The colour of potassium dichromate is due to

Options

A. d - d transition
B. transition in `K^+` ion
C. ligand- to - metal charge transfer
D. metal to ligand charge transfer

Answer

To determine the color of potassium dichromate (K2Cr2O7), we need to analyze the oxidation state of chromium in the compound and understand the electronic transitions that contribute to its color.

### Step-by-Step Solution:

1. **Identify the Chemical Formula**:
- Potassium dichromate is represented as K2Cr2O7.

2. **Determine the Oxidation State of Chromium**:
- The oxidation state of potassium (K) is +1.
- Let the oxidation state of chromium (Cr) be \( x \).
- The oxidation state of oxygen (O) is -2.
- The equation for the oxidation states can be set up as follows:
\[ 2(+1) + 2x + 7(-2) = 0 \]
Simplifying this gives:
\[ 2 + 2x - 14 = 0 \implies 2x - 12 = 0 \implies 2x = 12 \implies x = +6 \]
- Thus, the oxidation state of chromium in potassium dichromate is +6.

3. **Analyze the Electronic Configuration**:
- The atomic number of chromium is 24, and its ground state electronic configuration is:
\[ \text{Cr: } [Ar] 3d^5 4s^1 \]
- In the +6 oxidation state, the electronic configuration changes to:
\[ \text{Cr}^{6+}: 1s^2 2s^2 2p^6 3s^2 3p^6 \]
- This indicates that there are no unpaired electrons in the d-orbitals.

4. **Determine the Cause of Color**:
- The color of potassium dichromate is not due to d-d transitions (which require unpaired electrons) since there are none in the +6 state.
- Instead, the color arises from **ligand-to-metal charge transfer (LMCT)**. This occurs when electrons are transferred from the ligand (in this case, oxygen) to the metal (chromium).
- Specifically, the charge transfer is from the 2p orbital of oxygen to the 3d orbital of chromium.

5. **Conclusion**:
- The color of potassium dichromate is due to ligand-to-metal charge transfer.

### Final Answer:
The color of potassium dichromate is due to **ligand-to-metal charge transfer**.

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