In the structure of the dichromate ion, there is a
(a) linear symmetrical Cr-O-Cr bond.
(b) non-line

Q: Why isn't the Cr-O-Cr bond linear like in carbon dioxide?

Unlike the carbon atom in $CO_2$ which is $sp$ hybridized with no lone pairs, the bridging oxygen in dichromate has lone pairs. These lone pairs exert repulsion, pushing the chromium atoms down and creating a bent geometry with an angle of about $126^\circ$.

Q: Are all the Cr-O bonds in the dichromate ion of the same length?

No! The two bridging Cr-O bonds are longer (about 179 pm) because they are single bonds. The six terminal Cr-O bonds are shorter (about 163 pm) because resonance gives them significant double-bond character.

Q: What is the hybridization of the chromium atoms in dichromate?

The chromium atoms are in a +6 oxidation state and are tetrahedrally coordinated by four oxygen atoms. They utilize $d^3s$ or $sd^3$ hybridization to form these tetrahedral structures.

Q: Does the dichromate ion have a dipole moment?

Yes. Because the Cr-O-Cr bridge is bent and not linear, the individual bond dipoles do not perfectly cancel out, giving the overall ion a net dipole moment.

Q: Why is the dichromate ion colored if Cr(VI) has a $d^0$ configuration?

The intense orange color of the dichromate ion is not due to d-d transitions (since there are no d-electrons). Instead, it is caused by Ligand-to-Metal Charge Transfer (LMCT), where an electron from an oxygen lone pair is temporarily excited into an empty d-orbital of chromium.

Question

In the structure of the dichromate ion, there is a

Options:

(A) linear symmetrical $Cr-O-Cr$ bond.

(B) non-linear symmetrical $Cr-O-Cr$ bond.

(C) linear unsymmetrical $Cr-O-Cr$ bond.

(D) non-linear unsymmetrical $Cr-O-Cr$ bond.

Sigma Percentile · Accepted Answer

### The Anatomy of the Dichromate Ion

The dichromate ion, mathematically represented as $Cr_2O_7^{2-}$, is a fascinating chemical species. It is essentially formed by two chromate ($CrO_4^{2-}$) tetrahedra that have decided to link together by sharing a single oxygen atom at one of their corners.

This shared oxygen atom acts as a bridge, connecting the two chromium centers and creating the $Cr-O-Cr$ linkage that defines the molecule's core structure.

### The Bent Reality: Why It Is Non-Linear

When we look at the central bridging oxygen atom, our first instinct might be to assume it forms a straight, $180^\circ$ line between the two chromium atoms. However, physical reality is quite different!

The central oxygen atom is $sp^3$ hybridized and possesses two lone pairs of electrons. These lone pairs exert significant repulsive forces on the $Cr-O$ bonds. Combined with the steric hindrance of the two bulky $CrO_3$ groups, this forces the molecule into a bent geometry.

The actual $\angle Cr-O-Cr$ bond angle is approximately $126^\circ$. Because this angle is far from $180^\circ$, we can definitively say that the $Cr-O-Cr$ bond is **non-linear**.

### Perfect Balance: The Symmetrical Bridge

Now, let's examine the bond lengths within this bridge. Are they skewed, or are they perfectly balanced?

If we look at the two chromium atoms, we see that they are in perfectly identical chemical environments. Each chromium is bonded to three terminal oxygen atoms and one bridging oxygen atom. Because there is no difference in their surroundings, there is no chemical reason for one bridging $Cr-O$ bond to be longer or stronger than the other.

Therefore, both bridging $Cr-O$ bonds are of exactly equal length. This perfect balance means that the $Cr-O-Cr$ bridge is **symmetrical**.

### The Terminal Oxygens and Resonance

To complete our understanding of the dichromate ion, we must consider the terminal oxygen atoms. Each chromium atom is bonded to three terminal oxygens.

While a static Lewis structure might show a mix of single and double bonds, the reality is that the negative charge is delocalized across the entire molecule through resonance. This resonance ensures that all six terminal $Cr-O$ bonds are completely equivalent to one another, further adding to the molecule's overall symmetry.

### The Final Verdict

By analyzing the geometry and the chemical environment, we have uncovered the true nature of the dichromate ion's core linkage.

The presence of lone pairs on the bridging oxygen creates a bent angle, making it **non-linear**. Simultaneously, the identical environments of the chromium atoms ensure the bond lengths are equal, making it **symmetrical**.

Thus, the $Cr-O-Cr$ bond is beautifully **non-linear and symmetrical**.

Animated Solution for Chemistry - d and f-Block Elements: In the structure of the dichromate ion, there is a

Select Answer:

Visualized Solution

The Dichromate Ion $C r_{2} O_{7}^{2 -}$

The Bridging Oxygen

Symmetry of the Bridge

Terminal Oxygen Atoms

Final Conclusion

The Sigma Insight: d-block Elements

The Anatomy of the Dichromate Ion

The Bent Reality

Perfect Balance

The Terminal Oxygens and Resonance

The Final Verdict

Similar Questions

The sum of the total number of bonds between chromium and oxygen atoms in chromate and dichromate ions is .........

The correct statement(s) about $Cr^{2 +}$ and $Mn^{3 +}$ is (are) [Atomic numbers of $Cr = 24$ and $Mn = 25$ ]

What would happen when a solution of potassium chromate is treated with an excess of dilute nitric acid?

The incorrect statement is :

The correct order of following $3 d$ -metal oxides, according to their oxidation numbers is (A) $CrO_{3}$ (B) $Fe_{2} O_{3}$ (C) $MnO_{2}$ (D) $V_{2} O_{5}$ (E) $Cu_{2} O$

The addition of dilute $NaOH$ to $Cr^{3 +}$ salt solution will give

In which of the following pairs, the outer most electronic configuration will be the same?

Fusion of $MnO_{2}$ with $KOH$ in presence of $O_{2}$ produces a salt $W$ . Alkaline solution of $W$ upon eletrolytic oxidation yields another salt $X$ . The manganese containing ions present in $W$ and $X$ , respectively, are $Y$ and $Z$ . Correct statement(s) is (are)

The colour of $KMnO_{4}$ is due to