The colour of `K_(2)Cr_(2)O_(7)`, and `Fe^(+2)` ions are respectively due to

Options

A. d-d transition and charge transfer spectra
B. Charge transfer spectra and d-d transition
C. crystal defects and charge transfer spectra
D. Charge transfer spectra and crystal defects

Answer

To solve the question regarding the colors of \( K_2Cr_2O_7 \) and \( Fe^{2+} \) ions, we can follow these steps:

### Step 1: Analyze the Color of \( K_2Cr_2O_7 \)

1. **Identify the Oxidation State of Chromium**:
- In \( K_2Cr_2O_7 \), the oxidation state of chromium (Cr) can be calculated as follows:
\[ 2X + 2(+1) + 7(-2) = 0 \]
\[ 2X + 2 - 14 = 0 \implies 2X - 12 = 0 \implies X = +6 \]
Thus, the oxidation state of chromium in \( K_2Cr_2O_7 \) is +6.

2. **Determine the Electron Configuration**:
- The electron configuration of chromium is \( [Ar] 4s^1 3d^5 \). After losing 6 electrons (to reach +6 oxidation state), the configuration becomes \( 4s^0 3d^0 \).

3. **Evaluate the Presence of Unpaired Electrons**:
- Since there are no electrons in the d-orbitals (3d), there are no unpaired electrons. Therefore, d-d transitions are not possible.

4. **Identify the Reason for Color**:
- Despite the absence of unpaired electrons, \( K_2Cr_2O_7 \) is red-orange in color due to charge transfer spectra.

### Step 2: Analyze the Color of \( Fe^{2+} \)

1. **Identify the Electron Configuration**:
- Iron (Fe) has the electron configuration \( [Ar] 4s^2 3d^6 \). When it loses 2 electrons to form \( Fe^{2+} \), the configuration becomes \( 3d^6 \).

2. **Evaluate the Presence of Unpaired Electrons**:
- In \( 3d^6 \), there are 4 unpaired electrons. This means that d-d transitions are possible.

3. **Identify the Reason for Color**:
- The \( Fe^{2+} \) ion is pale green in color, which is attributed to d-d transitions due to the presence of unpaired electrons.

### Step 3: Conclusion

- The color of \( K_2Cr_2O_7 \) is due to charge transfer spectra.
- The color of \( Fe^{2+} \) is due to d-d transitions.

### Final Answer

The colors of \( K_2Cr_2O_7 \) and \( Fe^{2+} \) ions are respectively due to:
- Charge transfer spectra (for \( K_2Cr_2O_7 \))
- d-d transition (for \( Fe^{2+} \))