Select the correct statement(s) about `Cr_(2)O_(7)^(2-)` and `CrO_(4)^(2-)`

Options

A. `Cr_(2)underset(OH)overset(H^(+))hArrCrO_(4)^(2-)`
B.
C. `theta gt 109^(@)28'`
D. Central 'O' is not purely `sp^(3)` hybridised.

Explanation

To solve the question regarding the correct statements about the ions \( \text{Cr}_2\text{O}_7^{2-} \) (dichromate) and \( \text{CrO}_4^{2-} \) (chromate), we will analyze the properties and structures of these ions step by step.

### Step 1: Identify the Ions
- **Dichromate Ion**: \( \text{Cr}_2\text{O}_7^{2-} \)
- **Chromate Ion**: \( \text{CrO}_4^{2-} \)

### Step 2: Understand the Hydrolysis Reaction
When chromate ions react with hydrogen ions (\( \text{H}^+ \)), they can convert to dichromate ions:
\[ \text{CrO}_4^{2-} + 2\text{H}^+ \rightarrow \text{HCrO}_4^{-} \rightarrow \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O} \]
This reaction indicates that chromate ions can be converted to dichromate ions in acidic conditions.

### Step 3: Analyze the Structures
- **Chromate Ion Structure**: The chromate ion has a tetrahedral structure with the chromium in the +6 oxidation state, surrounded by four oxygen atoms.
- **Dichromate Ion Structure**: The dichromate ion also has a tetrahedral structure but consists of two chromium atoms connected by bridging oxygen atoms.

### Step 4: Determine the Bond Angles
- The bond angle in the dichromate ion is approximately 131 degrees, which is greater than the typical tetrahedral angle of 109.5 degrees. This indicates that the structure is not purely tetrahedral due to the presence of multiple bonding interactions.

### Step 5: Hybridization
- The central atom in both ions (chromium) is not purely \( \text{sp}^3 \) hybridized due to the presence of double bonds and the geometry of the ions.

### Conclusion
Based on the analysis, the correct statements regarding \( \text{Cr}_2\text{O}_7^{2-} \) and \( \text{CrO}_4^{2-} \) are:
- Both ions exist in tetrahedral structures.
- The bond angle in dichromate is greater than 109 degrees.
- The central atom is not purely \( \text{sp}^3 \) hybridized.

### Correct Statements
The correct options are:
- B: The structure of chromate and dichromate is tetrahedral.
- C: The bond angle in dichromate is greater than 109 degrees.
- D: The central atom is not purely \( \text{sp}^3 \) hybridized.