Number of identical Cr-O bonds in dichromate ion `Cr_(2)O_(7)^(2-)` is :

Options

A. `4Cr-O` bonds are equivalent
B. `6Cr-O` bonds are equivalent
C. All `Cr-O` bonds are equivalent
D. None of `Cr-O` bonds are equivalent

Answer

To determine the number of identical Cr-O bonds in the dichromate ion \( \text{Cr}_2\text{O}_7^{2-} \), we can follow these steps:

### Step 1: Understand the Structure of Dichromate Ion
The dichromate ion has the formula \( \text{Cr}_2\text{O}_7^{2-} \). It consists of two chromium (Cr) atoms and seven oxygen (O) atoms. The structure can be represented as follows:

\[ \text{O} = \text{Cr} - \text{O} - \text{Cr} = \text{O} \]

### Step 2: Identify the Types of Cr-O Bonds
In the dichromate ion, there are two types of Cr-O bonds:
1. Terminal Cr-O bonds (the bonds between Cr and the outermost O atoms).
2. Central Cr-O bonds (the bonds between Cr and the O atoms in the middle).

### Step 3: Analyze Resonance
The dichromate ion exhibits resonance. This means that the double bonds and charges can shift among the oxygen atoms. The resonance structures show that the terminal Cr-O bonds can alternate between single and double bonds, leading to equal bond lengths.

### Step 4: Count the Identical Bonds
- There are 6 terminal Cr-O bonds that are involved in resonance, making them identical.
- The central Cr-O bonds do not participate in resonance and are different from the terminal bonds.

### Conclusion
Thus, the number of identical Cr-O bonds in the dichromate ion \( \text{Cr}_2\text{O}_7^{2-} \) is **6**.

### Final Answer
The number of identical Cr-O bonds in the dichromate ion \( \text{Cr}_2\text{O}_7^{2-} \) is **6**.

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