.

Additional Information:
1. Resonance is the delocalization of pi bonds, by shifting electrons. More the resonance structure more will be the stability.
2. These structures are similar in energy, bonding, and nonbonding pairs of electrons. Each form of structure is known as a canonical form.
3. The canonical forms of a molecule are also known as Resonating Structures.
Importance:
a) Resonance stabilizes the molecule
b) Resonance causes equality of bond lengths
c) Resonance hybrid always have the lowest energy than any canonical form

Note: The possibility to make a mistake is that you may choose option B. There are ${ 6 }$ equivalent bonds in dichromate ion, not four as all these bonds are in resonance.