How many Cr-O bonds are equivalent in chromate dianion ?

Answer

To determine how many Cr-O bonds are equivalent in the chromate dianion (Cr2O7^2-), we can analyze the structure and bonding of the ion step by step.

### Step-by-Step Solution:

1. **Identify the Structure of Chromate Dianion**:
The chromate dianion has the formula Cr2O7^2-. It consists of two chromium (Cr) atoms and seven oxygen (O) atoms.

2. **Draw the Lewis Structure**:
In the chromate ion, each chromium atom is bonded to oxygen atoms. The typical structure includes:
- Each chromium atom is bonded to three oxygen atoms through single bonds and one oxygen atom through a double bond.
- The overall structure has resonance, meaning that the double bonds can be placed between different oxygen atoms.

3. **Recognize Resonance**:
The Cr-O bonds in the chromate ion are subject to resonance. This means that the double bonds can shift among the oxygen atoms. For each chromium atom:
- There are three Cr-O bonds that can participate in resonance.
- The resonance allows these bonds to be considered equivalent.

4. **Count the Equivalent Bonds**:
- For the first chromium atom, there are three equivalent Cr-O bonds due to resonance.
- For the second chromium atom, there are also three equivalent Cr-O bonds due to resonance.
- Therefore, the total number of equivalent Cr-O bonds in the chromate dianion is 3 (from the first Cr) + 3 (from the second Cr) = 6 equivalent Cr-O bonds.

5. **Conclusion**:
The total number of equivalent Cr-O bonds in the chromate dianion is 6.

### Final Answer:
**6 Cr-O bonds are equivalent in the chromate dianion (Cr2O7^2-).**